Finding the prime factors

This lesson covers prime factorisation in Objective-C, with the aim of understanding a number-theory algorithm built on loops.

Prime factorisation expresses a number as a product of primes. Try dividing from 2 upwards, and whenever a number divides, keep dividing by it.

The sample code starts at d = 2 and, while n divides by d, records the factor with [factors addObject:@(d)] and keeps dividing with n /= d, increasing d by one when it no longer divides — repeating until n reaches 1.

A common early stumble is the nested loop structure. The outer loop increases the divisor d while the inner one keeps dividing for as long as it can — that two-stage arrangement is the key to understanding it.

It is an important idea spanning mathematics and computer science, and underpins the security of cryptography such as RSA.

📖 Reference code

void printPrimeFactors(int n) {
    NSMutableArray *factors = [NSMutableArray array];
    int d = 2;
    while (n > 1) {
        while (n % d == 0) {
            [factors addObject:@(d)];
            n /= d;
        }
        d++;
    }
    NSLog(@"%@", [factors componentsJoinedByString:@", "]);
}

printPrimeFactors(60);
printPrimeFactors(97);

✍️ Your code

Type your code, then press "Run"

🧪 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).