objective C tutorial

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Objective-C is general-purpose language that is developed on top of C Programming language by adding features of Small Talk programming language making it an object-oriented language. It is primarily used in developing iOS and Mac OS X operating systems as well as its applications.

Initially, Objective-C was developed by NeXT for its NeXTSTEP OS from whom it was taken over by Apple for its iOS and Mac OS X.

Object-Oriented Programming

Fully supports object-oriented programming, including the four pillars of object-oriented development:

• Encapsulation
• Data hiding
• Inheritance
• Polymorphism

Example Code

#import <Foundation/Foundation.h>

int main (int argc, const char * argv[])
{
   NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init];

   NSLog (@"hello world");
   [pool drain];
   return 0;
}

Foundation Framework

Foundation Framework provides large set of features and they are listed below.

• It includes a list of extended datatypes like NSArray, NSDictionary, NSSet and so on.
• It consists of a rich set of functions manipulating files, strings, etc.
• It provides features for URL handling, utilities like date formatting, data handling, error handling, etc.

Learning Objective-C

The most important thing to do when learning Objective-C is to focus on concepts and not get lost in language technical details.

The purpose of learning a programming language is to become a better programmer; that is, to become more effective at designing and implementing new systems and at maintaining old ones.

Use of Objective-C

Objective-C, as mentioned earlier, is used in iOS and Mac OS X. It has large base of iOS users and largely increasing Mac OS X users. And since Apple focuses on quality first and its wonderful for those who started learning Objective-C.

Local Environment Setup

If you are still willing to set up your environment for Objective-C programming language, you need the following two softwares available on your computer, (a) Text Editor and (b) The GCC Compiler.

Text Editor

This will be used to type your program. Examples of few editors include Windows Notepad, OS Edit command, Brief, Epsilon, EMACS, and vim or vi.

Name and version of text editor can vary on different operating systems. For example, Notepad will be used on Windows, and vim or vi can be used on windows as well as Linux or UNIX.

The files you create with your editor are called source files and contain program source code. The source files for Objective-C programs are typically named with the extension ".m".

Before starting your programming, make sure you have one text editor in place and you have enough experience to write a computer program, save it in a file, compile it and finally execute it.

The GCC Compiler

The source code written in source file is the human readable source for your program. It needs to be "compiled" to turn into machine language, so that your CPU can actually execute the program as per instructions given.

This GCC compiler will be used to compile your source code into final executable program. I assume you have basic knowledge about a programming language compiler.

GCC compiler is available for free on various platforms and the procedure to set up on various platforms is explained below.

Installation on UNIX/Linux

The initial step is install gcc along with gcc Objective-C package. This is done by:

$ su - 
$ yum install gcc
$ yum install gcc-objc

The next step is to set up package dependencies using following command:

$ yum install make libpng libpng-devel libtiff libtiff-devel libobjc libxml2 libxml2-devel libX11-devel libXt-devel libjpeg libjpeg-devel

In order to get full features of Objective-C, download and install GNUStep. This can be done by downloading the package from http://main.gnustep.org/resources/downloads.php.

Now, we need to switch to the downloaded folder and unpack the file by:

$ tar xvfz gnustep-startup-.tar.gz

Now, we need to switch to the folder gnustep-startup that gets created using:

$ cd gnustep-startup-

Next, we need to configure the build process:

$ ./configure

Then, we can build by:

$ make

We need to finally set up the environment by:

$ . /usr/GNUstep/System/Library/Makefiles/GNUstep.sh

We have a helloWorld.m Objective-C as follows:

#import <Foundation/Foundation.h>

int main (int argc, const char * argv[])
{
    NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init];
    NSLog (@"hello world");
    [pool drain];
    return 0;
}

Now, we can compile and run a Objective-C file say helloWorld.m by switching to folder containing the file using cd and then using the following steps:

$ gcc `gnustep-config --objc-flags` -L/usr/GNUstep/Local/Library/Libraries -lgnustep-base helloWorld.m -o helloWorld
$ ./helloWorld

We can see the following output:

2013-09-07 10:48:39.772 tutorialsPoint[12906] hello world

Installation on Mac OS

If you use Mac OS X, the easiest way to obtain GCC is to download the Xcode development environment from Apple's web site and follow the simple installation instructions. Once you have Xcode set up, you will be able to use GNU compiler for C/C++.

Xcode is currently available at developer.apple.com/technologies/tools/.

Installation on Windows

In order to run Objective-C program on windows, we need to install MinGW and GNUStep Core. Both are available at http://www.gnu.org/software/gnustep/windows/installer.html.

First, we need to install the MSYS/MinGW System package. Then, we need to install the GNUstep Core package. Both of which provide a windows installer, which is self-explanatory.

Then to use Objective-C and GNUstep by selecting Start -> All Programs -> GNUstep -> Shell

Switch to the folder containing helloWorld.m

We can compile the program by using:

$ gcc `gnustep-config --objc-flags` -L /GNUstep/System/Library/Libraries hello.m -o hello -lgnustep-base -lobjc

We can run the program by using:

./hello.exe

We get the following output:

2013-09-07 10:48:39.772 tutorialsPoint[1200] hello worldBefore we study basic building blocks of the Objective-C programming language, let us look a bare minimum Objective-C program structure so that we can take it as a reference in upcoming chapters.

***************************************

Objective-C Hello World Example

A Objective-C program basically consists of the following parts:

• Preprocessor Commands
• Interface
• Implementation
• Method
• Variables
• Statements & Expressions
• Comments

Let us look at a simple code that would print the words "Hello World":

#import <Foundation/Foundation.h>

@interface SampleClass:NSObject
- (void)sampleMethod;
@end

@implementation SampleClass

- (void)sampleMethod{
   NSLog(@"Hello, World! \n");
}

@end

int main()
{
   /* my first program in Objective-C */
   SampleClass *sampleClass = [[SampleClass alloc]init];
   [sampleClass sampleMethod];
   return 0;
}

Let us look various parts of the above program:

1. The first line of the program #import <Foundation/Foundation.h> is a preprocessor command, which tells a Objective-C compiler to include Foundation.h file before going to actual compilation.
2. The next line @interface SampleClass:NSObject shows how to create an interface. It inherits NSObject, which is the base class of all objects.
3. The next line - (void)sampleMethod; shows how to declare a method.
4. The next line @end marks the end of an interface.
5. The next line @implementation SampleClass shows how to implement the interface SampleClass.
6. The next line - (void)sampleMethod{} shows the implementation of the sampleMethod.
7. The next line @end marks the end of an implementation.
8. The next line int main() is the main function where program execution begins.
9. The next line /*...*/ will be ignored by the compiler and it has been put to add additional comments in the program. So such lines are called comments in the program.
10. The next line NSLog(...) is another function available in Objective-C which causes the message "Hello, World!" to be displayed on the screen.
11. The next line return 0; terminates main()function and returns the value 0.

Compile & Execute Objective-C Program:

Now when we compile and run the program, we will get the following result.

2013-09-07 22:38:27.932 demo[28001] Hello, World!

You have seen a basic structure of Objective-C program, so it will be easy to understand other basic building blocks of the Objective-C programming language.

Tokens in Objective-C

A Objective-C program consists of various tokens and a token is either a keyword, an identifier, a constant, a string literal, or a symbol. For example, the following Objective-C statement consists of six tokens:

NSLog(@"Hello, World! \n");

The individual tokens are:

NSLog
@
(
"Hello, World! \n"
)
;

Semicolons ;

In Objective-C program, the semicolon is a statement terminator. That is, each individual statement must be ended with a semicolon. It indicates the end of one logical entity.

For example, following are two different statements:

NSLog(@"Hello, World! \n");
return 0;

Comments

Comments are like helping text in your Objective-C program and they are ignored by the compiler. They start with /* and terminate with the characters */ as shown below:

/* my first program in Objective-C */

You can not have comments with in comments and they do not occur within a string or character literals.

Identifiers

An Objective-C identifier is a name used to identify a variable, function, or any other user-defined item. An identifier starts with a letter A to Z or a to z or an underscore _ followed by zero or more letters, underscores, and digits (0 to 9).

Objective-C does not allow punctuation characters such as @, $, and % within identifiers. Objective-C is a case-sensitive programming language. Thus, Manpower and manpower are two different identifiers in Objective-C. Here are some examples of acceptable identifiers:

mohd       zara    abc   move_name  a_123
myname50   _temp   j     a23b9      retVal

Keywords

The following list shows few of the reserved words in Objective-C. These reserved words may not be used as constant or variable or any other identifier names.

auto	else	long	switch
break	enum	register	typedef
case	extern	return	union
char	float	short	unsigned
const	for	signed	void
continue	goto	sizeof	volatile
default	if	static	while
do	int	struct	_Packed
double	protocol	interface	implementation
NSObject	NSInteger	NSNumber	CGFloat
property	nonatomic;	retain	strong
weak	unsafe_unretained;	readwrite	readonly

Whitespace in Objective-C

A line containing only whitespace, possibly with a comment, is known as a blank line, and an Objective-C compiler totally ignores it.

Whitespace is the term used in Objective-C to describe blanks, tabs, newline characters and comments. Whitespace separates one part of a statement from another and enables the compiler to identify where one element in a statement, such as int, ends and the next element begins. Therefore, in the following statement:

int age;

There must be at least one whitespace character (usually a space) between int and age for the compiler to be able to distinguish them. On the other hand, in the following statement,

fruit = apples + oranges;   // get the total fruit

no whitespace characters are necessary between fruit and =, or between = and apples, although you are free to include some if you wish for readability purpose.

In the Objective-C programming language, data types refer to an extensive system used for declaring variables or functions of different types. The type of a variable determines how much space it occupies in storage and how the bit pattern stored is interpreted.

The types in Objective-C can be classified as follows:

S.N.	Types and Description
1	Basic Types: They are arithmetic types and consist of the two types: (a) integer types and (b) floating-point types.
2	Enumerated types: They are again arithmetic types and they are used to define variables that can only be assigned certain discrete integer values throughout the program.
3	The type void: The type specifier void indicates that no value is available.
4	Derived types: They include (a) Pointer types, (b) Array types, (c) Structure types, (d) Union types and (e) Function types.

