Pointer arithmetic

Learn to do arithmetic on pointers with + and -.

Pointer arithmetic

You can add to or subtract from a pointer. ptr + 1 advances by the size of the pointer's type.

int arr[3] = {10, 20, 30};
int *ptr = arr;
printf("%d\n", *(ptr + 1));  /* prints 20 */

If an int is 4 bytes, ptr + 1 jumps to the next int; the address goes up by 4. This is the basis of using pointers with arrays.

Walking an array with a pointer

Pointer arithmetic lets you walk an array without using an index. ptr++ moves to the next element each step.

int arr[3] = {10, 20, 30};
int *ptr = arr;
for (int i = 0; i < 3; i++) {
    printf("%d\n", *ptr);
    ptr++;
}

This avoids index math; but the pointer must not go past the array bounds.

Exercise Print an array with pointer arithmetic

Define int arr[3] = {10, 20, 30};. Start with int *ptr = arr;. In a for loop, print each element with *(ptr + i) (i = 0, 1, 2). Output should be three lines: 10, 20, 30.

Show example solution

#include <stdio.h>

int main(void) {
    int arr[3] = {10, 20, 30};
    int *ptr = arr;
    for (int i = 0; i < 3; i++) {
        printf("%d\n", *(ptr + i));
    }
    return 0;
}

Test yourself

For int *ptr, how many bytes does ptr + 1 advance? (int is 4 bytes)
- 1 byte
- 0 bytes
- 8 bytes
- 4 bytes
Show answer

4 bytes — If int is 4 bytes, ptr + 1 jumps to the next int; 4 bytes forward.
Match pointer arithmetic

ptr + 1

next element
ptr - 1

previous element
ptr++

move forward
*(ptr + n)

nth element

Access the second element

int arr[3] = {10, 20, 30};
int *ptr = arr;
printf("%d\n", ____);  /* prints 20 */

Show answer

int arr[3] = {10, 20, 30};
int *ptr = arr;
printf("%d\n", *(ptr + 1));  /* prints 20 */

Next lesson: Summary - Pointer basics

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