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Package rand

import "math/rand/v2"
Overview
Index
Examples

Overview ▾

Package rand implements pseudo-random number generators suitable for tasks such as simulation, but it should not be used for security-sensitive work.

Random numbers are generated by a Source, usually wrapped in a Rand. Both types should be used by a single goroutine at a time: sharing among multiple goroutines requires some kind of synchronization.

Top-level functions, such as Float64 and Int, are safe for concurrent use by multiple goroutines.

This package's outputs might be easily predictable regardless of how it's seeded. For random numbers suitable for security-sensitive work, see the crypto/rand package.

▾ Example

Code:

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))])

▾ Example (Rand)

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 Uint64(), Uint64().
// Using a fixed seed will produce the same output on every run.
r := rand.New(rand.NewPCG(1, 2))

// 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())

// Int32, Int64, and Uint32 generate values of the given width.
// The Int method (not shown) is like either Int32 or Int64
// depending on the size of 'int'.
show("Int32", r.Int32(), r.Int32(), r.Int32())
show("Int64", r.Int64(), r.Int64(), r.Int64())
show("Uint32", r.Uint32(), r.Uint32(), r.Uint32())

// IntN, Int32N, and Int64N 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("Int32N(10)", r.Int32N(10), r.Int32N(10), r.Int32N(10))
show("Int64N(10)", r.Int64N(10), r.Int64N(10), r.Int64N(10))

// Perm generates a random permutation of the numbers [0, n).
show("Perm", r.Perm(5), r.Perm(5), r.Perm(5))

Output:

Float32     0.95955694          0.8076733            0.8135684
Float64     0.4297927436037299  0.797802349388613    0.3883664855410056
ExpFloat64  0.43463410545541104 0.5513632046504593   0.7426404617374481
NormFloat64 -0.9303318111676635 -0.04750789419852852 0.22248301107582735
Int32       2020777787          260808523            851126509
Int64       5231057920893523323 4257872588489500903  158397175702351138
Uint32      314478343           1418758728           208955345
IntN(10)    6                   2                    0
Int32N(10)  3                   7                    7
Int64N(10)  8                   9                    4
Perm        [0 3 1 4 2]         [4 1 2 0 3]          [4 3 2 0 1]

Index ▾

func ExpFloat64() float64
func Float32() float32
func Float64() float64
func Int() int
func Int32() int32
func Int32N(n int32) int32
func Int64() int64
func Int64N(n int64) int64
func IntN(n int) int
func N[Int intType](n Int) Int
func NormFloat64() float64
func Perm(n int) []int
func Shuffle(n int, swap func(i, j int))
func Uint() uint
func Uint32() uint32
func Uint32N(n uint32) uint32
func Uint64() uint64
func Uint64N(n uint64) uint64
func UintN(n uint) uint
type ChaCha8
    func NewChaCha8(seed [32]byte) *ChaCha8
    func (c *ChaCha8) MarshalBinary() ([]byte, error)
    func (c *ChaCha8) Read(p []byte) (n int, err error)
    func (c *ChaCha8) Seed(seed [32]byte)
    func (c *ChaCha8) Uint64() uint64
    func (c *ChaCha8) UnmarshalBinary(data []byte) error
type PCG
    func NewPCG(seed1, seed2 uint64) *PCG
    func (p *PCG) MarshalBinary() ([]byte, error)
    func (p *PCG) Seed(seed1, seed2 uint64)
    func (p *PCG) Uint64() uint64
    func (p *PCG) UnmarshalBinary(data []byte) error
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) Int32() int32
    func (r *Rand) Int32N(n int32) int32
    func (r *Rand) Int64() int64
    func (r *Rand) Int64N(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) Shuffle(n int, swap func(i, j int))
    func (r *Rand) Uint() uint
    func (r *Rand) Uint32() uint32
    func (r *Rand) Uint32N(n uint32) uint32
    func (r *Rand) Uint64() uint64
    func (r *Rand) Uint64N(n uint64) uint64
    func (r *Rand) UintN(n uint) uint
type Source
type Zipf
    func NewZipf(r *Rand, s float64, v float64, imax uint64) *Zipf
    func (z *Zipf) Uint64() uint64

