rand
packagestandard library
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Documentation¶
Overview¶
Package rand implements pseudo-random number generators suitable for taskssuch as simulation, but it should not be used for security-sensitive work.
Random numbers are generated by aSource, usually wrapped in aRand.Both types should be used by a single goroutine at a time: sharing amongmultiple goroutines requires some kind of synchronization.
Top-level functions, such asFloat64 andInt,are safe for concurrent use by multiple goroutines.
This package's outputs might be easily predictable regardless of how it'sseeded. For random numbers suitable for security-sensitive work, see thecrypto/rand package.
Example¶
package mainimport ("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))])}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.
package mainimport ("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))}Output:Float32 0.2635776 0.6358173 0.6718283Float64 0.628605430454327 0.4504798828572669 0.9562755949377957ExpFloat64 0.3362240648200941 1.4256072328483647 0.24354758816173044NormFloat64 0.17233959114940064 1.577014951434847 0.04259129641113857Int31 1501292890 1486668269 182840835Int63 3546343826724305832 5724354148158589552 5239846799706671610Uint32 2760229429 296659907 1922395059Intn(10) 1 2 5Int31n(10) 4 7 8Int63n(10) 7 6 3Perm [1 4 2 3 0] [4 2 1 3 0] [1 2 4 0 3]
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)deprecated
- func Seed(seed int64)deprecated
- func Shuffle(n int, swap func(i, j int))
- func Uint32() uint32
- func Uint64() uint64
- type 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
- type Source64
- type Zipf
Examples¶
Constants¶
This section is empty.
Variables¶
This section is empty.
Functions¶
funcExpFloat64¶
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 defaultSource.To produce a distribution with a different rate parameter,callers can adjust the output using:
sample = ExpFloat64() / desiredRateParameter
funcFloat32¶
func Float32()float32
Float32 returns, as a float32, a pseudo-random number in the half-open interval [0.0,1.0)from the defaultSource.
funcFloat64¶
func Float64()float64
Float64 returns, as a float64, a pseudo-random number in the half-open interval [0.0,1.0)from the defaultSource.
funcInt31¶
func Int31()int32
Int31 returns a non-negative pseudo-random 31-bit integer as an int32from the defaultSource.
funcInt31n¶
Int31n returns, as an int32, a non-negative pseudo-random number in the half-open interval [0,n)from the defaultSource.It panics if n <= 0.
funcInt63¶
func Int63()int64
Int63 returns a non-negative pseudo-random 63-bit integer as an int64from the defaultSource.
funcInt63n¶
Int63n returns, as an int64, a non-negative pseudo-random number in the half-open interval [0,n)from the defaultSource.It panics if n <= 0.
funcIntn¶
Intn returns, as an int, a non-negative pseudo-random number in the half-open interval [0,n)from the defaultSource.It panics if n <= 0.
Example¶
package mainimport ("fmt""math/rand")func main() {fmt.Println(rand.Intn(100))fmt.Println(rand.Intn(100))fmt.Println(rand.Intn(100))}funcNormFloat64¶
func NormFloat64()float64
NormFloat64 returns a normally distributed float64 in the range[-math.MaxFloat64, +[math.MaxFloat64]] withstandard normal distribution (mean = 0, stddev = 1)from the defaultSource.To produce a different normal distribution, callers canadjust the output using:
sample = NormFloat64() * desiredStdDev + desiredMean
funcPerm¶
Perm returns, as a slice of n ints, a pseudo-random permutation of the integersin the half-open interval [0,n) from the defaultSource.
Example¶
package mainimport ("fmt""math/rand")func main() {for _, value := range rand.Perm(3) {fmt.Println(value)}}Output:120
funcReaddeprecatedadded ingo1.6
Read generates len(p) random bytes from the defaultSource andwrites them into p. It always returns len(p) and a nil error.Read, unlike theRand.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, usemath/rand/v2.ChaCha8.Read.
funcSeeddeprecated
func Seed(seedint64)
Seed uses the provided seed value to initialize the default Source to adeterministic state. Seed values that have the same remainder whendivided by 2³¹-1 generate the same pseudo-random sequence.Seed, unlike theRand.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 environmentbefore making any calls to functions in this package.
Deprecated: As of Go 1.20 there is no reason to call Seed witha random value. Programs that call Seed with a known value to geta specific sequence of results should use New(NewSource(seed)) toobtain a local random generator.
As of Go 1.24Seed is a no-op. To restore the previous behavior setGODEBUG=randseednop=0.
funcShuffle¶added ingo1.10
Shuffle pseudo-randomizes the order of elements using the defaultSource.n is the number of elements. Shuffle panics if n < 0.swap swaps the elements with indexes i and j.
Example¶
package mainimport ("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)}Example (SlicesInUnison)¶
package mainimport ("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])}}Types¶
typeRand¶
type Rand struct {// contains filtered or unexported fields}A Rand is a source of random numbers.
func (*Rand)ExpFloat64¶
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¶
Float32 returns, as a float32, a pseudo-random number in the half-open interval [0.0,1.0).
func (*Rand)Float64¶
Float64 returns, as a float64, a pseudo-random number in the half-open interval [0.0,1.0).
func (*Rand)Int31n¶
Int31n returns, as an int32, a non-negative pseudo-random number in the half-open interval [0,n).It panics if n <= 0.
func (*Rand)Int63n¶
Int63n 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¶
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¶
NormFloat64 returns a normally distributed float64 inthe range -math.MaxFloat64 through +[math.MaxFloat64] inclusive,with standard normal distribution (mean = 0, stddev = 1).To produce a different normal distribution, callers canadjust the output using:
sample = NormFloat64() * desiredStdDev + desiredMean
func (*Rand)Perm¶
Perm returns, as a slice of n ints, a pseudo-random permutation of the integersin the half-open interval [0,n).
func (*Rand)Read¶added ingo1.6
Read generates len(p) random bytes and writes them into p. Italways returns len(p) and a nil error.Read should not be called concurrently with any other Rand method.
func (*Rand)Seed¶
Seed uses the provided seed value to initialize the generator to a deterministic state.Seed should not be called concurrently with any otherRand method.
func (*Rand)Shuffle¶added ingo1.10
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.
typeSource¶
A Source represents a source of uniformly-distributedpseudo-random int64 values in the range [0, 1<<63).
A Source is not safe for concurrent use by multiple goroutines.
typeSource64¶added ingo1.8
A Source64 is aSource that can also generateuniformly-distributed pseudo-random uint64 values inthe range [0, 1<<64) directly.If aRand r's underlyingSource s implements Source64,then r.Uint64 returns the result of one call to s.Uint64instead of making two calls to s.Int63.
Source Files
Directories¶
| Path | Synopsis |
|---|---|
Package rand implements pseudo-random number generators suitable for tasks such as simulation, but it should not be used for security-sensitive work. | Package rand implements pseudo-random number generators suitable for tasks such as simulation, but it should not be used for security-sensitive work. |