Challenge 2: apply_to_all With a print_doubled Callback — Possible Solution
====================================================================

#include <stdio.h>

void print_doubled(int x) {
    printf("%d\n", x * 2);
}

void apply_to_all(int *arr, int n, void (*fn)(int)) {
    for (int i = 0; i < n; i++) {
        fn(arr[i]);
    }
}

int main() {
    int nums[5] = {1, 2, 3, 4, 5};
    apply_to_all(nums, 5, print_doubled);
    return 0;
}

Output:

    2
    4
    6
    8
    10

WHY THIS WORKS AS AN ANSWER
------------------------------
apply_to_all follows the chapter's own callback pattern exactly --
its third parameter is a function pointer type matching
print_doubled's own signature (void, taking one int) -- and passing
print_doubled by name relies on the same function-to-pointer decay
Challenge 1 also used, letting apply_to_all invoke fn(arr[i]) once per
array element without knowing at compile time which specific function
it's calling.