Package files

chacha8.go exp.go normal.go pcg.go rand.go zipf.go

func ExpFloat64 1.22 

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 1.22 

func Float32() float32

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

func Float64 1.22 

func Float64() float64

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

func Int 1.22 

func Int() int

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

func Int32 1.22 

func Int32() int32

Int32 returns a non-negative pseudo-random 31-bit integer as an int32 from the default Source.

func Int32N 1.22 

func Int32N(n int32) int32

Int32N returns, as an int32, a pseudo-random number in the half-open interval [0,n) from the default Source. It panics if n <= 0.

func Int64 1.22 

func Int64() int64

Int64 returns a non-negative pseudo-random 63-bit integer as an int64 from the default Source.

func Int64N 1.22 

func Int64N(n int64) int64

Int64N returns, as an int64, a pseudo-random number in the half-open interval [0,n) from the default Source. It panics if n <= 0.

func IntN 1.22 

func IntN(n int) int

IntN returns, as an int, a pseudo-random number in the half-open interval [0,n) from the default Source. It panics if n <= 0.

▾ Example

Code:

fmt.Println(rand.IntN(100))
fmt.Println(rand.IntN(100))
fmt.Println(rand.IntN(100))

func N 

func N[Int intType](n Int) Int

N returns a pseudo-random number in the half-open interval [0,n) from the default Source. The type parameter Int can be any integer type. It panics if n <= 0.

▾ Example

Code:

// Print an int64 in the half-open interval [0, 100).
fmt.Println(rand.N(int64(100)))

// Sleep for a random duration between 0 and 100 milliseconds.
time.Sleep(rand.N(100 * time.Millisecond))

func NormFloat64 1.22 

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 1.22 

func Perm(n int) []int

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

▾ Example

Code:

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

Output:

1
2
0

func Shuffle 1.22 

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.

▾ Example

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)

▾ Example (SlicesInUnison)

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])
}

func Uint 1.23 

func Uint() uint

Uint returns a pseudo-random uint from the default Source.

func Uint32 1.22 

func Uint32() uint32

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

func Uint32N 1.22 

func Uint32N(n uint32) uint32

Uint32N returns, as a uint32, a pseudo-random number in the half-open interval [0,n) from the default Source. It panics if n <= 0.

func Uint64 1.22 

func Uint64() uint64

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

func Uint64N 1.22 

func Uint64N(n uint64) uint64

Uint64N returns, as a uint64, a pseudo-random number in the half-open interval [0,n) from the default Source. It panics if n <= 0.

func UintN 1.22 

func UintN(n uint) uint

UintN returns, as a uint, a pseudo-random number in the half-open interval [0,n) from the default Source. It panics if n <= 0.

type ChaCha8 1.22

A ChaCha8 is a ChaCha8-based cryptographically strong random number generator. 

type ChaCha8 struct {
    // contains filtered or unexported fields
}

func NewChaCha8 1.22 

func NewChaCha8(seed [32]byte) *ChaCha8

NewChaCha8 returns a new ChaCha8 seeded with the given seed.

func (*ChaCha8) MarshalBinary 1.22 

func (c *ChaCha8) MarshalBinary() ([]byte, error)

MarshalBinary implements the encoding.BinaryMarshaler interface.

func (*ChaCha8) Read 1.23 

func (c *ChaCha8) Read(p []byte) (n int, err error)

Read reads exactly len(p) bytes into p. It always returns len(p) and a nil error.

If calls to Read and Uint64 are interleaved, the order in which bits are returned by the two is undefined, and Read may return bits generated before the last call to Uint64.

func (*ChaCha8) Seed 1.22 

func (c *ChaCha8) Seed(seed [32]byte)

Seed resets the ChaCha8 to behave the same way as NewChaCha8(seed).

func (*ChaCha8) Uint64 1.22 

func (c *ChaCha8) Uint64() uint64

Uint64 returns a uniformly distributed random uint64 value.

func (*ChaCha8) UnmarshalBinary 1.22 

func (c *ChaCha8) UnmarshalBinary(data []byte) error

UnmarshalBinary implements the encoding.BinaryUnmarshaler interface.

type PCG 1.22

A PCG is a PCG generator with 128 bits of internal state. A zero PCG is equivalent to NewPCG(0, 0). 

