til/go/not-so-random.md

Not So Random

Go's rand package makes it easy to generate all sorts of pseudo-random numbers.

What they don't tell you though is that the default seed is 1. They do tell you that the numbers are pseud-random and that you need to use the Seed function to initialize the default source if different behavior is required for each run, though. So if you write a program like so:

package main

import "fmt"
import "math/rand"

func main() {
    stuff := []string{
        "one",
        "two",
        "three",
        "four",
    }
    fmt.Println(stuff[rand.Intn(len(stuff))])
}

and then run it, you will get output like:

three

and any subsequent runs of the program will continue to produce three. Not exactly what we are looking for.

If you want your program to be a little less predictable, you will want to seed it yourself, perhaps with the current time, instead of 1. Try adding the following to the beginning of the main function:

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

You'll also want to import the time package.

Things should appear to be a bit more random now.

source: Jake Worth and Stackoverflow