GO: Check if a number is prime

Checking if a number is prime in the fastest and most efficient way is a trivial task for every programming language. Go offers math package with built in function big to perform task in a basic but still efficient fashion.

Ints

For integer types, use ProbablyPrime(0) from package math/big. This primality test is 100% accurate for inputs less than 264.

Code example:

const n = 1212121
if big.NewInt(n).ProbablyPrime(0) {
    fmt.Println(n, "is prime")
} else {
    fmt.Println(n, "is not prime")
}

Larger numbers

For larger numbers, you need to provide the desired number of tests to ProbablyPrime(n). For n tests, the probability of returning true for a randomly chosen non-prime is at most (1/4)n. A common choice is to use n = 20; this gives a false positive rate 0.000,000,000,001.

z := new(big.Int)
fmt.Sscan("20828378347937489020982309820938029380947893749", z)
if z.ProbablyPrime(20) {
    fmt.Println(z, "is probably prime")
} else {
    fmt.Println(z, "is not prime")
}

Disclaimer: this tutorial doesn’t take into account algorithm efficiency optimization

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