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.

Answer 1

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())

Answer 2

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

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

source

Answer 3

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).

Answer 4

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]

Answer 5

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.

Answer 6

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