Pointers & Functions (Pass by Reference)

void doubleValue(int *ptr) {
    *ptr = *ptr * 2;
}

int main() {
    int number = 5;
    doubleValue(&number);   // pass the ADDRESS of number
    printf("%d\n", number);      // 10 - the original variable actually changed!
    return 0;
}

Compare this directly to Lesson 7's tryToChange() example, which failed to modify the original variable. The only difference here is passing a pointer instead of the value itself.

void swap(int *a, int *b) {
    int temp = *a;
    *a = *b;
    *b = temp;
}

int main() {
    int x = 5, y = 10;
    swap(&x, &y);
    printf("x=%d y=%d\n", x, y);   // x=10 y=5
    return 0;
}

Swapping two variables is impossible to do correctly with plain pass-by-value parameters in C — this is one of the classic, textbook examples of exactly why pointers exist.

A C function can only return a single value directly. Pointers let you work around this by having the function write multiple results directly into variables the caller provides:

void getMinMax(int arr[], int size, int *min, int *max) {
    *min = *max = arr[0];
    for (int i = 1; i < size; i++) {
        if (arr[i] < *min) *min = arr[i];
        if (arr[i] > *max) *max = arr[i];
    }
}

int main() {
    int nums[5] = {4, 8, 1, 9, 3};
    int min, max;
    getMinMax(nums, 5, &min, &max);
    printf("Min: %d Max: %d\n", min, max);
    return 0;
}

A pointer can itself be pointed to by another pointer, written int **ptr. This sounds exotic, but you'll encounter it most often when a function needs to modify a pointer variable itself (not just the value it points to) — a topic worth revisiting once you're comfortable with single-level pointers.

#include <stdio.h>

void swap(int *a, int *b) {
    int temp = *a;
    *a = *b;
    *b = temp;
}

int main() {
    int x = 3, y = 8;
    printf("Before: x=%d y=%d\n", x, y);
    swap(&x, &y);
    printf("After: x=%d y=%d\n", x, y);
    return 0;
}