The array types and structure types are referred to collectively as the aggregate types. The type of a function specifies the type of the function's return value. We will see basic types in the following section whereas other types will be covered in the upcoming chapters.

Integer Types

Following table gives you details about standard integer types with its storage sizes and value ranges:

Type	Storage size	Value range
char	1 byte	-128 to 127 or 0 to 255
unsigned char	1 byte	0 to 255
signed char	1 byte	-128 to 127
int	2 or 4 bytes	-32,768 to 32,767 or -2,147,483,648 to 2,147,483,647
unsigned int	2 or 4 bytes	0 to 65,535 or 0 to 4,294,967,295
short	2 bytes	-32,768 to 32,767
unsigned short	2 bytes	0 to 65,535
long	4 bytes	-2,147,483,648 to 2,147,483,647
unsigned long	4 bytes	0 to 4,294,967,295

To get the exact size of a type or a variable on a particular platform, you can use the sizeof operator. The expression sizeof(type) yields the storage size of the object or type in bytes. Following is an example to get the size of int type on any machine:

#import <Foundation/Foundation.h>

int main()
{
   NSLog(@"Storage size for int : %d \n", sizeof(int));
   
   return 0;
}

When you compile and execute the above program, it produces the following result on Linux:

2013-09-07 22:21:39.155 demo[1340] Storage size for int : 4 

Floating-Point Types

Following table gives you details about standard float-point types with storage sizes and value ranges and their precision:

Type	Storage size	Value range	Precision
float	4 byte	1.2E-38 to 3.4E+38	6 decimal places
double	8 byte	2.3E-308 to 1.7E+308	15 decimal places
long double	10 byte	3.4E-4932 to 1.1E+4932	19 decimal places

The header file float.h defines macros that allow you to use these values and other details about the binary representation of real numbers in your programs. Following example will print storage space taken by a float type and its range values:

#import <Foundation/Foundation.h>

int main()
{
   NSLog(@"Storage size for float : %d \n", sizeof(float));
   
   return 0;
}

When you compile and execute the above program, it produces the following result on Linux:

2013-09-07 22:22:21.729 demo[3927] Storage size for float : 4 

The void Type

The void type specifies that no value is available. It is used in three kinds of situations:

S.N.	Types and Description
1	Function returns as void There are various functions in Objective-C which do not return value or you can say they return void. A function with no return value has the return type as void. For example, void exit (int status);
2	Function arguments as void There are various functions in Objective-C which do not accept any parameter. A function with no parameter can accept as a void. For example, int rand(void);

The void type may not be understood to you at this point, so let us proceed and we will cover these concepts in upcoming chapters.

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A variable is nothing but a name given to a storage area that our programs can manipulate. Each variable in Objective-C has a specific type, which determines the size and layout of the variable's memory; the range of values that can be stored within that memory; and the set of operations that can be applied to the variable.

The name of a variable can be composed of letters, digits, and the underscore character. It must begin with either a letter or an underscore. Upper and lowercase letters are distinct because Objective-C is case-sensitive. Based on the basic types explained in previous chapter, there will be the following basic variable types:

Type	Description
char	Typically a single octet (one byte). This is an integer type.
int	The most natural size of integer for the machine.
float	A single-precision floating point value.
double	A double-precision floating point value.
void	Represents the absence of type.

Objective-C programming language also allows to define various other types of variables, which we will cover in subsequent chapters like Enumeration, Pointer, Array, Structure, Union, etc. For this chapter, let us study only basic variable types.

Variable Definition in Objective-C:

A variable definition means to tell the compiler where and how much to create the storage for the variable. A variable definition specifies a data type and contains a list of one or more variables of that type as follows:

type variable_list;

Here, type must be a valid Objective-C data type including char, w_char, int, float, double, bool or any user-defined object, etc., and variable_list may consist of one or more identifier names separated by commas. Some valid declarations are shown here:

int    i, j, k;
char   c, ch;
float  f, salary;
double d;

The line int i, j, k; both declares and defines the variables i, j and k; which instructs the compiler to create variables named i, j and k of type int.

Variables can be initialized (assigned an initial value) in their declaration. The initializer consists of an equal sign followed by a constant expression as follows:

type variable_name = value;

Some examples are:

extern int d = 3, f = 5;    // declaration of d and f. 
int d = 3, f = 5;           // definition and initializing d and f. 
byte z = 22;                // definition and initializes z. 
char x = 'x';               // the variable x has the value 'x'.

For definition without an initializer: variables with static storage duration are implicitly initialized with NULL (all bytes have the value 0); the initial value of all other variables is undefined.

Variable Declaration in Objective-C:

A variable declaration provides assurance to the compiler that there is one variable existing with the given type and name so that compiler proceed for further compilation without needing complete detail about the variable. A variable declaration has its meaning at the time of compilation only, compiler needs actual variable declaration at the time of linking of the program.

A variable declaration is useful when you are using multiple files and you define your variable in one of the files, which will be available at the time of linking of the program. You will use extern keyword to declare a variable at any place. Though you can declare a variable multiple times in your Objective-C program but it can be defined only once in a file, a function or a block of code.

Example

Try the following example, where variables have been declared at the top, but they have been defined and initialized inside the main function:

#import <Foundation/Foundation.h>

// Variable declaration:
extern int a, b;
extern int c;
extern float f;

int main ()
{
  /* variable definition: */
  int a, b;
  int c;
  float f;
 
  /* actual initialization */
  a = 10;
  b = 20;
  
  c = a + b;
  NSLog(@"value of c : %d \n", c);

  f = 70.0/3.0;
  NSLog(@"value of f : %f \n", f);
 
  return 0;
}

When the above code is compiled and executed, it produces the following result:

2013-09-07 22:43:31.695 demo[14019] value of c : 30 
2013-09-07 22:43:31.695 demo[14019] value of f : 23.333334 

Same concept applies on function declaration where you provide a function name at the time of its declaration and its actual definition can be given anywhere else. In the following example, it's explained using C function and as you know Objective-C supports C style functions also:

// function declaration
int func();

int main()
{
    // function call
    int i = func();
}

// function definition
int func()
{
    return 0;
}

Lvalues and Rvalues in Objective-C:

There are two kinds of expressions in Objective-C:

1. lvalue : Expressions that refer to a memory location is called "lvalue" expression. An lvalue may appear as either the left-hand or right-hand side of an assignment.
2. rvalue : The term rvalue refers to a data value that is stored at some address in memory. An rvalue is an expression that cannot have a value assigned to it which means an rvalue may appear on the right- but not left-hand side of an assignment.

Variables are lvalues and so may appear on the left-hand side of an assignment. Numeric literals are rvalues and so may not be assigned and can not appear on the left-hand side. Following is a valid statement:

int g = 20;

But following is not a valid statement and would generate compile-time error:

10 = 20;

The constants refer to fixed values that the program may not alter during its execution. These fixed values are also called literals.

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Constants can be of any of the basic data types like an integer constant, a floating constant, a character constant, or a string literal. There are also enumeration constants as well.

The constants are treated just like regular variables except that their values cannot be modified after their definition.

Integer literals

An integer literal can be a decimal, octal, or hexadecimal constant. A prefix specifies the base or radix: 0x or 0X for hexadecimal, 0 for octal, and nothing for decimal.

An integer literal can also have a suffix that is a combination of U and L, for unsigned and long, respectively. The suffix can be uppercase or lowercase and can be in any order.

Here are some examples of integer literals:

212         /* Legal */
215u        /* Legal */
0xFeeL      /* Legal */
078         /* Illegal: 8 is not an octal digit */
032UU       /* Illegal: cannot repeat a suffix */

Following are other examples of various types of Integer literals:

85         /* decimal */
0213       /* octal */
0x4b       /* hexadecimal */
30         /* int */
30u        /* unsigned int */
30l        /* long */
30ul       /* unsigned long */

Floating-point literals

A floating-point literal has an integer part, a decimal point, a fractional part, and an exponent part. You can represent floating point literals either in decimal form or exponential form.

While representing using decimal form, you must include the decimal point, the exponent, or both and while representing using exponential form, you must include the integer part, the fractional part, or both. The signed exponent is introduced by e or E.

Here are some examples of floating-point literals:

3.14159       /* Legal */
314159E-5L    /* Legal */
510E          /* Illegal: incomplete exponent */
210f          /* Illegal: no decimal or exponent */
.e55          /* Illegal: missing integer or fraction */

Character constants

Character literals are enclosed in single quotes e.g., 'x' and can be stored in a simple variable of char type.

A character literal can be a plain character (e.g., 'x'), an escape sequence (e.g., '\t'), or a universal character (e.g., '\u02C0').

There are certain characters in C when they are proceeded by a backslash they will have special meaning and they are used to represent like newline (\n) or tab (\t). Here, you have a list of some of such escape sequence codes:

Escape sequence	Meaning
\\	\ character
\'	' character
\"	" character
\?	? character
\a	Alert or bell
\b	Backspace
\f	Form feed
\n	Newline
\r	Carriage return
\t	Horizontal tab
\v	Vertical tab
\ooo	Octal number of one to three digits
\xhh . . .	Hexadecimal number of one or more digits

Following is the example to show few escape sequence characters:

#import <Foundation/Foundation.h>

int main()
{
   NSLog(@"Hello\tWorld\n\n");

   return 0;
}

When the above code is compiled and executed, it produces the following result:

2013-09-07 22:17:17.923 demo[17871] Hello World

String literals

String literals or constants are enclosed in double quotes "". A string contains characters that are similar to character literals: plain characters, escape sequences, and universal characters.

You can break a long line into multiple lines using string literals and separating them using whitespaces.

Here are some examples of string literals. All the three forms are identical strings.

