Flattening an Array

This lesson covers flattening a nested array in C, so you understand a way to handle a complex data structure more simply. It's for anyone searching "C array flatten implementation."

Expanding a two-dimensional array into a single "flat" array is called flattening. C has no dedicated function for this, so you implement it yourself with a for loop.

The example uses a nested for loop to pull elements from the two-dimensional array nested one at a time, packing them into a one-dimensional array, flat, with flat[idx++] = nested[i][j]. Notice the postfix increment, idx++, advancing the index at the same time as the assignment.

A common early mistake is not computing the result array's size ahead of time. Since there's no automatically-resizing array like in some other languages, you need to reserve the exact size beforehand.

In real projects, this is a genuinely useful, real-world technique for handling a nested data structure as a single unified list.

📖 Reference code

#include <stdio.h>

int main(void) {
    int nested[3][2] = {{1, 2}, {3, 4}, {5, 6}};
    int flat[6];
    int idx = 0;
    for (int i = 0; i < 3; i++) {
        for (int j = 0; j < 2; j++) {
            flat[idx++] = nested[i][j];
        }
    }
    for (int i = 0; i < 6; i++) {
        printf("%d", flat[i]);
        if (i < 5) printf(", ");
    }
    printf("\n");
    return 0;
}

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🧪 This site can't compile or run C directly, so it checks on the spot whether what you typed matches the reference code (scoring happens entirely in your browser — nothing is sent anywhere).