src/math/rand/example_test.go

// Copyright 2012 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

package rand_test

import (
	"fmt"
	"math/rand"
	"os"
	"strings"
	"text/tabwriter"
)

// These tests serve as an example but also make sure we don't change
// the output of the random number generator when given a fixed seed.

func Example() {
	// Seeding with the same value results in the same random sequence each run.
	// For different numbers, seed with a different value, such as
	// time.Now().UnixNano(), which yields a constantly-changing number.
	rand.Seed(42)
	answers := []string{
		"It is certain",
		"It is decidedly so",
		"Without a doubt",
		"Yes definitely",
		"You may rely on it",
		"As I see it yes",
		"Most likely",
		"Outlook good",
		"Yes",
		"Signs point to yes",
		"Reply hazy try again",
		"Ask again later",
		"Better not tell you now",
		"Cannot predict now",
		"Concentrate and ask again",
		"Don't count on it",
		"My reply is no",
		"My sources say no",
		"Outlook not so good",
		"Very doubtful",
	}
	fmt.Println("Magic 8-Ball says:", answers[rand.Intn(len(answers))])
	// Output: Magic 8-Ball says: As I see it yes
}

// This example shows the use of each of the methods on a *Rand.
// The use of the global functions is the same, without the receiver.
func Example_rand() {
	// Create and seed the generator.
	// Typically a non-fixed seed should be used, such as time.Now().UnixNano().
	// Using a fixed seed will produce the same output on every run.
	r := rand.New(rand.NewSource(99))

	// The tabwriter here helps us generate aligned output.
	w := tabwriter.NewWriter(os.Stdout, 1, 1, 1, ' ', 0)
	defer w.Flush()
	show := func(name string, v1, v2, v3 any) {
		fmt.Fprintf(w, "%s\t%v\t%v\t%v\n", name, v1, v2, v3)
	}

	// Float32 and Float64 values are in [0, 1).
	show("Float32", r.Float32(), r.Float32(), r.Float32())
	show("Float64", r.Float64(), r.Float64(), r.Float64())

	// ExpFloat64 values have an average of 1 but decay exponentially.
	show("ExpFloat64", r.ExpFloat64(), r.ExpFloat64(), r.ExpFloat64())

	// NormFloat64 values have an average of 0 and a standard deviation of 1.
	show("NormFloat64", r.NormFloat64(), r.NormFloat64(), r.NormFloat64())

	// Int31, Int63, and Uint32 generate values of the given width.
	// The Int method (not shown) is like either Int31 or Int63
	// depending on the size of 'int'.
	show("Int31", r.Int31(), r.Int31(), r.Int31())
	show("Int63", r.Int63(), r.Int63(), r.Int63())
	show("Uint32", r.Uint32(), r.Uint32(), r.Uint32())

	// Intn, Int31n, and Int63n limit their output to be < n.
	// They do so more carefully than using r.Int()%n.
	show("Intn(10)", r.Intn(10), r.Intn(10), r.Intn(10))
	show("Int31n(10)", r.Int31n(10), r.Int31n(10), r.Int31n(10))
	show("Int63n(10)", r.Int63n(10), r.Int63n(10), r.Int63n(10))

	// Perm generates a random permutation of the numbers [0, n).
	show("Perm", r.Perm(5), r.Perm(5), r.Perm(5))
	// Output:
	// Float32     0.2635776           0.6358173           0.6718283
	// Float64     0.628605430454327   0.4504798828572669  0.9562755949377957
	// ExpFloat64  0.3362240648200941  1.4256072328483647  0.24354758816173044
	// NormFloat64 0.17233959114940064 1.577014951434847   0.04259129641113857
	// Int31       1501292890          1486668269          182840835
	// Int63       3546343826724305832 5724354148158589552 5239846799706671610
	// Uint32      2760229429          296659907           1922395059
	// Intn(10)    1                   2                   5
	// Int31n(10)  4                   7                   8
	// Int63n(10)  7                   6                   3
	// Perm        [1 4 2 3 0]         [4 2 1 3 0]         [1 2 4 0 3]
}

func ExamplePerm() {
	for _, value := range rand.Perm(3) {
		fmt.Println(value)
	}

	// Unordered output: 1
	// 2
	// 0
}

func ExampleShuffle() {
	words := strings.Fields("ink runs from the corners of my mouth")
	rand.Shuffle(len(words), func(i, j int) {
		words[i], words[j] = words[j], words[i]
	})
	fmt.Println(words)

	// Output:
	// [mouth my the of runs corners from ink]
}

func ExampleShuffle_slicesInUnison() {
	numbers := []byte("12345")
	letters := []byte("ABCDE")
	// Shuffle numbers, swapping corresponding entries in letters at the same time.
	rand.Shuffle(len(numbers), func(i, j int) {
		numbers[i], numbers[j] = numbers[j], numbers[i]
		letters[i], letters[j] = letters[j], letters[i]
	})
	for i := range numbers {
		fmt.Printf("%c: %c\n", letters[i], numbers[i])
	}

	// Output:
	// C: 3
	// D: 4
	// A: 1
	// E: 5
	// B: 2
}

func ExampleIntn() {
	// Seeding with the same value results in the same random sequence each run.
	// For different numbers, seed with a different value, such as
	// time.Now().UnixNano(), which yields a constantly-changing number.
	rand.Seed(86)
	fmt.Println(rand.Intn(100))
	fmt.Println(rand.Intn(100))
	fmt.Println(rand.Intn(100))

	// Output:
	// 42
	// 76
	// 30
}