math/rand: Perm not working

[liamg]

Please answer these questions before submitting your issue. Thanks!

What version of Go are you using (go version)?

go version go1.8.3 linux/amd64

What operating system and processor architecture are you using (go env)?

GOARCH="amd64"
GOBIN=""
GOEXE=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOOS="linux"
GOPATH="/home/lgalvin/go"
GORACE=""
GOROOT="/usr/lib/go"
GOTOOLDIR="/usr/lib/go/pkg/tool/linux_amd64"
GCCGO="gccgo"
CC="gcc"
GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build653200321=/tmp/go-build -gno-record-gcc-switches"
CXX="g++"
CGO_ENABLED="1"
PKG_CONFIG="pkg-config"
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"

What did you do?

Tried to use Perm to retrieve random values.

package main

import (
	"fmt"
	"math/rand"
)

func main() {
	for _, value := range rand.Perm(3) {
		fmt.Println(value)
	}

	// Unordered output: 1
	// 2
	// 0
}

always returns:

0
1
2

The values are not randomised.

Run it here: https://play.golang.org/p/gOAw978ZNI

What did you expect to see?

Randomised values.

What did you see instead?

Same order each time:

0
1
2

[ALTree]

Same order each time

This is expected: Perm is pseudorandom, the output is deterministic if you perform the same call with the same randomness source. If you want the permutation to change you'll have to call Perm from a custom source using func (r *Rand) Perm(n int) []int.

Closing this, since it's not a bug.