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

func NewPCG 1.22 

func NewPCG(seed1, seed2 uint64) *PCG

NewPCG returns a new PCG seeded with the given values.

func (*PCG) MarshalBinary 1.22 

func (p *PCG) MarshalBinary() ([]byte, error)

MarshalBinary implements the encoding.BinaryMarshaler interface.

func (*PCG) Seed 1.22 

func (p *PCG) Seed(seed1, seed2 uint64)

Seed resets the PCG to behave the same way as NewPCG(seed1, seed2).

func (*PCG) Uint64 1.22 

func (p *PCG) Uint64() uint64

Uint64 return a uniformly-distributed random uint64 value.

func (*PCG) UnmarshalBinary 1.22 

func (p *PCG) UnmarshalBinary(data []byte) error

UnmarshalBinary implements the encoding.BinaryUnmarshaler interface.

type Rand 1.22

A Rand is a source of random numbers. 

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

func New 1.22 

func New(src Source) *Rand

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

func (*Rand) ExpFloat64 1.22 

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 1.22 

func (r *Rand) Float32() float32

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

func (*Rand) Float64 1.22 

func (r *Rand) Float64() float64

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

func (*Rand) Int 1.22 

func (r *Rand) Int() int

Int returns a non-negative pseudo-random int.

func (*Rand) Int32 1.22 

func (r *Rand) Int32() int32

Int32 returns a non-negative pseudo-random 31-bit integer as an int32.

func (*Rand) Int32N 1.22 

func (r *Rand) Int32N(n int32) int32

Int32N returns, as an int32, a non-negative pseudo-random number in the half-open interval [0,n). It panics if n <= 0.

func (*Rand) Int64 1.22 

func (r *Rand) Int64() int64

Int64 returns a non-negative pseudo-random 63-bit integer as an int64.

func (*Rand) Int64N 1.22 

func (r *Rand) Int64N(n int64) int64

Int64N returns, as an int64, a non-negative pseudo-random number in the half-open interval [0,n). It panics if n <= 0.

func (*Rand) IntN 1.22 

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

IntN returns, as an int, a non-negative pseudo-random number in the half-open interval [0,n). It panics if n <= 0.

func (*Rand) NormFloat64 1.22 

func (r *Rand) NormFloat64() float64

NormFloat64 returns a normally distributed float64 in the range -math.MaxFloat64 through +math.MaxFloat64 inclusive, 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 1.22 

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

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

func (*Rand) Shuffle 1.22 

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) Uint 1.23 

func (r *Rand) Uint() uint

Uint returns a pseudo-random uint.

func (*Rand) Uint32 1.22 

func (r *Rand) Uint32() uint32

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

func (*Rand) Uint32N 1.22 

func (r *Rand) Uint32N(n uint32) uint32

Uint32N returns, as a uint32, a non-negative pseudo-random number in the half-open interval [0,n). It panics if n == 0.

func (*Rand) Uint64 1.22 

func (r *Rand) Uint64() uint64

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

func (*Rand) Uint64N 1.22 

func (r *Rand) Uint64N(n uint64) uint64

Uint64N returns, as a uint64, a non-negative pseudo-random number in the half-open interval [0,n). It panics if n == 0.

func (*Rand) UintN 1.22 

func (r *Rand) UintN(n uint) uint

UintN returns, as a uint, a non-negative pseudo-random number in the half-open interval [0,n). It panics if n == 0.

type Source 1.22

A Source is a source of uniformly-distributed pseudo-random uint64 values in the range [0, 1<<64).

A Source is not safe for concurrent use by multiple goroutines. 

type Source interface {
    Uint64() uint64
}

type Zipf 1.22

A Zipf generates Zipf distributed variates. 

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

func NewZipf 1.22 

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 1.22 

func (z *Zipf) Uint64() uint64

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