Checking Whether a Number Is Prime

In this lesson you will learn how to test whether a number is prime in Go, covering a basic mathematical algorithm built on a loop. This is for anyone searching for "Go prime number check implementation".

To decide whether a number is prime (divisible only by 1 and itself), check whether it divides evenly by anything from 2 up to its square root. If nothing divides it, it is prime.

The sample code loops over the range for i := 2; float64(i) <= math.Sqrt(float64(n)); i++ and declares the number non-prime the moment it finds an i where n%i == 0. Stopping at the square root works because any larger divisor is always paired with a smaller one.

A common stumbling block for beginners is understanding why the square root is enough. Checking every number below n also gives the right answer, but narrowing the range cuts out wasted work and speeds things up.

This is an important mathematical property that also underpins cryptography, and there is a more advanced technique, the Sieve of Eratosthenes, for finding many primes efficiently at once.

📖 Reference code

package main

import (
	"fmt"
	"math"
)

func isPrime(n int) bool {
	if n < 2 {
		return false
	}
	for i := 2; float64(i) <= math.Sqrt(float64(n)); i++ {
		if n%i == 0 {
			return false
		}
	}
	return true
}

func main() {
	fmt.Println(isPrime(17))
	fmt.Println(isPrime(18))
}

✍️ Your code

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🧪 This site can't compile or run Go 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).