为什么每次重新启动程序时math / rand都返回相同的结果？

Question

I'm a little confused. I have a random string generator, here's the code:

package utils

import (
  "fmt"
  "math/rand"
)

var chars = []rune("abcdefghijklmnopqrstuvwxyz0123456789")

func RandSeq(n int) string {
  b := make([]rune, n)
  for i := range b {
    b[i] = chars[rand.Intn(len(chars))]
  }
  fmt.Println(string(b))
  return string(b)
}

Every time I run this, it will generate a new random string, but if I restart the server the results will repeat themselves. Here are some results:

go run main.go
fpllngzieyoh43e0133ols6k1hh2gdny
xxvi7hvszwk1b182tvjzjpezi4hx9gvm
kir0xcta0opsb5qipjzb3h3x9kcegta5
m1zcv5drxckn42gb50anxndsckjdwgfw
5japz01zicapy9eqixuc9uehq235v48c
51wgg1gypq4s9miwn1dxkjqd614m58f0
fyy29g6ujmxbouxshy2plmkmhlnmdbfh
f7kq8u26873eql4yyp7fyilbb72nrtlc

go run main.go
fpllngzieyoh43e0133ols6k1hh2gdny
xxvi7hvszwk1b182tvjzjpezi4hx9gvm
kir0xcta0opsb5qipjzb3h3x9kcegta5
m1zcv5drxckn42gb50anxndsckjdwgfw
5japz01zicapy9eqixuc9uehq235v48c

Any idea why this is happening?

Answer 1

You need to Seed the random number generator prior to generating any random numbers. A good value to use is the current unix timestamp:

import (
    "math/rand"
    "time"
)

func main() {
    rand.Seed(time.Now().UnixNano())
}

Note that you should only seed the RNG once at startup (or after forking), not before each random number generation.