Checking Whether a Number Is Prime

This lesson covers checking whether a number is prime in C, so you understand a basic math algorithm using a loop. It's for anyone searching "C prime check implementation."

Whether a number is prime (divisible only by 1 and itself) can be determined by checking whether it's divisible by any number from 2 up to its square root. If none of them divide it evenly, it's prime.

The example loops over the range for (int i = 2; i * i <= n; i++), deciding the number isn't prime the moment it finds an i where n % i == 0. Notice using the stdbool.h header to work with a bool return value.

A common early mistake is not knowing older C standards have no bool type. Including stdbool.h lets you use true/false, available from C99 onward.

It's an important mathematical property that underlies cryptography too, and there's also a more advanced technique, the Sieve of Eratosthenes, for efficiently finding many primes at once.

📖 Reference code

#include <stdio.h>
#include <stdbool.h>

bool isPrime(int n) {
    if (n < 2) return false;
    for (int i = 2; i * i <= n; i++) {
        if (n % i == 0) return false;
    }
    return true;
}

int main() {
    printf("%d\n", isPrime(17));
    printf("%d\n", isPrime(18));
    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).