[ << ] | [ < ] | [ Up ] | [ > ] | [ >> ] | [Top] | [Contents] | [Index] | [ ? ] |

#### 6.6.2.14 Random Number Generation

Pseudo-random numbers are generated from a random state object, which
can be created with `seed->random-state`

or
`datum->random-state`

. An external representation (i.e. one
which can written with `write`

and read with `read`

) of a
random state object can be obtained via
`random-state->datum`

. The `state` parameter to the
various functions below is optional, it defaults to the state object
in the `*random-state*`

variable.

- Scheme Procedure:
**copy-random-state***[state]* - C Function:
**scm_copy_random_state***(state)* Return a copy of the random state

`state`.

- Scheme Procedure:
**random***n [state]* - C Function:
**scm_random***(n, state)* Return a number in [0,

`n`).Accepts a positive integer or real n and returns a number of the same type between zero (inclusive) and

`n`(exclusive). The values returned have a uniform distribution.

- Scheme Procedure:
**random:exp***[state]* - C Function:
**scm_random_exp***(state)* Return an inexact real in an exponential distribution with mean 1. For an exponential distribution with mean

`u`use`(*`

.`u`(random:exp))

- Scheme Procedure:
**random:hollow-sphere!***vect [state]* - C Function:
**scm_random_hollow_sphere_x***(vect, state)* Fills

`vect`with inexact real random numbers the sum of whose squares is equal to 1.0. Thinking of`vect`as coordinates in space of dimension`n`*=*`(vector-length`

, the coordinates are uniformly distributed over the surface of the unit n-sphere.`vect`)

- Scheme Procedure:
**random:normal***[state]* - C Function:
**scm_random_normal***(state)* Return an inexact real in a normal distribution. The distribution used has mean 0 and standard deviation 1. For a normal distribution with mean

`m`and standard deviation`d`use`(+`

.`m`(*`d`(random:normal)))

- Scheme Procedure:
**random:normal-vector!***vect [state]* - C Function:
**scm_random_normal_vector_x***(vect, state)* Fills

`vect`with inexact real random numbers that are independent and standard normally distributed (i.e., with mean 0 and variance 1).

- Scheme Procedure:
**random:solid-sphere!***vect [state]* - C Function:
**scm_random_solid_sphere_x***(vect, state)* Fills

`vect`with inexact real random numbers the sum of whose squares is less than 1.0. Thinking of`vect`as coordinates in space of dimension`n`*=*`(vector-length`

, the coordinates are uniformly distributed within the unit`vect`)`n`-sphere.

- Scheme Procedure:
**random:uniform***[state]* - C Function:
**scm_random_uniform***(state)* Return a uniformly distributed inexact real random number in [0,1).

- Scheme Procedure:
**seed->random-state***seed* - C Function:
**scm_seed_to_random_state***(seed)* Return a new random state using

`seed`.

- Scheme Procedure:
**datum->random-state***datum* - C Function:
**scm_datum_to_random_state***(datum)* Return a new random state from

`datum`, which should have been obtained by`random-state->datum`

.

- Scheme Procedure:
**random-state->datum***state* - C Function:
**scm_random_state_to_datum***(state)* Return a datum representation of

`state`that may be written out and read back with the Scheme reader.

- Scheme Procedure:
**random-state-from-platform** - C Function:
**scm_random_state_from_platform***()* Construct a new random state seeded from a platform-specific source of entropy, appropriate for use in non-security-critical applications. Currently ‘

`/dev/urandom`’ is tried first, or else the seed is based on the time, date, process ID, an address from a freshly allocated heap cell, an address from the local stack frame, and a high-resolution timer if available.

- Variable:
***random-state*** The global random state used by the above functions when the

`state`parameter is not given.

Note that the initial value of `*random-state*`

is the same every
time Guile starts up. Therefore, if you don’t pass a `state`
parameter to the above procedures, and you don’t set
`*random-state*`

to `(seed->random-state your-seed)`

, where
`your-seed`

is something that *isn’t* the same every time,
you’ll get the same sequence of “random” numbers on every run.

For example, unless the relevant source code has changed, ```
(map
random (cdr (iota 30)))
```

, if the first use of random numbers since
Guile started up, will always give:

(map random (cdr (iota 19))) ⇒ (0 1 1 2 2 2 1 2 6 7 10 0 5 3 12 5 5 12)

To seed the random state in a sensible way for non-security-critical applications, do this during initialization of your program:

(set! *random-state* (random-state-from-platform))

[ << ] | [ < ] | [ Up ] | [ > ] | [ >> ] | [Top] | [Contents] | [Index] | [ ? ] |

This document was generated on *February 3, 2012* using *texi2html 5.0*.