"hello, dear"

"hello, \

dear"

"hello, " "d" "ear"

Defining Constants

There are two simple ways in C to define constants:

1. Using #define preprocessor.
2. Using const keyword.

The #define Preprocessor

Following is the form to use #define preprocessor to define a constant:

#define identifier value

Following example explains it in detail:

#import <Foundation/Foundation.h>

#define LENGTH 10   
#define WIDTH  5
#define NEWLINE '\n'

int main()
{

   int area;  
  
   area = LENGTH * WIDTH;
   NSLog(@"value of area : %d", area);
   NSLog(@"%c", NEWLINE);

   return 0;
}

When the above code is compiled and executed, it produces the following result:

2013-09-07 22:18:16.637 demo[21460] value of area : 50
2013-09-07 22:18:16.638 demo[21460] 

The const Keyword

You can use const prefix to declare constants with a specific type as follows:

const type variable = value;

Following example explains it in detail:

#import <Foundation/Foundation.h>

int main()
{
   const int  LENGTH = 10;
   const int  WIDTH  = 5;
   const char NEWLINE = '\n';
   int area;  
   
   area = LENGTH * WIDTH;
   NSLog(@"value of area : %d", area);
   NSLog(@"%c", NEWLINE);

   return 0;
}

When the above code is compiled and executed, it produces the following result:

2013-09-07 22:19:24.780 demo[25621] value of area : 50
2013-09-07 22:19:24.781 demo[25621] 

Note that it is a good programming practice to define constants in CAPITALS.

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An operator is a symbol that tells the compiler to perform specific mathematical or logical manipulations. Objective-C language is rich in built-in operators and provides following types of operators:

• Arithmetic Operators
• Relational Operators
• Logical Operators
• Bitwise Operators
• Assignment Operators
• Misc Operators

This tutorial will explain the arithmetic, relational, logical, bitwise, assignment and other operators one by one.

Arithmetic Operators

Following table shows all the arithmetic operators supported by Objective-C language. Assume variable A holds 10 and variable B holds 20, then:

Show Examples

Operator	Description	Example
+	Adds two operands	A + B will give 30
-	Subtracts second operand from the first	A - B will give -10
*	Multiplies both operands	A * B will give 200
/	Divides numerator by denominator	B / A will give 2
%	Modulus Operator and remainder of after an integer division	B % A will give 0
++	Increment operator increases integer value by one	A++ will give 11
--	Decrement operator decreases integer value by one	A-- will give 9

Relational Operators

Following table shows all the relational operators supported by Objective-C language. Assume variable A holds 10 and variable B holds 20, then:

Show Examples

Operator	Description	Example
==	Checks if the values of two operands are equal or not; if yes, then condition becomes true.	(A == B) is not true.
!=	Checks if the values of two operands are equal or not; if values are not equal, then condition becomes true.	(A != B) is true.
>	Checks if the value of left operand is greater than the value of right operand; if yes, then condition becomes true.	(A > B) is not true.
<	Checks if the value of left operand is less than the value of right operand; if yes, then condition becomes true.	(A < B) is true.
>=	Checks if the value of left operand is greater than or equal to the value of right operand; if yes, then condition becomes true.	(A >= B) is not true.
<=	Checks if the value of left operand is less than or equal to the value of right operand; if yes, then condition becomes true.	(A <= B) is true.

Logical Operators

Following table shows all the logical operators supported by Objective-C language. Assume variable A holds 1 and variable B holds 0, then:

Show Examples

Operator	Description	Example
&&	Called Logical AND operator. If both the operands are non zero then condition becomes true.	(A && B) is false.
||	Called Logical OR Operator. If any of the two operands is non zero then condition becomes true.	(A || B) is true.
!	Called Logical NOT Operator. Use to reverses the logical state of its operand. If a condition is true, then Logical NOT operator will make false.	!(A && B) is true.

Bitwise Operators

Bitwise operator works on bits and perform bit by bit operation. The truth tables for &, |, and ^ are as follows:

p	q	p & q	p | q	p ^ q
0	0	0	0	0
0	1	0	1	1
1	1	1	1	0
1	0	0	1	1

Assume if A = 60; and B = 13; now in binary format they will be as follows:

A = 0011 1100

B = 0000 1101

-----------------

A&B = 0000 1100

A|B = 0011 1101

A^B = 0011 0001

~A  = 1100 0011

The Bitwise operators supported by Objective-C language are listed in the following table. Assume variable A holds 60 and variable B holds 13 then:

Show Examples

Operator	Description	Example
&	Binary AND Operator copies a bit to the result if it exists in both operands.	(A & B) will give 12, which is 0000 1100
|	Binary OR Operator copies a bit if it exists in either operand.	(A | B) will give 61, which is 0011 1101
^	Binary XOR Operator copies the bit if it is set in one operand but not both.	(A ^ B) will give 49, which is 0011 0001
~	Binary Ones Complement Operator is unary and has the effect of 'flipping' bits.	(~A ) will give -61, which is 1100 0011 in 2's complement form.
<<	Binary Left Shift Operator. The left operands value is moved left by the number of bits specified by the right operand.	A << 2 will give 240, which is 1111 0000
>>	Binary Right Shift Operator. The left operands value is moved right by the number of bits specified by the right operand.	A >> 2 will give 15, which is 0000 1111

Assignment Operators

There are following assignment operators supported by Objective-C language:

Show Examples

Operator	Description	Example
=	Simple assignment operator, Assigns values from right side operands to left side operand	C = A + B will assign value of A + B into C
+=	Add AND assignment operator, It adds right operand to the left operand and assigns the result to left operand	C += A is equivalent to C = C + A
-=	Subtract AND assignment operator, It subtracts right operand from the left operand and assigns the result to left operand	C -= A is equivalent to C = C - A
*=	Multiply AND assignment operator, It multiplies right operand with the left operand and assigns the result to left operand	C *= A is equivalent to C = C * A
/=	Divide AND assignment operator, It divides left operand with the right operand and assigns the result to left operand	C /= A is equivalent to C = C / A
%=	Modulus AND assignment operator, It takes modulus using two operands and assigns the result to left operand	C %= A is equivalent to C = C % A
<<=	Left shift AND assignment operator	C <<= 2 is same as C = C << 2
>>=	Right shift AND assignment operator	C >>= 2 is same as C = C >> 2
&=	Bitwise AND assignment operator	C &= 2 is same as C = C & 2
^=	bitwise exclusive OR and assignment operator	C ^= 2 is same as C = C ^ 2
|=	bitwise inclusive OR and assignment operator	C |= 2 is same as C = C | 2

Misc Operators ↦ sizeof & ternary

There are few other important operators including sizeof and ? : supported by Objective-C Language.

Show Examples

Operator	Description	Example
sizeof()	Returns the size of an variable.	sizeof(a), where a is integer, will return 4.
&	Returns the address of an variable.	&a; will give actual address of the variable.
*	Pointer to a variable.	*a; will pointer to a variable.
? :	Conditional Expression	If Condition is true ? Then value X : Otherwise value Y

Operators Precedence in Objective-C

Operator precedence determines the grouping of terms in an expression. This affects how an expression is evaluated. Certain operators have higher precedence than others; for example, the multiplication operator has higher precedence than the addition operator:

For example, x = 7 + 3 * 2; here, x is assigned 13, not 20 because operator * has higher precedence than +, so it first gets multiplied with 3*2 and then adds into 7.

Here, operators with the highest precedence appear at the top of the table, those with the lowest appear at the bottom. Within an expression, higher precedence operators will be evaluated first.

Show Examples

Category	Operator	Associativity
Postfix	() [] -> . ++ - -	Left to right
Unary	+ - ! ~ ++ - - (type)* & sizeof	Right to left
Multiplicative	* / %	Left to right
Additive	+ -	Left to right
Shift	<< >>	Left to right
Relational	< <= > >=	Left to right
Equality	== !=	Left to right
Bitwise XOR	^	Left to right
Bitwise OR	|	Left to right
Logical AND	&&	Left to right
Logical OR	||	Left to right
Conditional	?:	Right to left
Assignment	= += -= *= /= %=>>= <<= &= ^= |=	Right to left
Comma	,	Left to right

There may be a situation, when you need to execute a block of code several number of times. In general, statements are executed sequentially: The first statement in a function is executed first, followed by the second, and so on.

Programming languages provide various control structures that allow for more complicated execution paths.

A loop statement allows us to execute a statement or group of statements multiple times and following is the general form of a loop statement in most of the programming languages:

Objective-C programming language provides the following types of loop to handle looping requirements. Click the following links to check their details.

Loop Type	Description
while loop	Repeats a statement or group of statements while a given condition is true. It tests the condition before executing the loop body.
for loop	Execute a sequence of statements multiple times and abbreviates the code that manages the loop variable.
do...while loop	Like a while statement, except that it tests the condition at the end of the loop body.
nested loops	You can use one or more loops inside any another while, for or do..while loop.

Loop Control Statements:

Loop control statements change execution from its normal sequence. When execution leaves a scope, all automatic objects that were created in that scope are destroyed.

Objective-C supports the following control statements. Click the following links to check their details.

Control Statement	Description
break statement	Terminates the loop or switch statement and transfers execution to the statement immediately following the loop or switch.
continue statement	Causes the loop to skip the remainder of its body and immediately retest its condition prior to reiterating.

The Infinite Loop:

A loop becomes infinite loop if a condition never becomes false. The for loop is traditionally used for this purpose. Since none of the three expressions that form the for loop are required, you can make an endless loop by leaving the conditional expression empty.

#import <Foundation/Foundation.h>
 
int main ()
{

   for( ; ; )
   {
      NSLog(@"This loop will run forever.\n");
   }

   return 0;
}

When the conditional expression is absent, it is assumed to be true. You may have an initialization and increment expression, but Objective-C programmers more commonly use the for(;;) construct to signify an infinite loop.

****************************************************************

Decision making structures require that the programmer specify one or more conditions to be evaluated or tested by the program, along with a statement or statements to be executed if the condition is determined to be true, and optionally, other statements to be executed if the condition is determined to be false.

