C Pointer Arithmetic

We can perform arithmetic operations on the pointers like addition, subtraction, etc. However, as we know that pointer contains the address, the result of an arithmetic operation performed on the pointer will also be a pointer if the other operand is of type integer. In pointer-from-pointer subtraction, the result will be an integer value. Following arithmetic operations are possible on the pointer in C language:

• Increment
• Decrement
• Addition
• Subtraction

 

Incrementing Pointer in C

If we increment a pointer by 1, the pointer will start pointing to the immediate next location. This is somewhat different from the general arithmetic since the value of the pointer will get increased by the size of the data type to which the pointer is pointing.

We can traverse an array by using the increment operation on a pointer which will keep pointing to every element of the array, perform some operation on that, and update itself in a loop.

The Rule to increment the pointer is given below:

new_address= current_address + i * size_of(data type)

Where i is the number by which the pointer get increased.

32-bit

For 32-bit int variable, it will be incremented by 2 bytes.

64-bit

For 64-bit int variable, it will be incremented by 4 bytes.

Let's see the example of incrementing pointer variable on 64-bit architecture.

#include<stdio.h>
#include<conio.h>
int main(){  
int number=50;        
int *p;//pointer to int      
p=&number;//stores the address of number variable        
printf("Address of p variable is %u \n",p);        
p=p+1;        
printf("After increment: Address of p variable is %u \n",p); // in our case, p will get incremented by 4 bytes.      
return 0;  
}    

 Output

 

Traversing an array by using pointer

    #include<stdio.h>
    #include<conio.h>
    void main ()  
    {  
        int arr[5] = {1, 2, 3, 4, 5};  
        int *p = arr;  
        int i;  
        printf("printing array elements...\n");  
        for(i = 0; i< 5; i++)  
        {  
            printf("%d  ",*(p+i));  
        }  
    }

   

Output

 

 

 

Decrementing Pointer in C

Like increment, we can decrement a pointer variable. If we decrement a pointer, it will start pointing to the previous location. The formula of decrementing the pointer is given below:

 new_address= current_address -  i * size_of(data type)

 32-bit

For 32-bit int variable, it will be decremented by 2 bytes.

64-bit

For 64-bit int variable, it will be decremented by 4 bytes.

Let's see the example of decrementing pointer variable on 64-bit OS.

#include<stdio.h>
#include<conio.h>
void main(){            
int number=50;        
int *p;//pointer to int      
p=&number;//stores the address of number variable        
printf("Address of p variable is %u \n",p);        
p=p-1;       
printf("After decrement: Address of p variable is %u \n",p); // P will now point to the immidiate previous location.         
}      

Output

 

 

C Pointer Addition

We can add a value to the pointer variable. The formula of adding value to pointer is given below:

new_address= current_address + (number * size_of(data type)) 

32-bit

For 32-bit int variable, it will add 2 * number.

64-bit

For 64-bit int variable, it will add 4 * number.

Let's see the example of adding value to pointer variable on 64-bit architecture.

    #include<stdio.h>  
    int main(){  
    int number=50;        
    int *p;//pointer to int      
    p=&number;//stores the address of number variable        
    printf("Address of p variable is %u \n",p);        
    p=p+3;   //adding 3 to pointer variable    
    printf("After adding 3: Address of p variable is %u \n",p);       
    return 0;  
    }    

 Output

C Pointer Subtraction

Like pointer addition, we can subtract a value from the pointer variable. Subtracting any number from a pointer will give an address. The formula of subtracting value from the pointer variable is given below:

new_address= current_address - (number * size_of(data type))  

32-bit

For 32-bit int variable, it will subtract 2 * number.

64-bit

For 64-bit int variable, it will subtract 4 * number.

Let's see the example of subtracting value from the pointer variable on 64-bit architecture.

    #include<stdio.h>  
    int main(){  
    int number=50;        
    int *p;//pointer to int      
    p=&number;//stores the address of number variable        
    printf("Address of p variable is %u \n",p);        
    p=p-3; //subtracting 3 from pointer variable    
    printf("After subtracting 3: Address of p variable is %u \n",p);        
    return 0;  
    } 

Output 

 

We can see after subtracting 3 from the pointer variable, it is 12 (4*3) less than the previous address value.

Illegal arithmetic with pointers

There are various operations which can not be performed on pointers. Since, pointer stores address hence we must ignore the operations which may lead to an illegal address, for example, addition, and multiplication. A list of such operations is given below.

• Address + Address = illegal
• Address * Address = illegal
• Address % Address = illegal
• Address / Address = illegal
• Address & Address = illegal
• Address ^ Address = illegal
• Address | Address = illegal
• ~Address = illegal