C Pointer Arithmetic

Pointer arithmetic means changing the address stored in a pointer by using operations like addition or subtraction. This is useful when working with arrays or other blocks of memory.

Increment (++) and Decrement (–)

When you increment or decrement a pointer, it doesn’t just move by 1 byte. Instead, it moves by the size of the type it points to.

For example, if a pointer points to an integer (which is usually 4 bytes in C), incrementing the pointer will move it by 4 bytes to the next integer.

Example: 


#include <stdio.h>

int main() {
        int arr[3] = {1, 2, 3};   // Array of 3 integers
        int *ptr = arr;               // Pointer points to arr[0]
        
        printf("%d\n", *ptr);   // Output: 1 (points to the first element)
        
          ptr++;    // Move the pointer to the next element (arr[1])
        
        printf("%d\n", *ptr); // Output: 2 (now points to the second element)

    return 0;
}

Output:

1
2

Explanation:

  • ptr initially points to the first element (arr[0]), which is 1.
  • After ptr++, ptr now points to the second element (arr[1]), which is 2.

Adding an Integer to a Pointer

When you add a number to a pointer, the pointer moves by that number of elements, not just bytes. So, adding 3 to a pointer will move it by 3 elements forward, not just 3 bytes.

Example:


#include <stdio.h>

int main() {
        int arr[5] = {1, 2, 3, 4, 5};
        int *ptr = arr;            // Pointer points to arr[0]
        
        ptr = ptr + 3;  // Move pointer 3 elements forward (arr[3])
        
        printf("%d\n", *ptr);  // Output: 4 (now points to arr[3])


    return 0;
}

Output:

4

Explanation:

Initially, ptr points to arr[0] (1).
ptr + 3 moves the pointer 3 elements ahead, so it points to arr[3] (4).

Subtracting an Integer from a Pointer

You can subtract an integer from a pointer, and this moves the pointer backward by that number of elements.


#include <stdio.h>

int main() {
        int arr[5] = {1, 2, 3, 4, 5};
        int *ptr = &arr[3];        // Pointer points to arr[3] (4)
        
        ptr = ptr - 1;             // Move pointer 1 element backward (arr[2])
        
        printf("%d\n", *ptr);      // Output: 3 (now points to arr[2])

       return 0;
}

Output:

3

Explanation:

  1. ptr initially points to arr[3] is 4.
  2. After ptr – 1, it moves 1 elements backward to arr[2] is 3.

Pointer Subtraction (Between Pointers)

You can also subtract one pointer from another. This will tell you how many elements are between the two pointers.

Example:


#include <stdio.h>

int main() {
    int arr[5] = {1, 2, 3, 4, 5};
    int *ptr1 = &arr[0];       // Points to arr[0] (1)
    int *ptr2 = &arr[4];       // Points to arr[4] (5)
    
    int diff = ptr2 - ptr1;    // Difference in elements between ptr2 and ptr1
    
    printf("%d\n", diff);


    return 0;
}

Output:

4

C Pointer Arithmetic – Interview Questions

Q 1: What is pointer arithmetic?
Ans: Performing operations like increment or decrement on pointers.
Q 2: What happens when a pointer is incremented?
Ans: It moves to the next memory location based on data type size.
Q 3: Can we add two pointers?
Ans: No, adding two pointers is not allowed.
Q 4: Is pointer subtraction allowed?
Ans: Yes, between two pointers pointing to the same array.
Q 5: Where is pointer arithmetic commonly used?
Ans: In array traversal.

C Pointer arithmetic – Objective Questions (MCQs)

Q1. Which arithmetic operations are allowed on pointers?





Q2. If p is an integer pointer pointing to address 1000, then p + 2 points to:

(Assume int size = 4 bytes)





Q3. What happens when you subtract two pointers pointing into the same array?





Q4. Which of the following expressions is invalid?





Q5. What is the output of:

int a[3] = {10, 20, 30};
int *p = a;
p++;
printf("%d", *p);





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