Chez Scheme provides a built-in procedure random that is often
helpful in modelling real-world phenomena that do not follow any obvious
algorithms and are not predictable.
When the random procedure is given an exact positive integer
as argument, it returns some non-negative integer less than the argument,
but not always the same one. Over a long series of calls, you'll get back
each non-negative integer less than the argument about equally often. So,
for example, the call (random 20) always yields some integer
in the range from 0 to 19, and over a long series of calls you can expect
each of those values to be returned about one-twentieth of the time.
(random 20) ===> 4 (random 20) ===> 17 (random 20) ===> 16
When the argument to random is an inexact positive real
(typically specified by a numeral containing a decimal point),
random returns some non-negative real value less than the
argument. Over a long series of identical calls, the values returned will
be distributed approximately evenly throughout the interval from 0.0
(inclusive) to the value of the argument (exclusive).
(random 1.5) ===> 0.046003446339158316 (random 1.5) ===> 1.3329912409120568 (random 1.5) ===> 1.4584015323556465 (random 1.5) ===> 0.14671341559491913 (random 1.5) ===> 1.1500817949427606
A typical use of random is to simulate the rolling of an
ordinary six-sided die. The roll-a-die procedure returns a
random integer in the range from 1 to 6:
(define roll-a-die
(lambda ()
(+ 1 (random 6))))
Note that roll-a-die takes no arguments. To invoke it, just
write its name between parentheses: (roll-a-die).
Another common application of random is to select an element
at random from a non-empty list. The random-element procedure
performs this operation on a given list:
(define random-element
(lambda (ls)
(list-ref ls (random (length ls)))))
(random-element '(a b c d e f g)) ===> c
(random-element '(a b c d e f g)) ===> f
(random-element '(a b c d e f g)) ===> b
(random-element '(111 112 114 118 126)) ===> 112
(random-element '(only)) ===> only
In English: To choose a random element from a list, obtain a random non-negative integer less than the length of that list and select the element at that position in the list.
Define a toss-a-coin procedure that takes no arguments and
returns either the string "heads" or the string
"tails", about equally often over a long series of calls.
Define a procedure named roll-two-dice that takes no
arguments, simulates the independent rolling of two dice, and returns the
total of the numbers on the faces that turn up.
Define a procedure named random-Boolean-list that takes one
argument, a non-negative integer, and constructs and returns a list of the
length specified by that argument. Each element of the list should be one
of the Boolean values #t and #f, chosen
independently, at random, and with equal probability. For instance, the
call (random-Boolean-list 4) might return the value (#t
#t #f #t).
According to the Des Moines Register (September 10, 1997, page 1M), registered voters in Iowa have declared the following party affiliations:
Democratic Party 558801 Reform Party 81 Republican Party 583897 unaffiliated 583002 -------------------------- total 1725781
Define a procedure named random-voter-affiliation that takes
no arguments and returns the symbol Democrat 558801/1725781 of
the time, the symbol Reform 81/1725781 of the time, the symbol
Republican 583897/1725781 of the time, and the symbol
mugwump 583002/1725781 of the time.
This document is available on the World Wide Web as
http://www.math.grin.edu/~stone/courses/scheme/random-number-generation.html
created February 7, 1997
last revised September 11, 1997