- take(+Length, +List, -Prefix) is semidet
- True if Prefix is a list of length Length and that is the prefix of
List.
Re-exported predicates
The following predicates are exported from this file while their implementation is defined in imported modules or non-module files loaded by this module.
- member(?Elem, ?List)
- True if Elem is a member of List. The SWI-Prolog definition
differs from the classical one. Our definition avoids unpacking
each list element twice and provides determinism on the last
element. E.g. this is deterministic:
member(X, [One]).
- author
- - Gertjan van Noord
- append(?List1, ?List2, ?List1AndList2)
- List1AndList2 is the concatenation of List1 and List2
- append(+ListOfLists, ?List)
- Concatenate a list of lists. Is true if ListOfLists is a list of
lists, and List is the concatenation of these lists.
- Arguments:
-
ListOfLists | - must be a list of possibly partial lists |
- prefix(?Part, ?Whole)
- True iff Part is a leading substring of Whole. This is the same
as
append(Part, _, Whole)
.
- select(?Elem, ?List1, ?List2)
- Is true when List1, with Elem removed, results in List2. This
implementation is determinsitic if the last element of List1 has
been selected.
- selectchk(+Elem, +List, -Rest) is semidet
- Semi-deterministic removal of first element in List that unifies
with Elem.
- select(?X, ?XList, ?Y, ?YList) is nondet
- Select from two lists at the same position. True if XList is
unifiable with YList apart a single element at the same position
that is unified with X in XList and with Y in YList. A typical use
for this predicate is to replace an element, as shown in the
example below. All possible substitutions are performed on
backtracking.
?- select(b, [a,b,c,b], 2, X).
X = [a, 2, c, b] ;
X = [a, b, c, 2] ;
false.
- See also
- - selectchk/4 provides a semidet version.
- selectchk(?X, ?XList, ?Y, ?YList) is semidet
- Semi-deterministic version of select/4.
- nextto(?X, ?Y, ?List)
- True if Y directly follows X in List.
- delete(+List1, @Elem, -List2) is det
- Delete matching elements from a list. True when List2 is a list
with all elements from List1 except for those that unify with
Elem. Matching Elem with elements of List1 is uses
\+ Elem \=
H
, which implies that Elem is not changed.
- See also
- - select/3, subtract/3.
- deprecated
- - There are too many ways in which one might want to
delete elements from a list to justify the name.
Think of matching (= vs. ==), delete first/all,
be deterministic or not.
- nth0(?Index, ?List, ?Elem)
- True when Elem is the Index'th element of List. Counting starts
at 0.
- Errors
- -
type_error(integer, Index)
if Index is not an integer or
unbound.
- See also
- - nth1/3.
- nth1(?Index, ?List, ?Elem)
- Is true when Elem is the Index'th element of List. Counting
starts at 1.
- See also
- - nth0/3.
- nth0(?N, ?List, ?Elem, ?Rest) is det
- Select/insert element at index. True when Elem is the N'th
(0-based) element of List and Rest is the remainder (as in by
select/3) of List. For example:
?- nth0(I, [a,b,c], E, R).
I = 0, E = a, R = [b, c] ;
I = 1, E = b, R = [a, c] ;
I = 2, E = c, R = [a, b] ;
false.
?- nth0(1, L, a1, [a,b]).
L = [a, a1, b].
- nth1(?N, ?List, ?Elem, ?Rest) is det
- As nth0/4, but counting starts at 1.
- last(?List, ?Last)
- Succeeds when Last is the last element of List. This
predicate is
semidet
if List is a list and multi
if List is
a partial list.
- Compatibility
- - There is no de-facto standard for the argument order of
last/2. Be careful when porting code or use
append(_, [Last], List)
as a portable alternative.
- proper_length(@List, -Length) is semidet
- True when Length is the number of elements in the proper list
List. This is equivalent to
proper_length(List, Length) :-
is_list(List),
length(List, Length).
- same_length(?List1, ?List2)
- Is true when List1 and List2 are lists with the same number of
elements. The predicate is deterministic if at least one of the
arguments is a proper list. It is non-deterministic if both
arguments are partial lists.
- See also
- - length/2
- reverse(?List1, ?List2)
- Is true when the elements of List2 are in reverse order compared to
List1. This predicate is deterministic if either list is a proper
list. If both lists are partial lists backtracking generates
increasingly long lists.
- permutation(?Xs, ?Ys) is nondet
- True when Xs is a permutation of Ys. This can solve for Ys given
Xs or Xs given Ys, or even enumerate Xs and Ys together. The
predicate permutation/2 is primarily intended to generate
permutations. Note that a list of length N has N! permutations,
and unbounded permutation generation becomes prohibitively
expensive, even for rather short lists (10! = 3,628,800).
If both Xs and Ys are provided and both lists have equal length
the order is |Xs|^2. Simply testing whether Xs is a permutation
of Ys can be achieved in order log(|Xs|) using msort/2 as
illustrated below with the semidet
predicate is_permutation/2:
is_permutation(Xs, Ys) :-
msort(Xs, Sorted),
msort(Ys, Sorted).
The example below illustrates that Xs and Ys being proper lists
is not a sufficient condition to use the above replacement.
?- permutation([1,2], [X,Y]).
