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]         


        

Answer 1

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).