golang rand.Int(). Why every time same values?

Question

I have a code:

package main

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

func main() {
    fmt.Println(strconv.Itoa(rand.Int()))
    fmt.Println(strconv.Itoa(rand.Int()))
    fmt.Println(strconv.Itoa(rand.Int()))
    fmt.Println(strconv.Itoa(rand.Int()))
    fmt.Println(strconv.Itoa(rand.Int()))
    fmt.Println(strconv.Itoa(rand.Int()))
    fmt.Println(strconv.Itoa(rand.Int()))
    fmt.Println(strconv.Itoa(rand.Int()))
    fmt.Println(strconv.Itoa(rand.Int()))
    fmt.Println(strconv.Itoa(rand.Int()))
    fmt.Println(strconv.Itoa(rand.Int()))
    fmt.Println(strconv.Itoa(rand.Int()))
    fmt.Println(strconv.Itoa(rand.Int()))
    fmt.Println(strconv.Itoa(rand.Int()))
}

When I run it (go run code.go), I get every time same values:

5577006791947779410
8674665223082153551
6129484611666145821
4037200794235010051
3916589616287113937
6334824724549167320
605394647632969758
1443635317331776148
894385949183117216
2775422040480279449
4751997750760398084
7504504064263669287
1976235410884491574
3510942875414458836

Second try:

package main

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

func main() {
    fmt.Println(strconv.Itoa(rand.Intn(100)))
    fmt.Println(strconv.Itoa(rand.Intn(100)))
    fmt.Println(strconv.Itoa(rand.Intn(100)))
    fmt.Println(strconv.Itoa(rand.Intn(100)))
    fmt.Println(strconv.Itoa(rand.Intn(100)))
    fmt.Println(strconv.Itoa(rand.Intn(100)))
    fmt.Println(strconv.Itoa(rand.Intn(100)))
}

Same behaviour. Every time's

81
87
47
59
81
18
25

Is this a joke? Why it happens? Here is no description about non-random same result. I can see only pseudo-random term without explanation what that means.

^{Looks like even bash is more logical and stable...}

This is a C way

Answer 1

You need to seed it. It says right in the docs

Random numbers are generated by a Source. Top-level functions, such as Float64 and Int, use a default shared Source that produces a deterministic sequence of values each time a program is run. Use the Seed function to initialize the default Source if different behavior is required for each run.

Generally

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

Answer 2

You don't appear to have called Seed for math/rand before using the generator.

If Seed is not called, the generator behaves as if seeded by Seed(1). That is not a joke - actually for a PRNG to be deterministic and repeatable is desirable in many cases.

For different numbers, seed with a different value, such as time.Now().UnixNano().

Answer 3

Addition to the other answers, good explanation found about this in medium article go-how-are-random-numbers-generated

Go implements two packages to generate random numbers:

• a pseudo-random number generator (PRNG) in the package math/rand cryptographic
• pseudorandom number generator (CPRNG), implemented in crypto/rand

If both generate random numbers, your choice will be based on a tradeoff between genuinely random numbers and performance.

As other answers explain, math/rand packages populate random numbers reading from a source. So if you need random numbers, you need to set random seed calling rand.Seed()

In other case you can use crypto/rand package to generate random numbers. It generates random numbers which can not be deterministic. But performance is bit lower than math/rand package.

I have added sample code below for that. you can run and see different out put here.

package main

import (
    "crypto/rand"
    "math/big"
)

func main() {
    for i := 0; i < 4; i++  {
        n, _ := rand.Int(rand.Reader, big.NewInt(100))
        println(n.Int64())
    }
}