Pick a random value from a Go Slice

Question

Situation:

I've a slice of values and need to pick up a randomly chosen value from it. Then I want to concatenate it with a fixed string. This is my code so far:

func main() {
//create the reasons slice and append reasons to it
reasons := make([]string, 0)
reasons = append(reasons,
    "Locked out",
    "Pipes broke",
    "Food poisoning",
    "Not feeling well")

message := fmt.Sprint("Gonna work from home...", pick a random reason )
}

Question:

Is there a built-in function, which can help me by doing the "pick a random reason" part?

Answer 1

Use function Intn from rand package to select a random index.

import (
  "math/rand"
  "time"
)

// ...

rand.Seed(time.Now().Unix()) // initialize global pseudo random generator
message := fmt.Sprint("Gonna work from home...", reasons[rand.Intn(len(reasons))])

Other solution is to use Rand object.

s := rand.NewSource(time.Now().Unix())
r := rand.New(s) // initialize local pseudorandom generator 
r.Intn(len(reasons))

Answer 2

Just pick a random integer mod slice length:

rand.Seed(time.Now().Unix())
reasons := []string{
    "Locked out",
    "Pipes broke",
    "Food poisoning",
    "Not feeling well",
}
n := rand.Int() % len(reasons)
fmt.Print("Gonna work from home...", reasons[n])

Playground: http://play.golang.org/p/fEHElLJrEZ. (Note the commend about rand.Seed.)

Answer 3

Since this still shows up in Google's top results for Golang random string generation, I wanted to share what I have been working with.

Here is the solution I am using:

package main

import (
  "fmt"
  "strings"
  "time"
)

var (
  opts  = strings.Split("option1,option2,option3", ",")
  start = time.Now()
)

func main() {
  fmt.Println(getRandomOpt(), time.Since(start))
  fmt.Println(getRandomOpt(), time.Since(start))
  fmt.Println(getRandomOpt(), time.Since(start))
  fmt.Println(getRandomOpt(), time.Since(start))
  fmt.Println(getRandomOpt(), time.Since(start))
  fmt.Println(getRandomOpt(), time.Since(start))
  fmt.Println(getRandomOpt(), time.Since(start))
  fmt.Println(getRandomOpt(), time.Since(start))
  fmt.Println(getRandomOpt(), time.Since(start))
}

func getRandomOpt() string {
  len := len(opts)
  n := uint32(0)
  if len > 0 {
    n = getRandomUint32() % uint32(len)
  }
  return opts[n]
}

func getRandomUint32() uint32 {
  x := time.Now().UnixNano()
  return uint32((x >> 32) ^ x)
}

And results:

option2 665ns
option1 41.406µs
option1 44.817µs
option3 47.329µs
option1 49.725µs
option3 52µs
option2 54.393µs
option2 56.798µs
option1 59.098µs

Source wise, I copied getRandomUint32() from fastrand: https://github.com/valyala/fastrand

And the solution proposed above. Performance isn't all that different, but I wanted to share results.

package main

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

var (
  opts  = strings.Split("option1,option2,option3", ",")
  start = time.Now()
)

func main() {
  rand.Seed(start.Unix())
  fmt.Println(getRandomOpt(), time.Since(start))
  fmt.Println(getRandomOpt(), time.Since(start))
  fmt.Println(getRandomOpt(), time.Since(start))
  fmt.Println(getRandomOpt(), time.Since(start))
  fmt.Println(getRandomOpt(), time.Since(start))
  fmt.Println(getRandomOpt(), time.Since(start))
  fmt.Println(getRandomOpt(), time.Since(start))
  fmt.Println(getRandomOpt(), time.Since(start))
  fmt.Println(getRandomOpt(), time.Since(start))
}

func getRandomOpt() string {
  return opts[rand.Intn(len(opts))]
}

And results:

option3 11.865µs
option2 48.415µs
option3 52.809µs
option1 55.536µs
option3 58.191µs
option3 60.793µs
option1 63.391µs
option2 65.982µs
option2 68.601µs

These results were only run a few times locally and I grabbed what appeared to be the median result. There is certainly more work to be done in terms of iterations and whatnot, but I just wanted to share what I have.