Package rand

import "math/rand"
Overview
Index
Examples
Documentation

Overview

Package rand implements pseudo-random number generators.

Random numbers are generated by a Source. Top-level functions, such as Float64 and Int, use a default shared Source that produces a deterministic sequence of values each time a program is run. Use the Seed function to initialize the default Source if different behavior is required for each run. The default Source is safe for concurrent use by multiple goroutines, but Sources created by NewSource are not.

For random numbers suitable for security-sensitive work, see the crypto/rand package.

Code:

rand.Seed(42) // Try changing this number!
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.

Code:

// 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.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        [0 1 4 2 3]         [0 4 3 1 2]         [1 2 3 0 4]

Index

func ExpFloat64() float64
func Float32() float32
func Float64() float64
func Int() int
func Int31() int32
func Int31n(n int32) int32
func Int63() int64
func Int63n(n int64) int64
func Intn(n int) int
func NormFloat64() float64
func Perm(n int) []int
func Read(p []byte) (n int, err error)
func Seed(seed int64)
func Shuffle(n int, swap func(i, j int))
func Uint32() uint32
func Uint64() uint64
type Rand
func New(src Source) *Rand
func (r *Rand) ExpFloat64() float64
func (r *Rand) Float32() float32
func (r *Rand) Float64() float64
func (r *Rand) Int() int
func (r *Rand) Int31() int32
func (r *Rand) Int31n(n int32) int32
func (r *Rand) Int63() int64
func (r *Rand) Int63n(n int64) int64
func (r *Rand) Intn(n int) int
func (r *Rand) NormFloat64() float64
func (r *Rand) Perm(n int) []int
func (r *Rand) Read(p []byte) (n int, err error)
func (r *Rand) Seed(seed int64)
func (r *Rand) Shuffle(n int, swap func(i, j int))
func (r *Rand) Uint32() uint32
func (r *Rand) Uint64() uint64
type Source
func NewSource(seed int64) Source
type Source64
type Zipf
func NewZipf(r *Rand, s float64, v float64, imax uint64) *Zipf
func (z *Zipf) Uint64() uint64

Examples

Package
Package (Rand)
Perm
Shuffle
Shuffle (SlicesInUnison)

Documentation

func ExpFloat64

func ExpFloat64() float64

ExpFloat64 returns an exponentially distributed float64 in the range (0, +math.MaxFloat64] with an exponential distribution whose rate parameter (lambda) is 1 and whose mean is 1/lambda (1) from the default Source. To produce a distribution with a different rate parameter, callers can adjust the output using:

sample = ExpFloat64() / desiredRateParameter

func Float32

func Float32() float32

Float32 returns, as a float32, a pseudo-random number in [0.0,1.0) from the default Source.

func Float64

func Float64() float64

Float64 returns, as a float64, a pseudo-random number in [0.0,1.0) from the default Source.

func Int

func Int() int

Int returns a non-negative pseudo-random int from the default Source.

func Int31

func Int31() int32

Int31 returns a non-negative pseudo-random 31-bit integer as an int32 from the default Source.

func Int31n

func Int31n(n int32) int32

Int31n returns, as an int32, a non-negative pseudo-random number in [0,n) from the default Source. It panics if n <= 0.

func Int63

func Int63() int64

Int63 returns a non-negative pseudo-random 63-bit integer as an int64 from the default Source.

func Int63n

func Int63n(n int64) int64

Int63n returns, as an int64, a non-negative pseudo-random number in [0,n) from the default Source. It panics if n <= 0.

func Intn

func Intn(n int) int

Intn returns, as an int, a non-negative pseudo-random number in [0,n) from the default Source. It panics if n <= 0.

func NormFloat64

func NormFloat64() float64

NormFloat64 returns a normally distributed float64 in the range [-math.MaxFloat64, +math.MaxFloat64] with standard normal distribution (mean = 0, stddev = 1) from the default Source. To produce a different normal distribution, callers can adjust the output using:

sample = NormFloat64() * desiredStdDev + desiredMean

func Perm

func Perm(n int) []int

Perm returns, as a slice of n ints, a pseudo-random permutation of the integers [0,n) from the default Source.

Code:

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

Unordered output:

1
2
0

func Read

func Read(p []byte) (n int, err error)

Read generates len(p) random bytes from the default Source and writes them into p. It always returns len(p) and a nil error. Read, unlike the Rand.Read method, is safe for concurrent use.

func Seed

func Seed(seed int64)

Seed uses the provided seed value to initialize the default Source to a deterministic state. If Seed is not called, the generator behaves as if seeded by Seed(1). Seed values that have the same remainder when divided by 2^31-1 generate the same pseudo-random sequence. Seed, unlike the Rand.Seed method, is safe for concurrent use.

func Shuffle

func Shuffle(n int, swap func(i, j int))

Shuffle pseudo-randomizes the order of elements using the default Source. n is the number of elements. Shuffle panics if n < 0. swap swaps the elements with indexes i and j.

