Pointers

Pointers :-

                   A pointer is a special type of variable which is used to store the memory address of basic datatype, of array , structure, union etc .

           Till now , we only have used simple undirection variables in which we can store/retrieve values . But now we will discuss a special type of variables which are used to store the memory location of a variable. Its hard to understand and manipulate values on the basis of there address(with pointers) , but this is the most powerful concept of C programming which

• Provide Flexibility to the program
• As we deal with memory addresses , therefore increase execution speed
• It allows a function to reflect back more from one value at a time
• It allow us to pass array , structureetc as arguments to functions

Before starting the concept of pointers , I want to introduce you with two operators

1. 1)Address operator ‘&’     :- This is use to get the address of a variable .
2. 2)Indirection operator ‘*’   :- It is use to manipulate the value stored in variable which is being pointed from the pointer variable .

 Now Let’s start the concept of pointer variables :-

A pointer variable is a special type of variable that contains the address of another variable of same data type 
Syntax :-

            data_type   *ptr ;

here

data_type         the data type of variable which this pointer will point

ptr                    name of pointer variable

*                      Indirection operator

Now I tell you how a variable is stored in memory .

Example :-

            int   x = 5 ;

            int   *y ;

            y = &x ;

see following diagram.

Here

x is simple variable of integer data type

y is pointer variable which is used to store the address of an integer variable x

2002 is the memory address of variable x (Assumed values)

2004 is the memory address of pointer variable y (Assumed values)

Now consider the following statements .

 One more important thing , the variable , and pointer variable which we want to use to store the address of that variable must be of same data type , because it can arise syntax and logical errors . I discuss it later .

 Now you can understand that how pointers are used to store the address of a variable, But now I tell you how a pointer can store the address of another pointer i.e.

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Poniter to Pointer :-

                                    As we know every variable declared in a program is stored at some memory location and has a uniqueaddress , same as simple variables pointers also have there address in memory . Therefore a pointer to pointer variable can be defined which can store the address of a pointer variable .

 Syntax:-

          data_type   **y ;

here we have used **to declare a pointer to pointer variable

I show you with the help of an example.

            int x ;

            int *y ;

            int **k ;

            y = &x ;

            k = &y ;

 Now see the following figure .

 Here

 x is simple variable

y is pointer variable , which is used to store the address of a variable

k is a pointer to pointer variable , which is used to store the address of a pointer variable

2002 , 2004 , 2006 are memory addresses of x , y , k .

Now consider the following statements .

Now lets move to next topic , which is Pointer Arithmetic .

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Pointer Arithmetic :-

                                 We all are much familiar with the word arithmetic . We can also perform some arithmetic operations on pointers , like Incrementing , Decrementing , Add/suntract some values . But as we know we are dealing with pointers and memory addresses these are manipulated in different manner . for example :-

            int x ;

            int *p1 ;

            x=5 ;

            p1 = &x ;

 Here we have an integer variable with name ‘x’ and a pointer ‘p1’ pointing towards ‘x’

 Now if we perform Increment operation on x

 x++ ;

 its result is x = 6

 and now if we perform increment operation on pointer p1

 (Let memory address of x = 2002   i.e. p1 = 2002)

 p1++

 the result comes p1 = 2004

 Now the question arises   HOW ?

 I tell you , we know that ‘x’ is an integer variable which takes 2 Bytes in memory and p1 is pointer which contains the address of ‘x’ . When we increment the pointer it goes to the next variable position in memory which is at a difference of 2 Bytes from ‘x’

 See following Figure .

Thus if we use pointer for long and float so it will increment the address from 4 as float and double data type takes 4 Bytes in memory .

Following are the valid operations on Pointers .

 1)Increment and Decrement operation :-

 We can perform pre-Increment (++p) , post-Increment (p++) , Pre-Decrement (- - p) , Post-Decrement (p- - ) operations on Pointers .

 2)Addition and Subtraction of a number from pointer :-

    We can also Add , subtract a number from a pointer variable like p = p + 1 but it work differently .

 Let   *p is an integer pointer variable and contain address 2002(Assumed value) , Now we perform operation .

  

	Statements	Output
P = 2002	p = p + 1 ;	P = 2004
P = 2002	p = p + 4 ;	P = 2010
P = 2002	p = p – 1 ;	P = 2000
P = 2002	p = p – 4 ;	P = 1994

 (I don’t think that I need to explain it again as above )

 There are some more operations valid on pointers , these are :-

1)p1 > p2

1. 2)p1 < p2
2. 3)p1 == p2
3. 4)p1 != p2
4. 5)p1 >= p2
5. 6)p1 <= p2

These operations are valid if p1 and p2 are of same data type .

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Pointers and Array :-

                                   Pointers and array are very closely related with each other . The name of the array is infact a pointer which points to the first element of the array , i.e. the Base address or starting address of the array .