Introduction to Pointers in C

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Introduction to Pointers in C

Pointer Notation

Consider the declaration,

int i = 3 ;

This declaration tells the C compiler to:

  • Reserve space in memory to hold the integer value.
  • Associate the name i with this memory location.
  • Store the value 3 at this location.

We may represent i’s location in memory by the following memory map

location map of a variable
location map of a variable

What this means

We see that the computer has selected memory location 65524 as the place to store the value 3.

The location number 65524 is not a number to be relied upon, because some other time the computer may choose a different location for storing the value 3.

The important point is, i’s address in memory is a number.

We can print this address number through the following program:

main( )

{

int i = 3 ;

printf ( “\nAddress of i = %u”, &i ) ;

printf ( “\nValue of i = %d”, i ) ;

}

The output of the above program would be: Address of i = 65524 Value of i = 3

Look at the first printf( ) statement carefully.

  • ‘&’ used in this statement is C’s ‘address of’ operator.
  • The expression &i returns the address of the variable i, which in this case happens to be 65524. Since 65524 represents an address, there is no question of a sign being associated with it.
  • Hence it is printed out using %u, which is a format specifier for printing an unsigned integer.
  • We have been using the ‘&’ operator all the time in the scanf( ) statement.
understanding c pointers
understanding c pointers

Now pointer comes

The other pointer operator available in C is ‘*’, called ‘value at address’ operator. It gives the value stored at a particular address. The ‘value at address’ operator is also called ‘indirection’ operator.

Let ‘s look at the below program

main( )

{

int i = 3 ;

printf ( “\nAddress of i = %u”, &i ) ;

printf ( “\nValue of i = %d”, i ) ;

printf ( “\nValue of i = %d”, *( &i ) ) ;

}

The output of the above program would be: Address of i = 65524 Value of i = 3 Value of i = 3

Note that printing the value of *( &i ) is same as printing the value of i.

The expression &i gives the address of the variable i. This address can be collected in a variable, by saying, j = &i ;

But please note that j is not an ordinary variable like any other integer variable.

It is a variable that contains the address of other variable (i in this case). Since j is a variable the compiler must provide it space in the memory.

Once again, the following memory map would illustrate the contents of i and j.

pointer in c

pointer in c

As we can see in the above example, i’s value is 3 and j’s value is i’s address

But, we can’t use j in a program without declaring it. And since j is a variable that contains the address of i, it is declared as, int *j ;

int *j ;

This declaration tells the compiler that j will be used to store the address of an integer value.

In other words j points to an integer.

Now how do we justify the usage of * in the declaration. Let see..

int *j ;

Now see the meaning of *. It stands for ‘value at address’. Thus, int *j would mean, the value at the address contained in j is an int.

Here is a program that demonstrates the relationships we have been discussing.

main( )

{

int i = 3 ; int *j ;

j = &i ;

printf ( “\nAddress of i = %u”, &i ) ;

printf ( “\nAddress of i = %u”, j ) ;

printf ( “\nAddress of j = %u”, &j ) ;

printf ( “\nValue of j = %u”, j ) ;

printf ( “\nValue of i = %d”, i ) ;

printf ( “\nValue of i = %d”, *( &i ) ) ;

printf ( “\nValue of i = %d”, *j ) ;

}

Output of the above program:

Address of i = 65524

Address of i = 65524

Address of j = 65522

Value of j = 65524

Value of i = 3

Value of i = 3

Value of i = 3

Look at the following declarations,

int *alpha ;

char *ch ;

float *s ;

Here, alpha, ch and s are declared as pointer variables, i.e. variables capable of holding addresses.

Remember that, addresses (location nos.) are always going to be whole numbers, therefore pointers always contain whole numbers.

Now we can put these two facts together and say—pointers are variables that contain addresses, and since addresses are always whole numbers, pointers would always contain whole numbers.

The declaration float *s does not mean that s is going to contain a floating-point value. What it means is, s is going to contain the address of a floating-point value.

Similarly, char *ch means that ch is going to contain the address of a char value. Or in other words, the value at address stored in ch is going to be a char.

Pointer is a variable that contains address of another variable.

 

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