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library(random): Random numbers |

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- The SWI-Prolog library
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- library(random): Random numbers
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- The SWI-Prolog library
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- Reference manual

- author
- R.A. O'Keefe, V.S. Costa, L. Damas, Jan Wielemaker
- See also
- Built-in function random/1:
A is
`random(10)`

This library is derived from the DEC10 library random. Later, the core random generator was moved to C. The current version uses the SWI-Prolog arithmetic functions to realise this library. These functions are based on the GMP library.

- [det]
**random**(`-R:float`) - Binds
`R`to a new random float in the*open*interval (0.0,1.0). - [semidet]
**random_between**(`+L:int, +U:int, -R:int`) - Binds
`R`to a random integer in [`L`,`U`] (i.e., including both`L`and`U`). Fails silently if`U``<``L`. - [det]
**random**(`+L:int, +U:int, -R:int`) - [det]
**random**(`+L:float, +U:float, -R:float`) - Generate a random integer or float in a range. If
`L`and`U`are both integers,`R`is a random integer in the half open interval [`L`,`U`). If`L`and`U`are both floats,`R`is a float in the open interval (`L`,`U`).- deprecated
- Please use random/1 for generating a random float and random_between/3 for generating a random integer. Note that random_between/3 includes the upper bound, while this predicate excludes it.

- [det]
**setrand**(`+State`) - [det]
**getrand**(`-State`) - Query/set the state of the random generator. This is intended for
restarting the generator at a known state only. The predicate setrand/1
accepts an opaque term returned by
getrand/1. This term
may be asserted, written and read. The application may not make other
assumptions about this term.
For compatibility reasons with older versions of this library, setrand/1 also accepts a term

`rand(A,B,C)`

, where A, B and C are integers in the range 1..30,000. This argument is used to seed the random generator. Deprecated.- Errors
`existence_error(random_state, _)`

is raised if the underlying infrastructure cannot fetch the random state. This is currently the case if SWI-Prolog is not compiled with the GMP library.- See also
- set_random/1 and random_property/1 provide the SWI-Prolog native implementation.

- [semidet]
**maybe** - Succeed/fail with equal probability (variant of maybe/1).
- [semidet]
**maybe**(`+P`) - Succeed with probability
`P`, fail with probability 1-`P` - [semidet]
**maybe**(`+K, +N`) - Succeed with probability
`K`/`N`(variant of maybe/1) - [semidet]
**random_perm2**(`?A, ?B, ?X, ?Y`) - Does
`X`=`A`,`Y`=`B`or`X`=`B`,`Y`=`A`with equal probability. - [semidet]
**random_member**(`-X, +List:list`) `X`is a random member of`List`. Equivalent to random_between(1,`|`

`List``|`

), followed by nth1/3. Fails of`List`is the empty list.- Compatibility
- Quintus and SICStus libraries.

- [semidet]
**random_select**(`-X, +List, -Rest`) - [det]
**random_select**(`+X, -List, +Rest`) - Randomly select or insert an element. Either
`List`or`Rest`must be a list. Fails if`List`is the empty list.- Compatibility
- Quintus and SICStus libraries.

- [det]
**randset**(`+K:int, +N:int, -S:list(int)`) `S`is a sorted list of`K`unique random integers in the range 1..`N`. Implemented by enumerating 1..`N`and deciding whether or not the number should be part of the set. For example:?- randset(5, 5, S). S = [1, 2, 3, 4, 5]. (always) ?- randset(5, 20, S). S = [2, 7, 10, 19, 20].

- See also
- randseq/3.
- bug
- Slow if
`N`is large and`K`is small.

- [det]
**randseq**(`+K:int, +N:int, -List:list(int)`) - S is a list of
`K`unique random integers in the range 1..`N`. The order is random. Works as if defined by the following code.randseq(K, N, List) :- randset(K, N, Set), random_permutation(Set, List).

- See also
- randset/3.

- [det]
**random_permutation**(`+List, -Permutation`) - [det]
**random_permutation**(`-List, +Permutation`) `Permutation`is a random permutation of`List`. This is intended to process the elements of`List`in random order. The predicate is symmetric.- Errors
- instantiation_error,
`type_error(list, _)`

.

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