random-numbers.asciidoc

Generating Random Numbers

by Ryan Neufeld

Problem

You need to generate a random number.

Solution

Clojure has a number of pseudo-random number generating functions available for your disposal.

For generating random floating point numbers from 0.0 up to (but not including) 1.0, use rand.

(rand)
;; -> 0.0249306187447903

(rand)
;; -> 0.9242089829055088

For generating random integers use rand-int.

;; Emulating a six-sided die
(defn roll-d6 []
  (inc (rand-int 6)))

(roll-d6)
;; -> 1

(roll-d6)
;; -> 3

Discussion

In addition to generating a number from 0.0 to 1.0, rand also accepts an optional argument that specifies the exclusive maximum value. For example, (rand 5) would return a floating point number ranging from 0.0 (inclusive) to 5.0 (exclusive).

(rand-int 5), on the other hand, would return a random integer between 0 (inclusive) and 5 (exclusive). At first blush rand-int might seem like an ideal way to select a random element from a vector or list. This is a lot of ceremony though, use rand-nth instead to get a random element from any sequential collect (i.e. the collection responds to nth.)

(rand-nth [1 2 3])
;; -> 1

(rand-nth '(:a :b :c))
;; -> :c

This won’t work for sets or hash-maps however. If you want to retrieve a random element from a non-sequential collection like a set, use seq to transform that collection into a sequence before calling rand-nth on it.

(rand-nth (seq #{:heads :tails}))
;; -> :heads

If you’re trying to randomly sort a collection, use shuffle to receive a random permutation of your collection.

(shuffle [1 2 3 4 5 6])
;; -> [3 1 4 5 2 6]