Generating random numbers over a range in Go

Question

All the integer functions in math/rand generate non-negative numbers.

rand.Int() int              // [0, MaxInt]
rand.Int31() int32          // [0, MaxInt32]
rand.Int31n(n int32) int32  // [0, n)
rand.Int63() int64          // [0, MaxInt64]
rand.Int63n(n int64) int64  // [0, n)
rand.Intn(n int) int        // [0, n)

I would like to generate random numbers in the range [-m, n). In other words, I would like to generate a mix of positive and negative numbers.

Your specification of range may be obvious to you and may be mathematically unambiguous, but most people are heavily attracted by symmetry, so many will think of the interval `[min,max]`. Well, maybe a matter of taste... — Wolf, Dec 22 '16 at 12:33
In other words: Your *`mix of positive and negative numbers`* will be unbalanced. — Wolf, Dec 22 '16 at 12:37

ryboe · Accepted Answer · 2022-10-26T04:42:32.563

96

I found this example at Go Cookbook, which is equivalent to rand.Range(min, max int) (if that function existed):

rand.Intn(max - min) + min

Don't forget to seed the PRNG before calling any rand function.

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

edited Oct 26 '22 at 04:42

answered May 10 '14 at 04:26

ryboe

2,435
1
20
21

1

`rand.Intn(2 - 1) + 1` will always return 1 – Timo Huovinen Aug 30 '20 at 18:07
12

The question asks for random numbers in the range [min, max). The only number in range [1, 2) is 1. – ryboe Sep 01 '20 at 02:33
@TimoHuovinen range [min, max) – zimmer Sep 10 '20 at 08:26
This is what I was missing https://stackoverflow.com/questions/4396290/what-does-this-square-bracket-and-parenthesis-bracket-notation-mean-first1-last#:~:text=A%20bracket%20means%20that%20end,but%20ends%20just%20before%20last1%20. – Timo Huovinen Sep 10 '20 at 16:11
1

import "math/rand" – Ryan Loggerythm Jun 17 '22 at 21:07

score 21 · Answer 2 · answered Feb 05 '19 at 14:27

21

This will generate random numbers within given range [a, b]

rand.Seed(time.Now().UnixNano())
n := a + rand.Intn(b-a+1)

source

answered Feb 05 '19 at 14:27

Asalle

1,239
19
41

1

This is the simplest answer that actually gives you the maximum value. – Andy Foster Mar 07 '19 at 14:05
For clarity a = min and b = max. Maybe obvious to some but it tripped me out – Carlo Nyte Jul 08 '22 at 15:14

Wolf · Answer 3 · 2020-05-03T17:56:16.950

10

As to prevent repeating min and max over and over again, I suggest to switch range and random in thinking about it. This is what I found to work as expected:

package main

import (
    "fmt"
    "math/rand"
)

// range specification, note that min <= max
type IntRange struct {
    min, max int
}

// get next random value within the interval including min and max
func (ir *IntRange) NextRandom(r* rand.Rand) int {
    return r.Intn(ir.max - ir.min +1) + ir.min
}

func main() {
    r := rand.New(rand.NewSource(55))
    ir := IntRange{-1,1}
    for i := 0; i<10; i++ {
        fmt.Println(ir.NextRandom(r))
    }
}

See on Go Playground

Specifying the range

The solution you found in the Cookbook misses to exactly specify how min and max work, but of course it meets your specification ([-min, max)). I decided to specify the range as a closed interval ([-min, max], that means its borders are included in the valid range). Compared to my understanding of the Cookbook description:

gives you that random number within any two positive numbers that you specify (in this case, 1 and 6).

_{(which can be found below the code snippet in the Golang Cookbook)}

the Cookbook implementation is off by one (which of course brings it in good company with lots of programs that are helpful at first glance).

edited May 03 '20 at 17:56

answered Dec 22 '16 at 12:24

Wolf

9,679
7
62
108

2

Good catch, @Wolf--I noticed the same issue with the Cookbook example. (This should have more votes, but the upvoters appear to have left SO a long time ago.) – SexxLuthor May 11 '17 at 06:14
...well, you are still here ;) I think that my variation should be understood as a side note because the OP explicitly referred to `the range [-m, n)` which is the understanding of ranges that is most common to programmers, mostly not to users... BTW: isolating the interval *may* be interesting, sometimes helpful, but definitely challenging for good naming: Today, I'm a little bit confused about `IntRange`s that produce `NextRandom` values. – Wolf May 11 '17 at 14:29
it returns always the same results https://play.golang.org/p/pXZvtcmxjOq – rok Jul 29 '21 at 05:15
1

@rok The implementation of the go playground fakes the time, see [Inside the Go Playground - The Go Blog](https://blog.golang.org/playground) for the details, so I decided to go that way instead of [*failing on fake randomization*](https://stackoverflow.com/q/48360260/2932052) – Wolf Aug 09 '21 at 13:06

score 2 · Answer 4 · answered Jul 09 '17 at 18:26

2

A small utility I wrote for generating random slices(very much like python range)

Code - https://github.com/alok87/goutils/blob/master/pkg/random/random.go

import "github.com/alok87/goutils/pkg/random"
random.RangeInt(2, 100, 5)

[3, 10, 30, 56, 67]

answered Jul 09 '17 at 18:26

Alok Kumar Singh

2,331
3
18
37

1

it's always returning the same numbers https://play.golang.org/p/oO6nBlQdWTX – rok Jul 29 '21 at 05:10

score 2 · Answer 5 · answered Mar 09 '20 at 08:52

2

Solution that worked for me is: j = rand.Intn(600) - 100 where m is 100 and n is 500, it will generate numbers from -100 to 499.

answered Mar 09 '20 at 08:52

score 1 · Answer 6 · answered Dec 19 '22 at 20:11

1

This worked for me (Generate random number between 1 and 10)...

import "math/rand"

rand.Seed(time.Now().UnixNano())
randId := rand.Intn(10-1) + 1

answered Dec 19 '22 at 20:11

MD Golam Rakib

66
4

score 0 · Answer 7 · answered Sep 03 '23 at 08:23

I solved this problem using for cycle. For example, you need to generate a number between 50 and 100:

val := rand.Intn(100) + 1    // determined the upper bound of the cycle
for val < 50 {
val = rand.Intn(100) + 1}   //   repeat the generation if the number is less than 50

Generating random numbers over a range in Go

7 Answers7

Specifying the range