🔢 Generate a random number in Go

April 22, 2021 introduction random

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To generate a random number in Go (actually, it’s pseudo-random, but in this article, we will refer to it as random), you just need to use the math/rand package.

Examples

Generate random integer number (also in a given range)

package main

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

func randInt(min, max int) int {
    return min + rand.Intn(max-min)
}

func main() {
    rand.Seed(time.Now().UnixNano())
    fmt.Printf("random integer: %d\n", rand.Int())
    fmt.Printf("random integer in range [5, 10): %d\n", randInt(5, 10))
}

Generate random float64 number (also in a given range)

package main

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

func randFloat(min, max float64) float64 {
    return min + rand.Float64()*(max-min)
}

func main() {
    rand.Seed(time.Now().UnixNano())
    fmt.Printf("random float64: %f\n", rand.Float64())
    fmt.Printf("random float64 in range [2.4, 3.2): %f\n", randFloat(2.4, 3.2))
}

How it works

Seed
```
rand.Seed(time.Now().UnixNano())
```
In the first line of the main function, we set seed to initialize a pseudorandom number generator. The default math/rand number generator is deterministic, so it will give the same output sequence for the same seed value. You can check this by removing the first line of the main function and running the program a couple of times - we always get the same “random” numbers. It is because the algorithm produces new values by performing some operations on the previous value, and when the initial value (the seed value) stays the same, we get the same output numbers. To avoid this, we use current time - time.Now().UnixNano() as the seed.
Random number generating function
```
fmt.Printf("random integer: %d\n", rand.Int())
fmt.Printf("random float64: %f\n", rand.Float64())
```
After the seed initialization, we can generate a random number using one of the math/rand package functions, for example:
See math/rand package documentation for more information.
Random number in a range
```
func randInt(min, max int) int {
    return min + rand.Intn(max-min)
}

func randFloat(min, max float64) float64 {
    return min + rand.Float64()*(max-min)
}
```
There is no function in the math/rand package that generates random numbers in the [min, max) range, so we have to shift the results from the function rand.Intn() or rand.Float64() to achieve a similar result. The rand.Intn() can generate a value in the range [0, n), so to get a value in the range [min, max) we need to generate a value in the range [0, max-min) and add min to the result.

The rand.Float64() produces number in [0.0, 1.0). To get number in [min, max) range multiply the result by max-min and add min.