Following is the general form of a typical decision making structure found in most of the programming languages:

Objective-C programming language assumes any non-zero and non-null values as true, and if it is either zero or null, then it is assumed as false value.

Objective-C programming language provides following types of decision making statements. Click the following links to check their details:

Statement	Description
if statement	An if statement consists of a boolean expression followed by one or more statements.
if...else statement	An if statement can be followed by an optional else statement, which executes when the boolean expression is false.
nested if statements	You can use one if or else if statement inside another if or else if statement(s).
switch statement	A switch statement allows a variable to be tested for equality against a list of values.
nested switch statements	You can use one switch statement inside another switch statement(s).

The ? : Operator:

We have covered conditional operator ? : in previous chapter which can be used to replace if...else statements. It has the following general form:

Exp1 ? Exp2 : Exp3;

Where Exp1, Exp2, and Exp3 are expressions. Notice the use and placement of the colon.

The value of a ? expression is determined like this: Exp1 is evaluated. If it is true, then Exp2 is evaluated and becomes the value of the entire ? expression. If Exp1 is false, then Exp3 is evaluated and its value becomes the value of the expression.

******************************************************************

A function is a group of statements that together perform a task. Every Objective-C program has one C function, which is main(), and all of the most trivial programs can define additional functions.

You can divide up your code into separate functions. How you divide up your code among different functions is up to you, but logically the division usually is so each function performs a specific task.

A function declaration tells the compiler about a function's name, return type, and parameters. A function definition provides the actual body of the function.

Basically in Objective-C, we call the function as method.

The Objective-C foundation framework provides numerous built-in methods that your program can call. For example, method appendString() to append string to another string.

A method is known with various names like a function or a sub-routine or a procedure, etc.

Defining a Method

The general form of a method definition in Objective-C programming language is as follows:

- (return_type) method_name:( argumentType1 )argumentName1 
joiningArgument2:( argumentType2 )argumentName2 ... 
joiningArgumentn:( argumentTypen )argumentNamen 
{
   body of the function
}

A method definition in Objective-C programming language consists of a method header and a method body. Here are all the parts of a method:

• Return Type: A method may return a value. The return_type is the data type of the value the function returns. Some methods perform the desired operations without returning a value. In this case, the return_type is the keyword void.
• Method Name: This is the actual name of the method. The method name and the parameter list together constitute the method signature.
• Arguments: A argument is like a placeholder. When a function is invoked, you pass a value to the argument. This value is referred to as actual parameter or argument. The parameter list refers to the type, order, and number of the arguments of a method. Arguments are optional; that is, a method may contain no argument.
• Joining Argument: A joining argument is to make it easier to read and to make it clear while calling it.
• Method Body: The method body contains a collection of statements that define what the method does.

Example:

Following is the source code for a method called max(). This method takes two parameters num1 and num2 and returns the maximum between the two:

/* function returning the max between two numbers */
- (int) max:(int) num1 secondNumber:(int) num2 
{
   /* local variable declaration */
   int result;
 
   if (num1 > num2)
   {
      result = num1;
   }
   else
   {
      result = num2;
   }
 
   return result; 
}

Method Declarations:

A method declaration tells the compiler about a function name and how to call the method. The actual body of the function can be defined separately.

A method declaration has the following parts:

- (return_type) function_name:( argumentType1 )argumentName1 
joiningArgument2:( argumentType2 )argumentName2 ... 
joiningArgumentn:( argumentTypen )argumentNamen;

For the above-defined function max(), following is the method declaration:

-(int) max:(int)num1 andNum2:(int)num2;

Method declaration is required when you define a method in one source file and you call that method in another file. In such case you should declare the function at the top of the file calling the function.

Calling a method:

While creating a Objective-C method, you give a definition of what the function has to do. To use a method, you will have to call that function to perform the defined task.

When a program calls a function, program control is transferred to the called method. A called method performs defined task, and when its return statement is executed or when its function-ending closing brace is reached, it returns program control back to the main program.

To call a method, you simply need to pass the required parameters along with method name, and if method returns a value, then you can store returned value. For example:

#import <Foundation/Foundation.h>

@interface SampleClass:NSObject
/* method declaration */
- (int)max:(int)num1 andNum2:(int)num2;
@end

@implementation SampleClass

/* method returning the max between two numbers */
- (int)max:(int)num1 andNum2:(int)num2{
/* local variable declaration */
   int result;
 
   if (num1 > num2)
   {
      result = num1;
   }
   else
   {
      result = num2;
   }
 
   return result; 
}

@end

int main ()
{
   /* local variable definition */
   int a = 100;
   int b = 200;
   int ret;
   
   SampleClass *sampleClass = [[SampleClass alloc]init];

   /* calling a method to get max value */
   ret = [sampleClass max:a andNum2:b];
 
   NSLog(@"Max value is : %d\n", ret );
 
   return 0;
}

I kept max() function along with main() function and complied the source code. While running final executable, it would produce the following result:

2013-09-07 22:28:45.912 demo[26080] Max value is : 200

Function Arguments:

If a function is to use arguments, it must declare variables that accept the values of the arguments. These variables are called the formal parameters of the function.

The formal parameters behave like other local variables inside the function and are created upon entry into the function and destroyed upon exit.

While calling a function, there are two ways that arguments can be passed to a function:

Call Type	Description
Call by value	This method copies the actual value of an argument into the formal parameter of the function. In this case, changes made to the parameter inside the function have no effect on the argument.
Call by reference	This method copies the address of an argument into the formal parameter. Inside the function, the address is used to access the actual argument used in the call. This means that changes made to the parameter affect the argument.

By default, Objective-C uses call by value to pass arguments. In general, this means that code within a function cannot alter the arguments used to call the function, and above-mentioned example while calling max() function used the same method.

*****************************************************************

An Objective-C class defines an object that combines data with related behavior. Sometimes, it makes sense just to represent a single task or unit of behavior, rather than a collection of methods.

Blocks are a language-level feature added to C, Objective-C and C++ which allow you to create distinct segments of code that can be passed around to methods or functions as if they were values. Blocks are Objective-C objects which means they can be added to collections like NSArray or NSDictionary. They also have the ability to capture values from the enclosing scope, making them similar to closures or lambdas in other programming languages

Simple Block declaration syntax

returntype (^blockName)(argumentType);

Simple block implementation

returntype (^blockName)(argumentType)= ^{
};

Here is a simple example

void (^simpleBlock)(void) = ^{
    NSLog(@"This is a block");
};

We can invoke the block using

simpleBlock();

Blocks Take Arguments and Return Values

Blocks can also take arguments and return values just like methods and functions.

Here is a simple example to implement and invoke a block with arguments and return values.

double (^multiplyTwoValues)(double, double) = 
    ^(double firstValue, double secondValue) {
     return firstValue * secondValue;
    };
double result = multiplyTwoValues(2,4); 
NSLog(@"The result is %f", result);

Blocks using type definitions

Here is a simple example using typedef in block. Please note this sample doesn't work on the online compiler for now. Use XCode to run the same.

#import <Foundation/Foundation.h>

typedef void (^CompletionBlock)();
@interface SampleClass:NSObject
- (void)performActionWithCompletion:(CompletionBlock)completionBlock;
@end

@implementation SampleClass

- (void)performActionWithCompletion:(CompletionBlock)completionBlock{

    NSLog(@"Action Performed");
    completionBlock();
}

@end

int main()
{
    /* my first program in Objective-C */
    SampleClass *sampleClass = [[SampleClass alloc]init];
    [sampleClass performActionWithCompletion:^{
        NSLog(@"Completion is called to intimate action is performed.");
    }];
    
    return 0;
}

Let us compile and execute it, it will produce the following result:

2013-09-10 08:13:57.155 demo[284:303] Action Performed
2013-09-10 08:13:57.157 demo[284:303] Completion is called to intimate action is performed.

Blocks are used more in iOS applications and Mac OS X. So its more important to understand the usage of blocks.

***************************************************************

In Objective-C programming language, in order to save the basic data types like int, float, bool in object form,

Objective-C provides a range of methods to work with NSNumber and important ones are listed in following table.

S.N.	Method and Description
1	+ (NSNumber *)numberWithBool:(BOOL)value Creates and returns an NSNumber object containing a given value, treating it as a BOOL.
2	+ (NSNumber *)numberWithChar:(char)value Creates and returns an NSNumber object containing a given value, treating it as a signed char.
3	+ (NSNumber *)numberWithDouble:(double)value Creates and returns an NSNumber object containing a given value, treating it as a double.
4	+ (NSNumber *)numberWithFloat:(float)value Creates and returns an NSNumber object containing a given value, treating it as a float.
5	+ (NSNumber *)numberWithInt:(int)value Creates and returns an NSNumber object containing a given value, treating it as a signed int.
6	+ (NSNumber *)numberWithInteger:(NSInteger)value Creates and returns an NSNumber object containing a given value, treating it as an NSInteger.
7	- (BOOL)boolValue Returns the receiver's value as a BOOL.
8	- (char)charValue Returns the receiver's value as a char.
9	- (double)doubleValue Returns the receiver's value as a double.
10	- (float)floatValue Returns the receiver's value as a float.
11	- (NSInteger)integerValue Returns the receiver's value as an NSInteger.
12	- (int)intValue Returns the receiver's value as an int.
13	- (NSString *)stringValue Returns the receiver's value as a human-readable string.

Here is a simple example for using NSNumber which multiplies two numbers and returns the product.

#import <Foundation/Foundation.h>

@interface SampleClass:NSObject

- (NSNumber *)multiplyA:(NSNumber *)a withB:(NSNumber *)b;

@end

@implementation SampleClass

- (NSNumber *)multiplyA:(NSNumber *)a withB:(NSNumber *)b
{
   float number1 = [a floatValue];
   float number2 = [b floatValue];
   float product = number1 * number2;
   NSNumber *result = [NSNumber numberWithFloat:product];
   return result;
}

@end

int main()
{
   NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init];

   SampleClass *sampleClass = [[SampleClass alloc]init];
   NSNumber *a = [NSNumber numberWithFloat:10.5];
   NSNumber *b = [NSNumber numberWithFloat:10.0];   
   NSNumber *result = [sampleClass multiplyA:a withB:b];
   NSString *resultString = [result stringValue];
   NSLog(@"The product is %@",resultString);

   [pool drain];
   return 0;
}

Now when we compile and run the program, we will get the following result.

