src/pkg/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"
	"text/tabwriter"
)

// This test serves as an example but also makes sure we don't change
// the output of the random number generator when given a fixed seed.

// 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() {
	// 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 interface{}) {
		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.Int63(), r.Int63(), r.Int63())

	// 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      5927547564735367388 637072299495207830  4128311955958246186
	// 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]
}