Generating Random Numbers in Go

Question

I am trying to generate random numbers (integers) in Go, to no avail. I found the rand package in crypto/rand, which seems to be what I want, but I can't tell from the documentation how to use it. This is what I'm trying right now:

    b := []byte{}
    something, err := rand.Read(b)
    fmt.Printf("something = %v\n", something)
    fmt.Printf("err = %v\n", err)

But unfortunately this always outputs:

    something = 0
    err = <nil>

Is there a way to fix this so that it actually generates random numbers? Alternatively, is there a way to set the upper bound on the random numbers this generates?

Answer 1

As of 1 april 2012, after the release of the stable version of the lang, you can do the following:

package main


import "fmt"
import "time"
import "math/rand"

func main() {
  rand.Seed(time.Now().UnixNano()) // takes the current time in nanoseconds as the seed
  fmt.Println(rand.Intn(100)) // this gives you an int up to but not including 100
}

Answer 2

Depending on your use case, another option is the math/rand package. Don't do this if you're generating numbers that need to be completely unpredictable. It can be helpful if you need to get results that are reproducible, though -- just pass in the same seed you passed in the first time.

Here's the classic "seed the generator with the current time and generate a number" program:

package main

import (
    "fmt"
    "math/rand"
    "time"
)

func main() {
    rand.Seed(time.Now().Unix())
    fmt.Println(rand.Int())
}

Answer 3

crypto/rand provides only binary stream of random data, but you can read integers from it using encoding/binary:

package main

import "encoding/binary"
import "crypto/rand"

func main() {
    var n int32
    binary.Read(rand.Reader, binary.LittleEndian, &n)
    println(n)
}