Code:

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:

[my of the mouth corners from ink runs]

Code:

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
E: 5
B: 2
A: 1
D: 4

func Uint32

func Uint32() uint32

Uint32 returns a pseudo-random 32-bit value as a uint32 from the default Source.

func Uint64

func Uint64() uint64

Uint64 returns a pseudo-random 64-bit value as a uint64 from the default Source.

type Rand

type Rand struct {
    // contains filtered or unexported fields
}

A Rand is a source of random numbers.

func New

func New(src Source) *Rand

New returns a new Rand that uses random values from src to generate other random values.

func Rand.ExpFloat64

func (r *Rand) ExpFloat64() float64

ExpFloat64 returns an exponentially distributed float64 in the range (0, +math.MaxFloat64] with an exponential distribution whose rate parameter (lambda) is 1 and whose mean is 1/lambda (1). To produce a distribution with a different rate parameter, callers can adjust the output using:

sample = ExpFloat64() / desiredRateParameter

func Rand.Float32

func (r *Rand) Float32() float32

Float32 returns, as a float32, a pseudo-random number in [0.0,1.0).

func Rand.Float64

func (r *Rand) Float64() float64

Float64 returns, as a float64, a pseudo-random number in [0.0,1.0).

func Rand.Int

func (r *Rand) Int() int

Int returns a non-negative pseudo-random int.

func Rand.Int31

func (r *Rand) Int31() int32

Int31 returns a non-negative pseudo-random 31-bit integer as an int32.

func Rand.Int31n

func (r *Rand) Int31n(n int32) int32

Int31n returns, as an int32, a non-negative pseudo-random number in [0,n). It panics if n <= 0.

func Rand.Int63

func (r *Rand) Int63() int64

Int63 returns a non-negative pseudo-random 63-bit integer as an int64.

func Rand.Int63n

func (r *Rand) Int63n(n int64) int64

Int63n returns, as an int64, a non-negative pseudo-random number in [0,n). It panics if n <= 0.

func Rand.Intn

func (r *Rand) Intn(n int) int

Intn returns, as an int, a non-negative pseudo-random number in [0,n). It panics if n <= 0.

func Rand.NormFloat64

func (r *Rand) NormFloat64() float64

NormFloat64 returns a normally distributed float64 in the range [-math.MaxFloat64, +math.MaxFloat64] with standard normal distribution (mean = 0, stddev = 1). To produce a different normal distribution, callers can adjust the output using:

sample = NormFloat64() * desiredStdDev + desiredMean

func Rand.Perm

func (r *Rand) Perm(n int) []int

Perm returns, as a slice of n ints, a pseudo-random permutation of the integers [0,n).

func Rand.Read

func (r *Rand) Read(p []byte) (n int, err error)

Read generates len(p) random bytes and writes them into p. It always returns len(p) and a nil error. Read should not be called concurrently with any other Rand method.

func Rand.Seed

func (r *Rand) Seed(seed int64)

Seed uses the provided seed value to initialize the generator to a deterministic state. Seed should not be called concurrently with any other Rand method.

func Rand.Shuffle

func (r *Rand) Shuffle(n int, swap func(i, j int))

Shuffle pseudo-randomizes the order of elements. n is the number of elements. Shuffle panics if n < 0. swap swaps the elements with indexes i and j.

func Rand.Uint32

func (r *Rand) Uint32() uint32

Uint32 returns a pseudo-random 32-bit value as a uint32.

func Rand.Uint64

func (r *Rand) Uint64() uint64

Uint64 returns a pseudo-random 64-bit value as a uint64.

type Source

type Source interface {
    Int63() int64
    Seed(seed int64)
}

A Source represents a source of uniformly-distributed pseudo-random int64 values in the range [0, 1<<63).

func NewSource

func NewSource(seed int64) Source

NewSource returns a new pseudo-random Source seeded with the given value. Unlike the default Source used by top-level functions, this source is not safe for concurrent use by multiple goroutines.

type Source64

type Source64 interface {
    Source
    Uint64() uint64
}

A Source64 is a Source that can also generate uniformly-distributed pseudo-random uint64 values in the range [0, 1<<64) directly. If a Rand r's underlying Source s implements Source64, then r.Uint64 returns the result of one call to s.Uint64 instead of making two calls to s.Int63.

type Zipf

type Zipf struct {
    // contains filtered or unexported fields
}

A Zipf generates Zipf distributed variates.

func NewZipf

func NewZipf(r *Rand, s float64, v float64, imax uint64) *Zipf

NewZipf returns a Zipf variate generator. The generator generates values k ∈ [0, imax] such that P(k) is proportional to (v + k) ** (-s). Requirements: s > 1 and v >= 1.

func Zipf.Uint64

func (z *Zipf) Uint64() uint64

Uint64 returns a value drawn from the Zipf distribution described by the Zipf object.