2013-09-14 18:53:40.575 demo[16787] The product is 10

****************************************************************************

Objective-C programming language provides a data structure called the array, which can store a fixed-size sequential collection of elements of the same type. An array is used to store a collection of data, but it is often more useful to think of an array as a collection of variables of the same type.

Instead of declaring individual variables, such as number0, number1, ..., and number99, you declare one array variable such as numbers and use numbers[0], numbers[1], and ..., numbers[99] to represent individual variables. A specific element in an array is accessed by an index.

All arrays consist of contiguous memory locations. The lowest address corresponds to the first element and the highest address to the last element.

Declaring Arrays

To declare an array in Objective-C, a programmer specifies the type of the elements and the number of elements required by an array as follows:

type arrayName [ arraySize ];

This is called a single-dimensional array. The arraySize must be an integer constant greater than zero and type can be any valid Objective-C data type. For example, to declare a 10-element array called balance of type double, use this statement:

double balance[10];

Now, balance is a variable array, which is sufficient to hold up to 10 double numbers.

Initializing Arrays

You can initialize an array in Objective-C either one by one or using a single statement as follows:

double balance[5] = {1000.0, 2.0, 3.4, 17.0, 50.0};

The number of values between braces { } can not be larger than the number of elements that we declare for the array between square brackets [ ]. Following is an example to assign a single element of the array:

If you omit the size of the array, an array just big enough to hold the initialization is created. Therefore, if you write:

double balance[] = {1000.0, 2.0, 3.4, 17.0, 50.0};

You will create exactly the same array as you did in the previous example.

balance[4] = 50.0;

The above statement assigns element number 5th in the array a value of 50.0. Array with 4th index will be 5th, i.e., last element because all arrays have 0 as the index of their first element which is also called base index. Following is the pictorial representation of the same array we discussed above:

Accessing Array Elements

An element is accessed by indexing the array name. This is done by placing the index of the element within square brackets after the name of the array. For example:

double salary = balance[9];

The above statement will take 10th element from the array and assign the value to salary variable. Following is an example, which will use all the above mentioned three concepts viz. declaration, assignment and accessing arrays:

#import <Foundation/Foundation.h>
 
int main ()
{
   int n[ 10 ]; /* n is an array of 10 integers */
   int i,j;
 
   /* initialize elements of array n to 0 */         
   for ( i = 0; i < 10; i++ )
   {
      n[ i ] = i + 100; /* set element at location i to i + 100 */
   }
   
   /* output each array element's value */
   for (j = 0; j < 10; j++ )
   {
      NSLog(@"Element[%d] = %d\n", j, n[j] );
   }
 
   return 0;
}

When the above code is compiled and executed, it produces the following result:

2013-09-14 01:24:06.669 demo[16508] Element[0] = 100
2013-09-14 01:24:06.669 demo[16508] Element[1] = 101
2013-09-14 01:24:06.669 demo[16508] Element[2] = 102
2013-09-14 01:24:06.669 demo[16508] Element[3] = 103
2013-09-14 01:24:06.669 demo[16508] Element[4] = 104
2013-09-14 01:24:06.669 demo[16508] Element[5] = 105
2013-09-14 01:24:06.669 demo[16508] Element[6] = 106
2013-09-14 01:24:06.669 demo[16508] Element[7] = 107
2013-09-14 01:24:06.669 demo[16508] Element[8] = 108
2013-09-14 01:24:06.669 demo[16508] Element[9] = 109

Objective-C Arrays in Detail

Arrays are important to Objective-C and need lots of more details. There are following few important concepts related to array which should be clear to a Objective-C programmer:

Concept	Description
Multi-dimensional arrays	Objective-C supports multidimensional arrays. The simplest form of the multidimensional array is the two-dimensional array.
Passing arrays to functions	You can pass to the function a pointer to an array by specifying the array's name without an index.
Return array from a function	Objective-C allows a function to return an array.
Pointer to an array	You can generate a pointer to the first element of an array by simply specifying the array name, without any index.

********************************************************************************

Pointers in Objective-C are easy and fun to learn. Some Objective-C programming tasks are performed more easily with pointers, and other tasks, such as dynamic memory allocation, cannot be performed without using pointers. So it becomes necessary to learn pointers to become a perfect Objective-C programmer. Let's start learning them in simple and easy steps.

As you know, every variable is a memory location and every memory location has its address defined which can be accessed using ampersand (&) operator, which denotes an address in memory. Consider the following example, which will print the address of the variables defined:

#import <Foundation/Foundation.h>

int main ()
{
   int  var1;
   char var2[10];

   NSLog(@"Address of var1 variable: %x\n", &var1  );
   NSLog(@"Address of var2 variable: %x\n", &var2  );

   return 0;
}

When the above code is compiled and executed, it produces the result something as follows:

2013-09-13 03:18:45.727 demo[17552] Address of var1 variable: 1c0843fc
2013-09-13 03:18:45.728 demo[17552] Address of var2 variable: 1c0843f0

So, you understood what is memory address and how to access it, so base of the concept is over. Now let us see what is a pointer.

What Are Pointers?

A pointer is a variable whose value is the address of another variable, i.e., direct address of the memory location. Like any variable or constant, you must declare a pointer before you can use it to store any variable address. The general form of a pointer variable declaration is:

type *var-name;

Here, type is the pointer's base type; it must be a valid Objective-C data type and var-name is the name of the pointer variable. The asterisk * you used to declare a pointer is the same asterisk that you use for multiplication. However, in this statement the asterisk is being used to designate a variable as a pointer. Following are the valid pointer declaration:

int    *ip;    /* pointer to an integer */
double *dp;    /* pointer to a double */
float  *fp;    /* pointer to a float */
char   *ch     /* pointer to a character */

The actual data type of the value of all pointers, whether integer, float, character, or otherwise, is the same, a long hexadecimal number that represents a memory address. The only difference between pointers of different data types is the data type of the variable or constant that the pointer points to.

How to use Pointers?

There are few important operations, which we will do with the help of pointers very frequently. (a) we define a pointer variable, (b) assign the address of a variable to a pointer, and (c) finally access the value at the address available in the pointer variable. This is done by using unary operator * that returns the value of the variable located at the address specified by its operand. Following example makes use of these operations:

#import <Foundation/Foundation.h>

int main ()
{
   int  var = 20;   /* actual variable declaration */
   int  *ip;        /* pointer variable declaration */

   ip = &var;  /* store address of var in pointer variable*/

   NSLog(@"Address of var variable: %x\n", &var  );

   /* address stored in pointer variable */
   NSLog(@"Address stored in ip variable: %x\n", ip );

   /* access the value using the pointer */
   NSLog(@"Value of *ip variable: %d\n", *ip );

   return 0;
}

When the above code is compiled and executed, it produces the result something as follows:

2013-09-13 03:20:21.873 demo[24179] Address of var variable: 337ed41c
2013-09-13 03:20:21.873 demo[24179] Address stored in ip variable: 337ed41c
2013-09-13 03:20:21.874 demo[24179] Value of *ip variable: 20

NULL Pointers in Objective-C

It is always a good practice to assign a NULL value to a pointer variable in case you do not have exact address to be assigned. This is done at the time of variable declaration. A pointer that is assigned NULL is called a null pointer.

The NULL pointer is a constant with a value of zero defined in several standard libraries. Consider the following program:

#import <Foundation/Foundation.h>

int main ()
{
   int  *ptr = NULL;

   NSLog(@"The value of ptr is : %x\n", ptr  );
 
   return 0;
}

When the above code is compiled and executed, it produces the following result:

2013-09-13 03:21:19.447 demo[28027] The value of ptr is : 0

On most of the operating systems, programs are not permitted to access memory at address 0 because that memory is reserved by the operating system. However, the memory address 0 has special significance; it signals that the pointer is not intended to point to an accessible memory location. But by convention, if a pointer contains the null (zero) value, it is assumed to point to nothing.

To check for a null pointer, you can use an if statement as follows:

if(ptr)     /* succeeds if p is not null */
if(!ptr)    /* succeeds if p is null */

Objective-C Pointers in Detail:

Pointers have many but easy concepts and they are very important to Objective-C programming. There are following few important pointer concepts, which should be clear to a Objective-C programmer:

Concept	Description
Objective-C - Pointer arithmetic	There are four arithmetic operators that can be used on pointers: ++, --, +, -
Objective-C - Array of pointers	You can define arrays to hold a number of pointers.
Objective-C - Pointer to pointer	Objective-C allows you to have pointer on a pointer and so on.
Passing pointers to functions in Objective-C	Passing an argument by reference or by address both enable the passed argument to be changed in the calling function by the called function.
Return pointer from functions in Objective-C	Objective-C allows a function to return a pointer to local variable, static variable and dynamically allocated memory as well.

*********************************************************************************

The string in Objective-C programming language is represented using NSString and its subclass NSMutableString provides several ways for creating string objects. The simplest way to create a string object is to use the Objective-C @"..." construct:

NSString *greeting = @"Hello";

A simple example for creating and printing a string is shown below.

