SWI-Prolog 7.4.2 Reference Manual: Section A.28

## A.28 library(random): Random numbers

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).
  See also
    - setrand/1, getrand/1 may be used to fetch/set the state.
    - In SWI-Prolog, random/1 is implemented by the function random_float/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 the random_between/3 includes the upper bound, while this predicate excludes the upper bound.
[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, _)`.