Previous: Math Functions, Up: Numbers [Contents][Index]

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`

(see 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.

Previous: Math Functions, Up: Numbers [Contents][Index]