SWI-Prolog 9.2.9 Reference Manual: Section A.45

## A.45 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 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]random_subseq(+List, -Subseq, -Complement)
[semidet]random_subseq(-List, +Subseq, +Complement)
  Selects a random subsequence Subseq of List, with Complement containing all elements of List that were not selected. Each element of List is included with equal probability in either Subseq or Complement.

  random_subseq/3 may also be called with Subseq and Complement bound and List unbound, which will recreate List by randomly interleaving Subseq and Complement. This mode may fail randomly, matching SICStus behavior. The failure probability corresponds to the probability of the "forward" mode selecting a Subseq/Complement combination with different lengths.

  Compatibility
    SICStus 4
[det]randset(+K:int, +N:int, -S:list(int))
  S is a sorted list of K unique random integers in the range 1..N. The implementation uses different techniques depending on the ratio K/N. For small K/N it generates a set of K random numbers, removes the duplicates and adds more numbers until `|`S`|` is K. For a large K/N it enumerates 1..N and decides randomly to include the number or not. 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.
[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. Defined as

      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, _)`.
[det]random_numlist(+P, +L, +U, -List)
  Unify List with an ascending list of integers between L and U (inclusive). Each integer in the range L..U is included with probability P.
  Compatibility
    SICStus 4