X = 1, Y = 2 ;
X = 2, Y = 1 ;
false.
- Errors
- -
type_error(list, Arg)
if either argument is not a proper
or partial list.
- flatten(+NestedList, -FlatList) is det
- Is true if FlatList is a non-nested version of NestedList. Note
that empty lists are removed. In standard Prolog, this implies
that the atom '[]' is removed too. In SWI7,
[]
is distinct
from '[]'.
Ending up needing flatten/2 often indicates, like append/3 for
appending two lists, a bad design. Efficient code that generates
lists from generated small lists must use difference lists,
often possible through grammar rules for optimal readability.
- See also
- - append/2
- clumped(+Items, -Pairs)
- Pairs is a list of
Item-Count
pairs that represents the run
length encoding of Items. For example:
?- clumped([a,a,b,a,a,a,a,c,c,c], R).
R = [a-2, b-1, a-4, c-3].
- Compatibility
- - SICStus
- subseq(+List, -SubList, -Complement) is nondet
- subseq(-List, +SubList, +Complement) is nondet
- Is true when SubList contains a subset of the elements of List in
the same order and Complement contains all elements of List not in
SubList, also in the order they appear in List.
- Compatibility
- - SICStus. The SWI-Prolog version raises an error for less
instantiated modes as these do not terminate.
- max_member(-Max, +List) is semidet
- True when Max is the largest member in the standard order of
terms. Fails if List is empty.
- See also
- - compare/3
- - max_list/2 for the maximum of a list of numbers.
- min_member(-Min, +List) is semidet
- True when Min is the smallest member in the standard order of
terms. Fails if List is empty.
- See also
- - compare/3
- - min_list/2 for the minimum of a list of numbers.
- max_member(:Pred, -Max, +List) is semidet
- True when Max is the largest member according to Pred, which must be
a 2-argument callable that behaves like (@=<)/2. Fails if List is
empty. The following call is equivalent to max_member/2:
?- max_member(@=<, X, [6,1,8,4]).
X = 8.
- See also
- - max_list/2 for the maximum of a list of numbers.
- min_member(:Pred, -Min, +List) is semidet
- True when Min is the smallest member according to Pred, which must
be a 2-argument callable that behaves like (@=<)/2. Fails if List is
empty. The following call is equivalent to max_member/2:
?- min_member(@=<, X, [6,1,8,4]).
X = 1.
- See also
- - min_list/2 for the minimum of a list of numbers.
- sum_list(+List, -Sum) is det
- Sum is the result of adding all numbers in List.
- max_list(+List:list(number), -Max:number) is semidet
- True if Max is the largest number in List. Fails if List is
empty.
- See also
- - max_member/2.
- min_list(+List:list(number), -Min:number) is semidet
- True if Min is the smallest number in List. Fails if List is
empty.
- See also
- - min_member/2.
- numlist(+Low, +High, -List) is semidet
- List is a list [Low, Low+1, ... High]. Fails if High < Low.
- Errors
- -
type_error(integer, Low)
- -
type_error(integer, High)
- is_set(@Set) is semidet
- True if Set is a proper list without duplicates. Equivalence is
based on ==/2. The implementation uses sort/2, which implies
that the complexity is N*
log(N)
and the predicate may cause a
resource-error. There are no other error conditions.
- list_to_set(+List, ?Set) is det
- True when Set has the same elements as List in the same order.
The left-most copy of duplicate elements is retained. List may
contain variables. Elements E1 and E2 are considered
duplicates iff E1 == E2 holds. The complexity of the
implementation is N*
log(N)
.
- Errors
- - List is type-checked.
- See also
- - sort/2 can be used to create an ordered set. Many
set operations on ordered sets are order N rather than
order N**2. The list_to_set/2 predicate is more
expensive than sort/2 because it involves, two sorts
and a linear scan.
- Compatibility
- - Up to version 6.3.11, list_to_set/2 had complexity
N**2 and equality was tested using =/2.
- intersection(+Set1, +Set2, -Set3) is det
- True if Set3 unifies with the intersection of Set1 and Set2. The
complexity of this predicate is |Set1|*|Set2|. A set is defined to
be an unordered list without duplicates. Elements are considered
duplicates if they can be unified.
- See also
- - ord_intersection/3.
- union(+Set1, +Set2, -Set3) is det
- True if Set3 unifies with the union of the lists Set1 and Set2. The
complexity of this predicate is |Set1|*|Set2|. A set is defined to
be an unordered list without duplicates. Elements are considered
duplicates if they can be unified.
- See also
- - ord_union/3
- subset(+SubSet, +Set) is semidet
- True if all elements of SubSet belong to Set as well. Membership
test is based on memberchk/2. The complexity is |SubSet|*|Set|. A
set is defined to be an unordered list without duplicates.
Elements are considered duplicates if they can be unified.
- See also
- - ord_subset/2.
- subtract(+Set, +Delete, -Result) is det
- Delete all elements in Delete from Set. Deletion is based on
unification using memberchk/2. The complexity is |Delete|*|Set|. A
set is defined to be an unordered list without duplicates.
Elements are considered duplicates if they can be unified.
- See also
- - ord_subtract/3.
Undocumented predicates
The following predicates are exported, but not or incorrectly documented.
- memberchk(Arg1, Arg2)