Speeding Up Calculations with Memoization (Caching)

This lesson covers speeding up calculations in C with memoization (caching), so you understand how to avoid repeating the same work. It's for anyone searching "C memoization usage."

Memoization saves a calculation's result in a "storage box" once, and retrieves it from the box instead of recalculating when the same input comes up again. It's like "giving the same answer you already remembered, if you're asked the same question again."

The example stores completed calculations in a global array called cache, and the slowSquare() function checks cache[n] != -1 every time it's called to see if it's already been computed. From the second call with the same input onward, a "retrieved from cache" message appears instead.

A common early mistake is handling the cache array's initial value. You need to decide a value representing "not yet calculated" (-1 here) ahead of time, and design it to be distinguishable from a genuine calculation result.

In real projects, caching recursive calculations or API call results in particular can give a noticeably real speedup.

📖 Reference code

#include <stdio.h>

int cache[100];

int slowSquare(int n) {
    if (cache[n] != -1) {
        printf("Retrieved from cache: %d\n", n);
        return cache[n];
    }
    printf("Calculating: %d\n", n);
    int result = n * n;
    cache[n] = result;
    return result;
}

int main(void) {
    for (int i = 0; i < 100; i++) cache[i] = -1;
    printf("%d\n", slowSquare(5));
    printf("%d\n", slowSquare(5));
    return 0;
}

✍️ Your code

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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).