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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

limitis a positive integer, the value is chosen to be nonnegative and less thanlimit. Otherwise, the value might be any integer representable in Lisp, i.e., an integer between`most-negative-fixnum`

and`most-positive-fixnum`

(see Integer Basics).If

limitis`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

limitis a string, it means to choose a new seed based on the string's contents.