Why this repeats the same random number?

Question

I'm new to Go and not sure why it prints the same number for rand.Intn(n int) int for every run:

package main

import (
    "fmt"
    "math/rand"
)


func main() {
    fmt.Println(rand.Intn(10)) 
}

The docs says :

Intn returns, as an int, a non-negative pseudo-random number in [0,n) from the default Source. It panics if n <= 0.

And how do I properly seed the random number generation?

Answer 1

By calling the rand.Seed() function, passing it a (random) seed (typically the current unix timestamp). Quoting from math/rand package doc:

Top-level functions, such as Float64 and Int, use a default shared Source that produces a deterministic sequence of values each time a program is run. Use the Seed function to initialize the default Source if different behavior is required for each run.

Example:

rand.Seed(time.Now().UnixNano())

If rand.Seed() is not called, the generator behaves as if seeded by 1:

Seed uses the provided seed value to initialize the default Source to a deterministic state. If Seed is not called, the generator behaves as if seeded by Seed(1).