example_test.go

     1  // Copyright 2012 The Go Authors. All rights reserved.
     2  // Use of this source code is governed by a BSD-style
     3  // license that can be found in the LICENSE file.
     4  
     5  package rand_test
     6  
     7  import (
     8  	"fmt"
     9  	"math/rand"
    10  	"os"
    11  	"strings"
    12  	"text/tabwriter"
    13  )
    14  
    15  // These tests serve as an example but also make sure we don't change
    16  // the output of the random number generator when given a fixed seed.
    17  
    18  func Example() {
    19  	// Seeding with the same value results in the same random sequence each run.
    20  	// For different numbers, seed with a different value, such as
    21  	// time.Now().UnixNano(), which yields a constantly-changing number.
    22  	rand.Seed(42)
    23  	answers := []string{
    24  		"It is certain",
    25  		"It is decidedly so",
    26  		"Without a doubt",
    27  		"Yes definitely",
    28  		"You may rely on it",
    29  		"As I see it yes",
    30  		"Most likely",
    31  		"Outlook good",
    32  		"Yes",
    33  		"Signs point to yes",
    34  		"Reply hazy try again",
    35  		"Ask again later",
    36  		"Better not tell you now",
    37  		"Cannot predict now",
    38  		"Concentrate and ask again",
    39  		"Don't count on it",
    40  		"My reply is no",
    41  		"My sources say no",
    42  		"Outlook not so good",
    43  		"Very doubtful",
    44  	}
    45  	fmt.Println("Magic 8-Ball says:", answers[rand.Intn(len(answers))])
    46  	// Output: Magic 8-Ball says: As I see it yes
    47  }
    48  
    49  // This example shows the use of each of the methods on a *Rand.
    50  // The use of the global functions is the same, without the receiver.
    51  func Example_rand() {
    52  	// Create and seed the generator.
    53  	// Typically a non-fixed seed should be used, such as time.Now().UnixNano().
    54  	// Using a fixed seed will produce the same output on every run.
    55  	r := rand.New(rand.NewSource(99))
    56  
    57  	// The tabwriter here helps us generate aligned output.
    58  	w := tabwriter.NewWriter(os.Stdout, 1, 1, 1, ' ', 0)
    59  	defer w.Flush()
    60  	show := func(name string, v1, v2, v3 any) {
    61  		fmt.Fprintf(w, "%s\t%v\t%v\t%v\n", name, v1, v2, v3)
    62  	}
    63  
    64  	// Float32 and Float64 values are in [0, 1).
    65  	show("Float32", r.Float32(), r.Float32(), r.Float32())
    66  	show("Float64", r.Float64(), r.Float64(), r.Float64())
    67  
    68  	// ExpFloat64 values have an average of 1 but decay exponentially.
    69  	show("ExpFloat64", r.ExpFloat64(), r.ExpFloat64(), r.ExpFloat64())
    70  
    71  	// NormFloat64 values have an average of 0 and a standard deviation of 1.
    72  	show("NormFloat64", r.NormFloat64(), r.NormFloat64(), r.NormFloat64())
    73  
    74  	// Int31, Int63, and Uint32 generate values of the given width.
    75  	// The Int method (not shown) is like either Int31 or Int63
    76  	// depending on the size of 'int'.
    77  	show("Int31", r.Int31(), r.Int31(), r.Int31())
    78  	show("Int63", r.Int63(), r.Int63(), r.Int63())
    79  	show("Uint32", r.Uint32(), r.Uint32(), r.Uint32())
    80  
    81  	// Intn, Int31n, and Int63n limit their output to be < n.
    82  	// They do so more carefully than using r.Int()%n.
    83  	show("Intn(10)", r.Intn(10), r.Intn(10), r.Intn(10))
    84  	show("Int31n(10)", r.Int31n(10), r.Int31n(10), r.Int31n(10))
    85  	show("Int63n(10)", r.Int63n(10), r.Int63n(10), r.Int63n(10))
    86  
    87  	// Perm generates a random permutation of the numbers [0, n).
    88  	show("Perm", r.Perm(5), r.Perm(5), r.Perm(5))
    89  	// Output:
    90  	// Float32     0.2635776           0.6358173           0.6718283
    91  	// Float64     0.628605430454327   0.4504798828572669  0.9562755949377957
    92  	// ExpFloat64  0.3362240648200941  1.4256072328483647  0.24354758816173044
    93  	// NormFloat64 0.17233959114940064 1.577014951434847   0.04259129641113857
    94  	// Int31       1501292890          1486668269          182840835
    95  	// Int63       3546343826724305832 5724354148158589552 5239846799706671610
    96  	// Uint32      2760229429          296659907           1922395059
    97  	// Intn(10)    1                   2                   5
    98  	// Int31n(10)  4                   7                   8
    99  	// Int63n(10)  7                   6                   3
   100  	// Perm        [1 4 2 3 0]         [4 2 1 3 0]         [1 2 4 0 3]
   101  }
   102  
   103  func ExamplePerm() {
   104  	for _, value := range rand.Perm(3) {
   105  		fmt.Println(value)
   106  	}
   107  
   108  	// Unordered output: 1
   109  	// 2
   110  	// 0
   111  }
   112  
   113  func ExampleShuffle() {
   114  	words := strings.Fields("ink runs from the corners of my mouth")
   115  	rand.Shuffle(len(words), func(i, j int) {
   116  		words[i], words[j] = words[j], words[i]
   117  	})
   118  	fmt.Println(words)
   119  
   120  	// Output:
   121  	// [mouth my the of runs corners from ink]
   122  }
   123  
   124  func ExampleShuffle_slicesInUnison() {
   125  	numbers := []byte("12345")
   126  	letters := []byte("ABCDE")
   127  	// Shuffle numbers, swapping corresponding entries in letters at the same time.
   128  	rand.Shuffle(len(numbers), func(i, j int) {
   129  		numbers[i], numbers[j] = numbers[j], numbers[i]
   130  		letters[i], letters[j] = letters[j], letters[i]
   131  	})
   132  	for i := range numbers {
   133  		fmt.Printf("%c: %c\n", letters[i], numbers[i])
   134  	}
   135  
   136  	// Output:
   137  	// C: 3
   138  	// D: 4
   139  	// A: 1
   140  	// E: 5
   141  	// B: 2
   142  }
   143  
   144  func ExampleIntn() {
   145  	// Seeding with the same value results in the same random sequence each run.
   146  	// For different numbers, seed with a different value, such as
   147  	// time.Now().UnixNano(), which yields a constantly-changing number.
   148  	rand.Seed(86)
   149  	fmt.Println(rand.Intn(100))
   150  	fmt.Println(rand.Intn(100))
   151  	fmt.Println(rand.Intn(100))
   152  
   153  	// Output:
   154  	// 42
   155  	// 76
   156  	// 30
   157  }
   158
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