Animal Magic

Package math/rand provides support for generating pseudo-random numbers.

Here is how to generate a random integer between 0 and 99:

n := rand.Intn(100) // n is a random int, 0 <= n < 100

Function rand.Float64 returns a random floating point number between 0.0 and 1.0:

f := rand.Float64() // f is a random float64, 0.0 <= f < 1.0

There is also support for shuffling a slice (or other data structures):

x := []string{"a", "b", "c", "d", "e"}
// shuffling the slice put its elements into a random order
rand.Shuffle(len(x), func(i, j int) {
	x[i], x[j] = x[j], x[i]
})

Seeds

The number sequences generated by package math/rand are not truly random. Given a specific "seed" value, the results are entirely deterministic.

In Go 1.20+ the seed is automatically picked at random so you will see a different sequence of random numbers each time you run your program.

In prior versions of Go, the seed was 1 by default. So to get different sequences for various runs of the program, you had to manually seed the random number generator, e.g. with the current time, before retrieving any random numbers.

rand.Seed(time.Now().UnixNano())