#import <Foundation/Foundation.h>

int main ()
{
   NSString *greeting = @"Hello";

   NSLog(@"Greeting message: %@\n", greeting );

   return 0;
}

When the above code is compiled and executed, it produces result something as follows:

2013-09-11 01:21:39.922 demo[23926] Greeting message: Hello

Objective-C supports a wide range of methods for manipulate strings:

S.N.	Method & Purpose
1	- (NSString *)capitalizedString; Returns a capitalized representation of the receiver.
2	- (unichar)characterAtIndex:(NSUInteger)index; Returns the character at a given array position.
3	- (double)doubleValue; Returns the floating-point value of the receiver’s text as a double.
4	- (float)floatValue; Returns the floating-point value of the receiver’s text as a float.
5	- (BOOL)hasPrefix:(NSString *)aString; Returns a Boolean value that indicates whether a given string matches the beginning characters of the receiver.
6	- (BOOL)hasSuffix:(NSString *)aString; Returns a Boolean value that indicates whether a given string matches the ending characters of the receiver.
7	- (id)initWithFormat:(NSString *)format ...; Returns an NSString object initialized by using a given format string as a template into which the remaining argument values are substituted.
8	- (NSInteger)integerValue; Returns the NSInteger value of the receiver’s text.
9	- (BOOL)isEqualToString:(NSString *)aString; Returns a Boolean value that indicates whether a given string is equal to the receiver using a literal Unicode-based comparison.
10	- (NSUInteger)length; Returns the number of Unicode characters in the receiver.
11	- (NSString *)lowercaseString; Returns lowercased representation of the receiver.
12	- (NSRange)rangeOfString:(NSString *)aString; Finds and returns the range of the first occurrence of a given string within the receiver.
13	- (NSString *)stringByAppendingFormat:(NSString *)format ...; Returns a string made by appending to the receiver a string constructed from a given format string and the following arguments.
14	- (NSString *)stringByTrimmingCharactersInSet:(NSCharacterSet *)set; Returns a new string made by removing from both ends of the receiver characters contained in a given character set.
15	- (NSString *)substringFromIndex:(NSUInteger)anIndex; Returns a new string containing the characters of the receiver from the one at a given index to the end.

Following example makes use of few of the above-mentioned functions:

#import <Foundation/Foundation.h>

int main ()
{
   NSString *str1 = @"Hello";
   NSString *str2 = @"World";
   NSString *str3;
   int  len ;

   NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init];

   /* uppercase string */
   str3 = [str2 uppercaseString];
   NSLog(@"Uppercase String :  %@\n", str3 );

   /* concatenates str1 and str2 */
   str3 = [str1 stringByAppendingFormat:@"World"];
   NSLog(@"Concatenated string:   %@\n", str3 );

   /* total length of str3 after concatenation */
   len = [str3 length];
   NSLog(@"Length of Str3 :  %d\n", len );
    
   /* InitWithFormat */
    str3 = [[NSString alloc] initWithFormat:@"%@ %@",str1,str2]; 
    NSLog(@"Using initWithFormat:   %@\n", str3 );
    [pool drain];

   return 0;
}

When the above code is compiled and executed, it produces result something as follows:

2013-09-11 01:15:45.069 demo[30378] Uppercase String :  WORLD
2013-09-11 01:15:45.070 demo[30378] Concatenated string:   HelloWorld
2013-09-11 01:15:45.070 demo[30378] Length of Str3 :  10
2013-09-11 01:15:45.070 demo[30378] Using initWithFormat:   Hello World

You can find a complete list of Objective-C NSString related methods in NSString Class Reference.

****************************************************************

Objective-C arrays allow you to define type of variables that can hold several data items of the same kind but structure is another user-defined data type available in Objective-C programming which allows you to combine data items of different kinds.

Structures are used to represent a record, Suppose you want to keep track of your books in a library. You might want to track the following attributes about each book:

• Title
• Author
• Subject
• Book ID

Defining a Structure

To define a structure, you must use the struct statement. The struct statement defines a new data type, with more than one member for your program. The format of the struct statement is this:

struct [structure tag]
{
   member definition;
   member definition;
   ...
   member definition;
} [one or more structure variables];  

The structure tag is optional and each member definition is a normal variable definition, such as int i; or float f; or any other valid variable definition. At the end of the structure's definition, before the final semicolon, you can specify one or more structure variables but it is optional. Here is the way you would declare the Book structure:

struct Books
{
   NSString *title;
   NSString *author;
   NSString *subject;
   int   book_id;
} book;  

Accessing Structure Members

To access any member of a structure, we use the member access operator (.). The member access operator is coded as a period between the structure variable name and the structure member that we wish to access. You would use struct keyword to define variables of structure type. Following is the example to explain usage of structure:

#import <Foundation/Foundation.h>

struct Books
{
   NSString *title;
   NSString *author;
   NSString *subject;
   int   book_id;
};
 
int main( )
{
   struct Books Book1;        /* Declare Book1 of type Book */
   struct Books Book2;        /* Declare Book2 of type Book */
 
   /* book 1 specification */
   Book1.title = @"Objective-C Programming";
   Book1.author = @"Nuha Ali"; 
   Book1.subject = @"Objective-C Programming Tutorial";
   Book1.book_id = 6495407;

   /* book 2 specification */
   Book2.title = @"Telecom Billing";
   Book2.author = @"Zara Ali";
   Book2.subject = @"Telecom Billing Tutorial";
   Book2.book_id = 6495700;
 
   /* print Book1 info */
   NSLog(@"Book 1 title : %@\n", Book1.title);
   NSLog(@"Book 1 author : %@\n", Book1.author);
   NSLog(@"Book 1 subject : %@\n", Book1.subject);
   NSLog(@"Book 1 book_id : %d\n", Book1.book_id);

   /* print Book2 info */
   NSLog(@"Book 2 title : %@\n", Book2.title);
   NSLog(@"Book 2 author : %@\n", Book2.author);
   NSLog(@"Book 2 subject : %@\n", Book2.subject);
   NSLog(@"Book 2 book_id : %d\n", Book2.book_id);

   return 0;
}

When the above code is compiled and executed, it produces the following result:

2013-09-14 04:20:07.947 demo[20591] Book 1 title : Objective-C Programming
2013-09-14 04:20:07.947 demo[20591] Book 1 author : Nuha Ali
2013-09-14 04:20:07.947 demo[20591] Book 1 subject : Objective-C Programming Tutorial
2013-09-14 04:20:07.947 demo[20591] Book 1 book_id : 6495407
2013-09-14 04:20:07.947 demo[20591] Book 2 title : Telecom Billing
2013-09-14 04:20:07.947 demo[20591] Book 2 author : Zara Ali
2013-09-14 04:20:07.947 demo[20591] Book 2 subject : Telecom Billing Tutorial
2013-09-14 04:20:07.947 demo[20591] Book 2 book_id : 6495700

Structures as Function Arguments

You can pass a structure as a function argument in very similar way as you pass any other variable or pointer. You would access structure variables in the similar way as you have accessed in the above example:

#import <Foundation/Foundation.h>

struct Books
{
   NSString *title;
   NSString *author;
   NSString *subject;
   int   book_id;
};

@interface SampleClass:NSObject

/* function declaration */
- (void) printBook:( struct Books) book ;

@end

@implementation SampleClass 

- (void) printBook:( struct Books) book
{
   NSLog(@"Book title : %@\n", book.title);
   NSLog(@"Book author : %@\n", book.author);
   NSLog(@"Book subject : %@\n", book.subject);
   NSLog(@"Book book_id : %d\n", book.book_id);
}
@end

int main( )
{
   struct Books Book1;        /* Declare Book1 of type Book */
   struct Books Book2;        /* Declare Book2 of type Book */
 
   /* book 1 specification */
   Book1.title = @"Objective-C Programming";
   Book1.author = @"Nuha Ali"; 
   Book1.subject = @"Objective-C Programming Tutorial";
   Book1.book_id = 6495407;

   /* book 2 specification */
   Book2.title = @"Telecom Billing";
   Book2.author = @"Zara Ali";
   Book2.subject = @"Telecom Billing Tutorial";
   Book2.book_id = 6495700;
 
   SampleClass *sampleClass = [[SampleClass alloc]init];
   /* print Book1 info */
   [sampleClass printBook: Book1];

   /* Print Book2 info */
   [sampleClass printBook: Book2];

   return 0;
}

When the above code is compiled and executed, it produces the following result:

2013-09-14 04:34:45.725 demo[8060] Book title : Objective-C Programming
2013-09-14 04:34:45.725 demo[8060] Book author : Nuha Ali
2013-09-14 04:34:45.725 demo[8060] Book subject : Objective-C Programming Tutorial
2013-09-14 04:34:45.725 demo[8060] Book book_id : 6495407
2013-09-14 04:34:45.725 demo[8060] Book title : Telecom Billing
2013-09-14 04:34:45.725 demo[8060] Book author : Zara Ali
2013-09-14 04:34:45.725 demo[8060] Book subject : Telecom Billing Tutorial
2013-09-14 04:34:45.725 demo[8060] Book book_id : 6495700

Pointers to Structures

You can define pointers to structures in very similar way as you define pointer to any other variable as follows:

struct Books *struct_pointer;

Now, you can store the address of a structure variable in the above-defined pointer variable. To find the address of a structure variable, place the & operator before the structure's name as follows:

struct_pointer = &Book1;

To access the members of a structure using a pointer to that structure, you must use the -> operator as follows:

struct_pointer->title;

Let us re-write above example using structure pointer, hope this will be easy for you to understand the concept:

#import <Foundation/Foundation.h>

struct Books
{
   NSString *title;
   NSString *author;
   NSString *subject;
   int   book_id;
};

@interface SampleClass:NSObject

/* function declaration */
- (void) printBook:( struct Books *) book ;

@end

@implementation SampleClass 

- (void) printBook:( struct Books *) book
{
   NSLog(@"Book title : %@\n", book->title);
   NSLog(@"Book author : %@\n", book->author);
   NSLog(@"Book subject : %@\n", book->subject);
   NSLog(@"Book book_id : %d\n", book->book_id);
}
@end

int main( )
{
   struct Books Book1;        /* Declare Book1 of type Book */
   struct Books Book2;        /* Declare Book2 of type Book */
 
   /* book 1 specification */
   Book1.title = @"Objective-C Programming";
   Book1.author = @"Nuha Ali"; 
   Book1.subject = @"Objective-C Programming Tutorial";
   Book1.book_id = 6495407;

   /* book 2 specification */
   Book2.title = @"Telecom Billing";
   Book2.author = @"Zara Ali";
   Book2.subject = @"Telecom Billing Tutorial";
   Book2.book_id = 6495700;
 
   SampleClass *sampleClass = [[SampleClass alloc]init];
   /* print Book1 info by passing address of Book1 */
   [sampleClass printBook:&Book1];

   /* print Book2 info by passing address of Book2 */
   [sampleClass printBook:&Book2];

   return 0;
}

When the above code is compiled and executed, it produces the following result:

2013-09-14 04:38:13.942 demo[20745] Book title : Objective-C Programming
2013-09-14 04:38:13.942 demo[20745] Book author : Nuha Ali
2013-09-14 04:38:13.942 demo[20745] Book subject : Objective-C Programming Tutorial
2013-09-14 04:38:13.942 demo[20745] Book book_id : 6495407
2013-09-14 04:38:13.942 demo[20745] Book title : Telecom Billing
2013-09-14 04:38:13.942 demo[20745] Book author : Zara Ali
2013-09-14 04:38:13.942 demo[20745] Book subject : Telecom Billing Tutorial
2013-09-14 04:38:13.942 demo[20745] Book book_id : 6495700

Bit Fields

Bit Fields allow the packing of data in a structure. This is especially useful when memory or data storage is at a premium. Typical examples:

• Packing several objects into a machine word. e.g. 1 bit flags can be compacted.
• Reading external file formats -- non-standard file formats could be read in. E.g. 9 bit integers.

