`package `**rand**
Import Path
math/rand* (on go.dev)*
Dependency Relation
imports 5 packages, and imported by 6 packages
Involved Source Files
exp.go
normal.go
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.
rng.go
zipf.go
Code Examples
package main
import (
"fmt"
"math/rand"
)
func main() {
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))])
}
package main
import (
"fmt"
"math/rand"
)
func main() {
fmt.Println(rand.Intn(100))
fmt.Println(rand.Intn(100))
fmt.Println(rand.Intn(100))
}
package main
import (
"fmt"
"math/rand"
)
func main() {
for _, value := range rand.Perm(3) {
fmt.Println(value)
}
}
package main
import (
"fmt"
"math/rand"
"strings"
)
func main() {
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)
}
package main
import (
"fmt"
"math/rand"
)
func main() {
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])
}
}
package main
import (
"fmt"
"math/rand"
"os"
"text/tabwriter"
)
func main() {
// 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))
}
Package-Level Type Names* (total 4)*
A Rand is a source of random numbers.
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
Float32 returns, as a float32, a pseudo-random number in the half-open interval [0.0,1.0).
Float64 returns, as a float64, a pseudo-random number in the half-open interval [0.0,1.0).
Int returns a non-negative pseudo-random int.
Int31 returns a non-negative pseudo-random 31-bit integer as an int32.
Int31n returns, as an int32, a non-negative pseudo-random number in the half-open interval [0,n).
It panics if n <= 0.
Int63 returns a non-negative pseudo-random 63-bit integer as an int64.
Int63n returns, as an int64, a non-negative pseudo-random number in the half-open interval [0,n).
It panics if n <= 0.
Intn returns, as an int, a non-negative pseudo-random number in the half-open interval [0,n).
It panics if n <= 0.
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
Perm returns, as a slice of n ints, a pseudo-random permutation of the integers
in the half-open interval [0,n).
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.
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.
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.
Uint32 returns a pseudo-random 32-bit value as a uint32.
Uint64 returns a pseudo-random 64-bit value as a uint64.
*Rand : Source
*Rand : Source64
*Rand : math/rand/v2.Source
*Rand : io.Reader
func New(src Source) ***Rand**
func NewZipf(r ***Rand**, s float64, v float64, imax uint64) *Zipf
func math/big.(*Int).Rand(rnd ***Rand**, n *big.Int) *big.Int
func testing/quick.Value(t reflect.Type, rand ***Rand**) (value reflect.Value, ok bool)
func testing/quick.Generator.Generate(rand ***Rand**, size int) reflect.Value
A Source represents a source of uniformly-distributed
pseudo-random int64 values in the range [0, 1<<63).
A Source is not safe for concurrent use by multiple goroutines.
( Source) Int63() int64
( Source) Seed(seed int64)
*Rand
Source64 *(interface)*
func NewSource(seed int64) **Source**
func New(src **Source**) *Rand
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.
( Source64) Int63() int64
( Source64) Seed(seed int64)
( Source64) Uint64() uint64
*Rand
Source64 : Source
Source64 : math/rand/v2.Source
Package-Level Functions* (total 19)*
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
Float32 returns, as a float32, a pseudo-random number in the half-open interval [0.0,1.0)
from the default [Source].
Float64 returns, as a float64, a pseudo-random number in the half-open interval [0.0,1.0)
from the default [Source].
Int returns a non-negative pseudo-random int from the default [Source].
Int31 returns a non-negative pseudo-random 31-bit integer as an int32
from the default [Source].
Int31n returns, as an int32, a non-negative pseudo-random number in the half-open interval [0,n)
from the default [Source].
It panics if n <= 0.
Int63 returns a non-negative pseudo-random 63-bit integer as an int64
from the default [Source].
Int63n returns, as an int64, a non-negative pseudo-random number in the half-open interval [0,n)
from the default [Source].
It panics if n <= 0.
Intn returns, as an int, a non-negative pseudo-random number in the half-open interval [0,n)
from the default [Source].
It panics if n <= 0.
New returns a new [Rand] that uses random values from src
to generate other random values.
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.
The returned [Source] implements [Source64].
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.
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
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].
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.
Deprecated: For almost all use cases, [crypto/rand.Read] is more appropriate.
If a deterministic source is required, use [math/rand/v2.ChaCha8.Read].
Seed uses the provided seed value to initialize the default Source to a
deterministic state. Seed values that have the same remainder when
divided by 2³¹-1 generate the same pseudo-random sequence.
Seed, unlike the [Rand.Seed] method, is safe for concurrent use.
If Seed is not called, the generator is seeded randomly at program startup.
Prior to Go 1.20, the generator was seeded like Seed(1) at program startup.
To force the old behavior, call Seed(1) at program startup.
Alternately, set GODEBUG=randautoseed=0 in the environment
before making any calls to functions in this package.
Deprecated: As of Go 1.20 there is no reason to call Seed with
a random value. Programs that call Seed with a known value to get
a specific sequence of results should use New(NewSource(seed)) to
obtain a local random generator.
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.
Uint32 returns a pseudo-random 32-bit value as a uint32
from the default [Source].
Uint64 returns a pseudo-random 64-bit value as a uint64
from the default [Source].

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