Arrays as parameters in C: solved exercise

If you searched for a solved arrays as parameters exercise in C, here is the fundamental pattern: in C, arrays are passed to functions as a pointer to the first element, which means the function can modify the original array and the size must be passed separately.

This behavior — called array-to-pointer decay — is one of the first stumbling blocks for developers coming from Java or Python, where arrays carry their own size.

Problem statement

Write three functions:

fill(arr, n): fills the array with values 0, 2, 4, …, 2*(n-1).
sum(arr, n): returns the sum of all elements (without modifying the array).
print_array(arr, n): prints all elements separated by spaces.

Demonstrate that modifications made by fill are visible in main.

C solution

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#include <stdio.h>

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

int sum(const int arr[], int n) {
    int s = 0;
    for (int i = 0; i < n; i++) s += arr[i];
    return s;
}

void print_array(const int arr[], int n) {
    for (int i = 0; i < n; i++) printf("%d ", arr[i]);
    printf("\n");
}

int main(void) {
    int v[5];
    fill(v, 5);
    print_array(v, 5);
    printf("Sum: %d\n", sum(v, 5));
    return 0;
}

Expected output

Common mistakes

Trying to get the array size inside the function using sizeof(arr): it returns the size of the pointer (8 bytes on 64-bit), not the array. Size must always be passed as a parameter.
Forgetting const in functions that only read the array: the compiler cannot optimize or detect accidental modifications.
Passing &v instead of v: &v is a pointer to array (int (*)[5]), a different type from what the function expects.
Confusing stack arrays (int v[5]) with dynamic arrays (malloc): both are passed as a pointer, but their lifetimes differ.

Practical use

Passing arrays to functions is the foundation of sorting, searching, and transformation algorithms in C. Separating logic into functions with const allows the compiler to apply optimizations and helps the programmer reason about side effects.

Recommended next exercise

Guided practice and full book

If you want a complete path with progressive difficulty:

FAQ

Why doesn’t `sizeof(arr)` give the array size inside the function?

Because when an array is passed to a function, it decays to a pointer to its first element. The function receives only the memory address, not the length. sizeof on a pointer returns the pointer size (4 or 8 bytes depending on the architecture).

What is the difference between `int arr[]` and `int *arr` as a function parameter?

None in practice: both declarations are equivalent in the context of function parameters. C treats them identically. int arr[] is preferred for semantic clarity.

How do you pass a two-dimensional array to a function?

For matrices, you must specify the number of columns: void f(int mat[][COLS], int rows). The column count must be a compile-time constant, or use pointer-to-pointer for fully dynamic dimensions.

Arrays as parameters in C: solved exercise