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Generating Random Numbers

Problem

You want a random number from a given range. For example, you wish to randomly select one element from an array, simulate rolling a die in a game of chance, or generate a random password.

Solution

Use (random-integer n) function from SRFI27?. It returns a random exact integer in the range of 0 to n-1.

> (require (lib "27.ss" "srfi"))
> (random-integer 172)
58

Discussion

This code generates and prints a random integer between 25 and 75, inclusive:

> (let ((rand (+ (random-integer 51) 25)))
    (for-each display (list "Random = " rand "\n")))
Random = 40

In the example, we want to limit the range to 25 to 75, so we add 25 to the result of the random function. This means we really want a random number between 0 and 50, so we give the random function the argument 51 (since the generated number is always from 0 to n-1).

The canonical application for this kind of number generation is the random selection of an element from a vector:

> (define myvec #(#\A #\B #\C #\D #\1 #\2 #\3))
> (define elt
    (vector-ref myvec (random-integer 7))
> elt
#\B

Another common example is generating a random password. For this example, we will make use of SRFI 14 (Character Set Library) and SRFI 42 (List Comprehensions):

(require (lib "14.ss" "srfi")
         (lib "27.ss" "srfi")
         (lib "42.ss" "srfi"))

(define (generate-8-password)
  (string-append-ec (:range i 0 8)
                    (let* ((bigcs (substring (char-set->string char-set:ascii) 6 95))
                           (rand (random-integer (string-length bigcs))))
                      (substring bigcs rand (+ rand 1)))))

> (generate-8-password)
"n9R*g(la"

See Also

NumberRecipeBigRandomNumber