Objective-C allows us do this in a structure definition by putting :bit length after the variable. For example:

struct packed_struct {
  unsigned int f1:1;
  unsigned int f2:1;
  unsigned int f3:1;
  unsigned int f4:1;
  unsigned int type:4;
  unsigned int my_int:9;
} pack;

Here, the packed_struct contains 6 members: Four 1 bit flags f1..f3, a 4 bit type and a 9 bit my_int.

Objective-C automatically packs the above bit fields as compactly as possible, provided that the maximum length of the field is less than or equal to the integer word length of the computer. If this is not the case, then some compilers may allow memory overlap for the fields whilst other would store the next field in the next word.

*******************************************************************

The Objective-C Preprocessor is not part of the compiler, but is a separate step in the compilation process. In simplistic terms, an Objective-C Preprocessor is just a text substitution tool and it instructs compiler to do required pre-processing before actual compilation. We'll refer to the Objective-C Preprocessor as the OCPP.

All preprocessor commands begin with a pound symbol (#). It must be the first nonblank character, and for readability, a preprocessor directive should begin in first column. Following section lists down all important preprocessor directives:

Directive	Description
#define	Substitutes a preprocessor macro
#include	Inserts a particular header from another file
#undef	Undefines a preprocessor macro
#ifdef	Returns true if this macro is defined
#ifndef	Returns true if this macro is not defined
#if	Tests if a compile time condition is true
#else	The alternative for #if
#elif	#else an #if in one statement
#endif	Ends preprocessor conditional
#error	Prints error message on stderr
#pragma	Issues special commands to the compiler using a standardized method

Preprocessors Examples

Analyze the following examples to understand various directives.

#define MAX_ARRAY_LENGTH 20

This directive tells the OCPP to replace instances of MAX_ARRAY_LENGTH with 20. Use #define for constants to increase readability.

#import <Foundation/Foundation.h>
#include "myheader.h"

These directives tell the OCPP to get foundation.h from Foundation Framework and add the text to the current source file. The next line tells OCPP to get myheader.h from the local directory and add the content to the current source file.

#undef  FILE_SIZE
#define FILE_SIZE 42

This tells the OCPP to undefine existing FILE_SIZE and define it as 42.

#ifndef MESSAGE
   #define MESSAGE "You wish!"
#endif

This tells the OCPP to define MESSAGE only if MESSAGE isn't already defined.

#ifdef DEBUG
   /* Your debugging statements here */
#endif

This tells the OCPP to do the process the statements enclosed if DEBUG is defined. This is useful if you pass the -DDEBUG flag to gcc compiler at the time of compilation. This will define DEBUG, so you can turn debugging on and off on the fly during compilation.

Predefined Macros

ANSI C defines a number of macros. Although each one is available for your use in programming, the predefined macros should not be directly modified.

Macro	Description
__DATE__	The current date as a character literal in "MMM DD YYYY" format
__TIME__	The current time as a character literal in "HH:MM:SS" format
__FILE__	This contains the current filename as a string literal.
__LINE__	This contains the current line number as a decimal constant.
__STDC__	Defined as 1 when the compiler complies with the ANSI standard.

Let's try the following example:

#import <Foundation/Foundation.h>

int main()
{
   NSLog(@"File :%s\n", __FILE__ );
   NSLog(@"Date :%s\n", __DATE__ );
   NSLog(@"Time :%s\n", __TIME__ );
   NSLog(@"Line :%d\n", __LINE__ );
   NSLog(@"ANSI :%d\n", __STDC__ );
   
   return 0;
}

When the above code in a file main.m is compiled and executed, it produces the following result:

2013-09-14 04:46:14.859 demo[20683] File :main.m
2013-09-14 04:46:14.859 demo[20683] Date :Sep 14 2013
2013-09-14 04:46:14.859 demo[20683] Time :04:46:14
2013-09-14 04:46:14.859 demo[20683] Line :8
2013-09-14 04:46:14.859 demo[20683] ANSI :1

Preprocessor Operators

The Objective-C preprocessor offers following operators to help you in creating macros:

Macro Continuation (\)

A macro usually must be contained on a single line. The macro continuation operator is used to continue a macro that is too long for a single line. For example:

#define  message_for(a, b)  \
    NSLog(@#a " and " #b ": We love you!\n")

Stringize (#)

The stringize or number-sign operator ('#'), when used within a macro definition, converts a macro parameter into a string constant. This operator may be used only in a macro that has a specified argument or parameter list. For example:

#import <Foundation/Foundation.h>

#define  message_for(a, b)  \
    NSLog(@#a " and " #b ": We love you!\n")

int main(void)
{
   message_for(Carole, Debra);
   return 0;
}

When the above code is compiled and executed, it produces the following result:

2013-09-14 05:46:14.859 demo[20683] Carole and Debra: We love you!

Token Pasting (##)

The token-pasting operator (##) within a macro definition combines two arguments. It permits two separate tokens in the macro definition to be joined into a single token. For example:

#import <Foundation/Foundation.h>

#define tokenpaster(n) NSLog (@"token" #n " = %d", token##n)

int main(void)
{
   int token34 = 40;
   
   tokenpaster(34);
   return 0;
}

When the above code is compiled and executed, it produces the following result:

2013-09-14 05:48:14.859 demo[20683] token34 = 40

How it happened, because this example results in the following actual output from the preprocessor:

NSLog (@"token34 = %d", token34);

This example shows the concatenation of token##n into token34 and here we have used both stringize and token-pasting.

The defined() Operator

The preprocessor defined operator is used in constant expressions to determine if an identifier is defined using #define. If the specified identifier is defined, the value is true (non-zero). If the symbol is not defined, the value is false (zero). The defined operator is specified as follows:

#import <Foundation/Foundation.h>

#if !defined (MESSAGE)
   #define MESSAGE "You wish!"
#endif

int main(void)
{
   NSLog(@"Here is the message: %s\n", MESSAGE);  
   return 0;
}

When the above code is compiled and executed, it produces the following result:

2013-09-14 05:48:19.859 demo[20683] Here is the message: You wish!

Parameterized Macros

One of the powerful functions of the OCPP is the ability to simulate functions using parameterized macros. For example, we might have some code to square a number as follows:

int square(int x) {
   return x * x;
}

We can rewrite above code using a macro as follows:

#define square(x) ((x) * (x))

Macros with arguments must be defined using the #define directive before they can be used. The argument list is enclosed in parentheses and must immediately follow the macro name. Spaces are not allowed between macro name and open parenthesis. For example:

#import <Foundation/Foundation.h>

#define MAX(x,y) ((x) > (y) ? (x) : (y))

int main(void)
{
   NSLog(@"Max between 20 and 10 is %d\n", MAX(10, 20));  
   return 0;
}

When the above code is compiled and executed, it produces the following result:

2013-09-14 05:52:15.859 demo[20683] Max between 20 and 10 is 20

********************************************************************************

The Objective-C programming language provides a keyword called typedef, which you can use to give a type a new name. Following is an example to defi

ne a term BYTE for one-byte numbers:

typedef unsigned char BYTE;

After this type definition, the identifier BYTE can be used as an abbreviation for the type unsigned char, for example:.

BYTE  b1, b2;

By convention, uppercase letters are used for these definitions to remind the user that the type name is really a symbolic abbreviation, but you can use lowercase, as follows:

typedef unsigned char byte;

You can use typedef to give a name to user-defined data type as well. For example, you can use typedef with structure to define a new data type and then use that data type to define structure variables directly as follows:

#import <Foundation/Foundation.h>

typedef struct Books
{
   NSString *title;
   NSString *author;
   NSString *subject;
   int book_id;

} Book;
 
int main( )
{

   Book book;
   book.title = @"Objective-C Programming";
   book.author = @"TutorialsPoint";
   book.subject = @"Programming tutorial";
   book.book_id = 100;
   NSLog( @"Book title : %@\n", book.title);
   NSLog( @"Book author : %@\n", book.author);
   NSLog( @"Book subject : %@\n", book.subject);
   NSLog( @"Book Id : %d\n", book.book_id);

   return 0;
}

When the above code is compiled and executed, it produces the following result:

2013-09-12 12:21:53.745 demo[31183] Book title : Objective-C Programming
2013-09-12 12:21:53.745 demo[31183] Book author : TutorialsPoint
2013-09-12 12:21:53.745 demo[31183] Book subject : Programming tutorial
2013-09-12 12:21:53.745 demo[31183] Book Id : 100

typedef vs #define

The #define is a Objective-C directive, which is also used to define the aliases for various data types similar to typedef but with following differences:

• The typedef is limited to giving symbolic names to types only whereas #define can be used to define alias for values as well, like you can define 1 as ONE, etc.
• The typedef interpretation is performed by the compiler where as #define statements are processed by the pre-processor.

