Speeding up calculation with memoization (caching)

This lesson covers memoization in Objective-C, with the aim of understanding how to avoid repeating the same calculation.

Memoization stores a result once calculated and, when the same input arrives again, fetches it from storage instead of recalculating. Picture answering a repeated question from memory rather than working it out afresh.

The sample code uses an NSMutableDictionary as the cache and checks cache[key] each time slowSquare() is called. From the second identical input onwards you see the "Retrieved from cache" message.

A common early stumble is that a number used as a dictionary key has to be converted to an NSNumber. A primitive integer cannot be a key directly — you convert with the @(n) notation.

In professional work, caching recursive calculations and API results gives speed improvements you can feel.

📖 Reference code

NSMutableDictionary *cache = [NSMutableDictionary dictionary];

NSInteger slowSquare(NSInteger n, NSMutableDictionary *cache) {
    NSNumber *key = @(n);
    if (cache[key]) {
        NSLog(@"Retrieved from cache: %ld", (long)n);
        return [cache[key] integerValue];
    }
    NSLog(@"Calculating: %ld", (long)n);
    NSInteger result = n * n;
    cache[key] = @(result);
    return result;
}

NSLog(@"%ld", (long)slowSquare(5, cache));
NSLog(@"%ld", (long)slowSquare(5, cache));

✍️ Your code

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🧪 This site can't compile or run Objective-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).