Arrays and Functions

You can pass an array to a function. Because a function can't tell how big the array is, you always pass the size as well:

#include <stdio.h>

int sum(int arr[], int size) {
    int total = 0;
    for (int i = 0; i < size; i++) {
        total += arr[i];
    }
    return total;
}

int main() {
    int a[3] = {1, 2, 3};
    int b[5] = {10, 20, 30, 40, 50};
    printf("%d\n", sum(a, 3));
    printf("%d\n", sum(b, 5));
    return 0;
}

Output

6
150

Notice int arr[] has no number inside the brackets – it works for arrays of any size.

Functions can change an array

Normal variables are copied when you pass them (Module 6). Arrays are different: the function works on the original array. The usual way to write this is with a * (int *arr), which means the same as int arr[]. Module 9 explains why.

#include <stdio.h>

void doubleAll(int *arr, int size) {
    for (int i = 0; i < size; i++) {
        arr[i] = arr[i] * 2;
    }
}

int main() {
    int nums[4] = {1, 5, 7, 10};
    doubleAll(nums, 4);
    for (int i = 0; i < 4; i++) {
        printf("%d ", nums[i]);
    }
    printf("\n");
    return 0;
}

Output

2 10 14 20 

Remember: Pass an array together with its size. A function that changes the array changes the caller's array too.

Tip: In the challenges, write array parameters as int *arr. The simple browser C runner handles that form best.