lists.pl -- List Manipulation
This library provides commonly accepted basic predicates for list manipulation in the Prolog community. Some additional list manipulations are built-in. See e.g., memberchk/2, length/2.
The implementation of this library is copied from many places. These include: "The Craft of Prolog", the DEC-10 Prolog library (LISTRO.PL) and the YAP lists library. Some predicates are reimplemented based on their specification by Quintus and SICStus.
- 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:
- 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.
- prefix(?Part, ?Whole)
- True iff Part is a leading substring of Whole. This is the same
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
?- select(b, [a,b,c,b], 2, X). X = [a, 2, c, b] ; X = [a, b, c, 2] ; false.
- 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.
- nth0(?Index, ?List, ?Elem)
- True when Elem is the Index'th element of List. Counting starts at 0.
- nth1(?Index, ?List, ?Elem)
- Is true when Elem is the Index'th element of List. Counting starts at 1.
- 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
semidetif List is a list and
multiif List is a partial list.
- 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.
- 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
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.
- 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.
- clumped(+Items, -Pairs)
- Pairs is a list of
Item-Countpairs 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].
- max_member(-Max, +List) is semidet
- True when Max is the largest member in the standard order of terms. Fails if List is empty.
- min_member(-Min, +List) is semidet
- True when Min is the smallest member in the standard order of terms. Fails if List is empty.
- 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.
- 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.
- 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.
- min_list(+List:list(number), -Min:number) is semidet
- True if Min is the smallest number in List. Fails if List is empty.
- numlist(+Low, +High, -List) is semidet
- List is a list [Low, Low+1, ... High]. Fails if High < Low.
- 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*
- 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.
- 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.
- 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.
- 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.
The following predicates are exported, but not or incorrectly documented.