math/rand Package

import "math/rand"

// Returns a random integer in [0, n).
n := rand.Intn(100)

// Returns a random float64 in [0.0, 1.0).
f := rand.Float64()

// Go 1.20+: Create a local random number generator.
r := rand.New(rand.NewSource(42))  // Fixed with seed 42.
n2 := r.Intn(10)

// Shuffle a slice randomly.
s := []int{1, 2, 3, 4, 5}
rand.Shuffle(len(s), func(i, j int) { s[i], s[j] = s[j], s[i] })

package main

import (
    "fmt"
    "math/rand"
)

func main() {
    // --- Global random number generator (auto-seeded since Go 1.20) ---
    fmt.Println("--- Random integers ---")
    for i := 0; i < 5; i++ {
        fmt.Printf("rand.Intn(100) = %d\n", rand.Intn(100))
    }

    fmt.Println()

    // Random floating-point numbers.
    fmt.Println("--- Random float64 ---")
    for i := 0; i < 3; i++ {
        fmt.Printf("rand.Float64() = %.4f\n", rand.Float64())
    }

    fmt.Println()

    // Random integers in a custom range (e.g., a die roll from 1 to 6).
    fmt.Println("--- Dice roll (1–6) ---")
    for i := 0; i < 5; i++ {
        dice := rand.Intn(6) + 1  // Intn(6) returns 0–5, so add 1.
        fmt.Printf("Dice: %d\n", dice)
    }

    fmt.Println()

    // Shuffling a slice.
    fmt.Println("--- Shuffle ---")
    s := []int{1, 2, 3, 4, 5}
    fmt.Println("Before shuffle:", s)
    rand.Shuffle(len(s), func(i, j int) {
        s[i], s[j] = s[j], s[i]
    })
    fmt.Println("After shuffle:", s)

    fmt.Println()

    // Generate reproducible random numbers with a fixed seed.
    fmt.Println("--- Fixed seed (reproducible) ---")
    r1 := rand.New(rand.NewSource(42))
    r2 := rand.New(rand.NewSource(42))
    fmt.Println("r1 numbers:", r1.Intn(100), r1.Intn(100), r1.Intn(100))
    fmt.Println("r2 numbers:", r2.Intn(100), r2.Intn(100), r2.Intn(100))
    // Same seed produces the same results.
}

Since Go 1.20, the global random number generator is automatically seeded randomly on each run. This means the old initialization code rand.Seed(time.Now().UnixNano()) is no longer needed. If you need reproducible results in tests, create a local generator with rand.New(rand.NewSource(seed)).

The random numbers generated by math/rand are not cryptographically secure. For security-sensitive values such as passwords or tokens, use the crypto/rand package instead.

For general math functions, see 'math package'.