rand

package standard library
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 Go to latest Published: Sep 3, 2025 License: BSD-3-Clause Imports: 6 Imported by: 6,961

Documentation

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 
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. 

// 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