elisp: Random Numbers
3.10 Random Numbers
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A deterministic computer program cannot generate true random numbers.
For most purposes, “pseudo-random numbers” suffice. A series of
pseudo-random numbers is generated in a deterministic fashion. The
numbers are not truly random, but they have certain properties that
mimic a random series. For example, all possible values occur equally
often in a pseudo-random series.
Pseudo-random numbers are generated from a “seed value”. Starting
from any given seed, the ‘random’ function always generates the same
sequence of numbers. By default, Emacs initializes the random seed at
startup, in such a way that the sequence of values of ‘random’ (with
overwhelming likelihood) differs in each Emacs run.
Sometimes you want the random number sequence to be repeatable. For
example, when debugging a program whose behavior depends on the random
number sequence, it is helpful to get the same behavior in each program
run. To make the sequence repeat, execute ‘(random "")’. This sets the
seed to a constant value for your particular Emacs executable (though it
may differ for other Emacs builds). You can use other strings to choose
various seed values.
-- Function: random &optional limit
This function returns a pseudo-random integer. Repeated calls
return a series of pseudo-random integers.
If LIMIT is a positive integer, the value is chosen to be
nonnegative and less than LIMIT. Otherwise, the value might be any
integer representable in Lisp, i.e., an integer between
‘most-negative-fixnum’ and ‘most-positive-fixnum’ (
Integer
Basics).
If LIMIT is ‘t’, it means to choose a new seed as if Emacs were
restarting, typically from the system entropy. On systems lacking
entropy pools, choose the seed from less-random volatile data such
as the current time.
If LIMIT is a string, it means to choose a new seed based on the
string’s contents.
Info Catalog
elisp: Math Functions
elisp: Numbers