Following is a simplest usage of #define:

#import <Foundation/Foundation.h>
 
#define TRUE  1
#define FALSE 0
 
int main( )
{
   NSLog( @"Value of TRUE : %d\n", TRUE);
   NSLog( @"Value of FALSE : %d\n", FALSE);

   return 0;
}

When the above code is compiled and executed, it produces the following result:

2013-09-12 12:23:37.993 demo[5160] Value of TRUE : 1
2013-09-12 12:23:37.994 demo[5160] Value of FALSE : 0Type casting is a way to convert a variable from one data type to another data type. For example, if you want to store a long value into a simple integer then you can type cast long to int. You can convert values from one type to another explicitly using the cast operator as follows:

(type_name) expression

In Objective-C, we generally use CGFloat for doing floating point operation, which is derived from basic type of float in case of 32-bit and double in case of 64-bit. Consider the following example where the cast operator causes the division of one integer variable by another to be performed as a floating-point operation:

#import <Foundation/Foundation.h>

int main()
{
   int sum = 17, count = 5;
   CGFloat mean;

   mean = (CGFloat) sum / count;
   NSLog(@"Value of mean : %f\n", mean );

   return 0;
}

When the above code is compiled and executed, it produces the following result:

2013-09-11 01:35:40.047 demo[20634] Value of mean : 3.400000

It should be noted here that the cast operator has precedence over division, so the value of sum is first converted to type double and finally it gets divided by count yielding a double value.

Type conversions can be implicit which is performed by the compiler automatically or it can be specified explicitly through the use of the cast operator. It is considered good programming practice to use the cast operator whenever type conversions are necessary.

Integer Promotion

Integer promotion is the process by which values of integer type "smaller" than int or unsigned int are converted either to int or unsigned int. Consider an example of adding a character in an int:

#import <Foundation/Foundation.h>

int main()
{
   int  i = 17;
   char c = 'c'; /* ascii value is 99 */
   int sum;

   sum = i + c;
   NSLog(@"Value of sum : %d\n", sum );

   return 0;
}

When the above code is compiled and executed, it produces the following result:

2013-09-11 01:38:28.492 demo[980] Value of sum : 116

Here, value of sum is coming as 116 because compiler is doing integer promotion and converting the value of 'c' to ascii before performing actual addition operation.

Usual Arithmetic Conversion

The usual arithmetic conversions are implicitly performed to cast their values in a common type. Compiler first performs integer promotion, if operands still have different types then they are converted to the type that appears highest in the following hierarchy:

The usual arithmetic conversions are not performed for the assignment operators, nor for the logical operators && and ||. Let us take following example to understand the concept:

#import <Foundation/Foundation.h>

int main()
{
   int  i = 17;
   char c = 'c'; /* ascii value is 99 */
   CGFloat sum;

   sum = i + c;
   NSLog(@"Value of sum : %f\n", sum );
   return 0;
}

When the above code is compiled and executed, it produces the following result:

2013-09-11 01:41:39.192 demo[15351] Value of sum : 116.000000

Here, it is simple to understand that first c gets converted to integer but because final value is float, so usual arithmetic conversion applies and compiler converts i and c into float and add them yielding a float result.

***************************************************************

NSLog method

In order to print logs, we use the NSLog method in Objective-C programming language which we have used right from the Hello World example.

Let us look at a simple code that would print the words "Hello World":

#import <Foundation/Foundation.h>

int main()
{
   NSLog(@"Hello, World! \n");
   return 0;
}

Now, when we compile and run the program, we will get the following result.

2013-09-16 00:32:50.888 demo[16669] Hello, World! 

Disabling logs in Live apps

Since the NSLogs we use in our application, it will be printed in logs of device and it is not good to print logs in a live build. Hence, we use a type definition for printing logs and we can use them as shown below.

#import <Foundation/Foundation.h>

#if DEBUG == 0
#define DebugLog(...)
#elif DEBUG == 1
#define DebugLog(...) NSLog(__VA_ARGS__)
#endif

int main()
{
   DebugLog(@"Debug log, our custom addition gets \
   printed during debug only" );
   NSLog(@"NSLog gets printed always" );     
   return 0;
}

Now, when we compile and run the program in debug mode, we will get the following result.

2013-09-11 02:47:07.723 demo[618] Debug log, our custom addition gets printed during debug only
2013-09-11 02:47:07.723 demo[618] NSLog gets printed always

Now, when we compile and run the program in release mode, we will get the following result.

2013-09-11 02:47:45.248 demo[3158] NSLog gets printed always

**********************************************************************************n Objective-C programming, error handling is provided with NSError class available in Foundation framework. An NSError object encapsulates richer and more extensible error information than is possible using only an error code or error string. The core attributes of an NSError object are an error domain (represented by a string), a domain-specific error code and a user info dictionary containing application specific information.

NSError

Objective-C programs use NSError objects to convey information about runtime errors that users need to be informed about. In most cases, a program displays this error information in a dialog or sheet. But it may also interpret the information and either ask the user to attempt to recover from the error or attempt to correct the error on its own

NSError Object consists of:

• Domain: The error domain can be one of the predefined NSError domains or an arbitrary string describing a custom domain and domain must not be nil.
• Code: The error code for the error.
• User Info: The userInfo dictionary for the error and userInfo may be nil.

The following example shows how to create a custom error

NSString *domain = @"com.MyCompany.MyApplication.ErrorDomain";
NSString *desc = NSLocalizedString(@"Unable to complete the process", @"");
NSDictionary *userInfo = @{ NSLocalizedDescriptionKey : desc };
NSError *error = [NSError errorWithDomain:domain code:-101 userInfo:userInfo];

Here is complete code of the above error sample passed as reference to an pointer

#import <Foundation/Foundation.h>

@interface SampleClass:NSObject

-(NSString *) getEmployeeNameForID:(int) id withError:(NSError **)errorPtr;

@end

@implementation SampleClass

-(NSString *) getEmployeeNameForID:(int) id withError:(NSError **)errorPtr{
    if(id == 1)
    {
       return @"Employee Test Name";
    }
    else
    {
       NSString *domain = @"com.MyCompany.MyApplication.ErrorDomain";
       NSString *desc =@"Unable to complete the process";
       NSDictionary *userInfo = [[NSDictionary alloc] 
       initWithObjectsAndKeys:desc,
       @"NSLocalizedDescriptionKey",NULL];  
       *errorPtr = [NSError errorWithDomain:domain code:-101 
       userInfo:userInfo];
       return @"";
    }
}

@end


int main()
{
   NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init];
   SampleClass *sampleClass = [[SampleClass alloc]init];
   NSError *error = nil;
   NSString *name1 = [sampleClass getEmployeeNameForID:1 withError:&error];
  
   if(error)
   {
      NSLog(@"Error finding Name1: %@",error);
   }
   else
   {
      NSLog(@"Name1: %@",name1);
   }

   error = nil;

   NSString *name2 = [sampleClass getEmployeeNameForID:2 withError:&error];

   if(error)
   {
      NSLog(@"Error finding Name2: %@",error);
   }
   else
   {
      NSLog(@"Name2: %@",name2);
   }

   [pool drain];
   return 0;
   
}

In the above example, we return a name if the id is 1, otherwise we set the 

user defined error object.

When the above code is compiled and executed, it produces the following result:

2013-09-14 18:01:00.809 demo[27632] Name1: Employee Test Name
2013-09-14 18:01:00.809 demo[27632] Error finding Name2: Unable to complete the process

******************************************************************

It is possible to pass some values from the command line to your Objective-C programs when they are executed. These values are called command line arguments and many times they are important for your program, especially when you want to control your program from outside instead of hard coding those values inside the code.

The command line arguments are handled using main() function arguments where argc refers to the number of arguments passed, and argv[] is a pointer array, which points to each argument passed to the program. Following is a simple example, which checks if there is any argument supplied from the command line and take action accordingly:

#import <Foundation/Foundation.h>

int main( int argc, char *argv[] )  
{
   if( argc == 2 )
   {
      NSLog(@"The argument supplied is %s\n", argv[1]);
   }
   else if( argc > 2 )
   {
      NSLog(@"Too many arguments supplied.\n");
   }
   else
   {
      NSLog(@"One argument expected.\n");
   }
}

When the above code is compiled and executed with a single argument, say "testing", it produces the following result.

2013-09-13 03:01:17.333 demo[7640] The argument supplied is testing

When the above code is compiled and executed with two arguments, say testing1 and testing2, it produces the following result.

2013-09-13 03:01:18.333 demo[7640] Too many arguments supplied.

When the above code is compiled and executed without passing any argument, it produces the following result.

2013-09-13 03:01:18.333 demo[7640] One argument expected

It should be noted that argv[0] holds the name of the program itself and argv[1] is a pointer to the first command-line argument supplied, and *argv[n] is the last argument. If no arguments are supplied, argc will be one, otherwise if you pass one argument, then argc is set at 2.

You pass all the command line arguments separated by a space, but if argument itself has a space, then you can pass such arguments by putting them inside double quotes "" or single quotes ''. Let us re-write above example once again where we will print program name and we also pass a command-line argument by putting inside double quotes:

#import <Foundation/Foundation.h>

int main( int argc, char *argv[] )  
{
   NSLog(@"Program name %s\n", argv[0]);
 
   if( argc == 2 )
   {
      NSLog(@"The argument supplied is %s\n", argv[1]);
   }
   else if( argc > 2 )
   {
      NSLog(@"Too many arguments supplied.\n");
   }
   else
   {
      NSLog(@"One argument expected.\n");
   }
   return 0;
}

When the above code is compiled and executed with a single argument separated by space but inside double quotes say "Testing1 Testing2", it produces the following result.

2013-09-14 04:07:57.305 demo[8534] Program name demo
2013-09-14 04:07:57.305 demo[8534] The argument supplied is Testing1 Testing 2