C - Pointer arithmetic

Pointer Arithmetic in C

Pointers can be incremented, decremented, and offset with +/-. Movement is in units of the pointee type (e.g., p+1 for int* advances by sizeof(int) bytes).

Learning Objectives

• Use p++, p--, and p + k safely within array bounds.
• Understand how element size affects pointer movement.

Prerequisites

• C - Pointers
• C - Dereference operator

Example

#include <stdio.h>

int main(void) {
    int arr[3] = {10, 20, 30};
    int *p = arr; // &arr[0]
    printf("%d\n", *p); // 10
    p++;
    printf("%d\n", *p); // 20
}

More Examples

Iterate with offsets

#include <stdio.h>

int main(void) {
  int arr[] = {5, 15, 25, 35, 45};
  int *ptr = arr;
  int n = (int)(sizeof arr / sizeof arr[0]);
  for (int i = 0; i < n; i++) {
    printf("%d ", *(ptr + i));
  }
}

Reverse traversal

#include <stdio.h>

int main(void) {
  int arr[] = {2, 4, 6, 8, 10};
  int *ptr = arr + 4; // last element
  for (int i = 4; i >= 0; i--) {
    printf("%d ", *(ptr - i));
  }
}

Other types

#include <stdio.h>

int main(void) {
  double arr[] = {1.1, 2.2, 3.3};
  double *ptr = arr;
  ptr += 2;
  printf("%.1f\n", *ptr); // 3.3
}

Common Pitfalls

• Pointer arithmetic outside the bounds of the same array has undefined behavior.
• Don’t mix pointer arithmetic with freed memory.

Checks for Understanding

1. What does p + 1 do for a char* vs an int*?