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yap-6.3/library/lists.yap
2001-04-27 16:02:43 +00:00

293 lines
8.1 KiB
Prolog

% This file has been included as an YAP library by Vitor Santos Costa, 1999
%
% This file includes code from Bob Welham, Lawrence Byrd, and R. A. O'Keefe.
%
:- module(lists,[append/3,
delete/3,
is_list/1,
last/2,
member/2,
memberchk/2,
nextto/3,
nth/3,
nth/4,
nth0/3,
nth0/4,
permutation/2,
prefix/2,
remove_duplicates/2,
reverse/2,
same_length/2,
select/3,
sublist/2,
substitute/4,
sum_list/2,
suffix/2,
sumlist/2,
list_concat/2
]).
% append(Prefix, Suffix, Combined)
% is true when all three arguments are lists, and the members of Combined
% are the members of Prefix followed by the members of Suffix. It may be
% used to form Combined from a given Prefix and Suffix, or to take a given
% Combined apart. E.g. we could define member/2 (from SetUtl.Pl) as
% member(X, L) :- append(_, [X|_], L).
append([], L, L).
append([H|T], L, [H|R]) :-
append(T, L, R).
% delete(List, Elem, Residue)
% is true when List is a list, in which Elem may or may not occur, and
% Residue is a copy of List with all elements identical to Elem deleted.
delete([], _, []).
delete([Head|List], Elem, Residue) :-
Head == Elem, !,
delete(List, Elem, Residue).
delete([Head|List], Elem, [Head|Residue]) :-
delete(List, Elem, Residue).
% is_list(List)
% is true when List is a proper List
%
is_list(L) :- var(L), !, fail.
is_list([]).
is_list([_|List]) :- is_list(List).
% last(List, Last)
% is true when List is a List and Last is identical to its last element.
% This could be defined as last(L, X) :- append(_, [X], L).
last([H|List], Last) :-
last(List, H, Last).
last([], Last, Last).
last([H|List], _, Last) :-
last(List, H, Last).
% member(?Element, ?Set)
% is true when Set is a list, and Element occurs in it. It may be used
% to test for an element or to enumerate all the elements by backtracking.
% Indeed, it may be used to generate the Set!
member(Element, [Element|_]).
member(Element, [_|Rest]) :-
member(Element, Rest).
% memberchk(+Element, +Set)
% means the same thing, but may only be used to test whether a known
% Element occurs in a known Set. In return for this limited use, it
% is more efficient when it is applicable.
memberchk(Element, [Element|_]) :- !.
memberchk(Element, [_|Rest]) :-
memberchk(Element, Rest).
% nextto(X, Y, List)
% is true when X and Y appear side-by-side in List. It could be written as
% nextto(X, Y, List) :- append(_, [X,Y], List).
% It may be used to enumerate successive pairs from the list.
nextto(X,Y, [X,Y|_]).
nextto(X,Y, [_|List]) :-
nextto(X,Y, List).
% nth0(+N, +List, ?Elem) is true when Elem is the Nth member of List,
% counting the first as element 0. (That is, throw away the first
% N elements and unify Elem with the next.) It can only be used to
% select a particular element given the list and index. For that
% task it is more efficient than nmember.
% nth(+N, +List, ?Elem) is the same as nth0, except that it counts from
% 1, that is nth(1, [H|_], H).
nth0(0, [Head|_], Head) :- !.
nth0(N, [_|Tail], Elem) :-
nonvar(N), !,
M is N-1,
nth0(M, Tail, Elem).
nth0(N,[_|T],Item) :- % Clause added KJ 4-5-87 to allow mode
nth0(M,T,Item),
N is M + 1.
nth(1, [Head|_], Head) :- !.
nth(N, [_|Tail], Elem) :-
nonvar(N), !,
M is N-1, % should be succ(M, N)
nth(M, Tail, Elem).
nth(N,[_|T],Item) :- % Clause added KJ 4-5-87 to allow mode
% nth(-,+,+)
nth(M,T,Item),
N is M + 1.
% nth0(+N, ?List, ?Elem, ?Rest) unifies Elem with the Nth element of List,
% counting from 0, and Rest with the other elements. It can be used
% to select the Nth element of List (yielding Elem and Rest), or to
% insert Elem before the Nth (counting from 1) element of Rest, when
% it yields List, e.g. nth0(2, List, c, [a,b,d,e]) unifies List with
% [a,b,c,d,e]. nth is the same except that it counts from 1. nth
% can be used to insert Elem after the Nth element of Rest.
nth0(0, [Head|Tail], Head, Tail) :- !.
nth0(N, [Head|Tail], Elem, [Head|Rest]) :-
nonvar(N),
M is N-1,
nth0(M, Tail, Elem, Rest).
nth0(N, [Head|Tail], Elem, [Head|Rest]) :- % Clause added KJ 4-5-87
var(N), % to allow mode
nth0(M, Tail, Elem, Rest), % nth0(-,+,+,?).
N is M+1.
nth(1, [Head|Tail], Head, Tail) :- !.
nth(N, [Head|Tail], Elem, [Head|Rest]) :-
nonvar(N),
M is N-1,
nth(M, Tail, Elem, Rest).
nth(N, [Head|Tail], Elem, [Head|Rest]) :- % Clause added KJ 4-5-87
var(N), % to allow mode
nth(M, Tail, Elem, Rest), % nth(-,+,+,?).
N is M+1.
% permutation(List, Perm)
% is true when List and Perm are permutations of each other. Of course,
% if you just want to test that, the best way is to keysort/2 the two
% lists and see if the results are the same. Or you could use list_to_bag
% (from BagUtl.Pl) to see if they convert to the same bag. The point of
% perm is to generate permutations. The arguments may be either way round,
% the only effect will be the order in which the permutations are tried.
% Be careful: this is quite efficient, but the number of permutations of an
% N-element list is N!, even for a 7-element list that is 5040.
permutation([], []).
permutation(List, [First|Perm]) :-
select(First, List, Rest), % tries each List element in turn
permutation(Rest, Perm).
% prefix(Part, Whole) iff Part is a leading substring of Whole
prefix([], _).
prefix([Elem | Rest_of_part], [Elem | Rest_of_whole]) :-
prefix(Rest_of_part, Rest_of_whole).
% remove_dups(List, Pruned)
% removes duplicated elements from List. Beware: if the List has
% non-ground elements, the result may surprise you.
remove_duplicates(List, Pruned) :-
sort(List, Pruned).
% reverse(List, Reversed)
% is true when List and Reversed are lists with the same elements
% but in opposite orders. rev/2 is a synonym for reverse/2.
reverse(List, Reversed) :-
reverse(List, [], Reversed).
reverse([], Reversed, Reversed).
reverse([Head|Tail], Sofar, Reversed) :-
reverse(Tail, [Head|Sofar], Reversed).
% same_length(?List1, ?List2)
% is true when List1 and List2 are both lists and have the same number
% of elements. No relation between the values of their elements is
% implied.
% Modes same_length(-,+) and same_length(+,-) generate either list given
% the other; mode same_length(-,-) generates two lists of the same length,
% in which case the arguments will be bound to lists of length 0, 1, 2, ...
same_length([], []).
same_length([_|List1], [_|List2]) :-
same_length(List1, List2).
% select(?Element, ?Set, ?Residue)
% is true when Set is a list, Element occurs in Set, and Residue is
% everything in Set except Element (things stay in the same order).
select(Element, [Element|Rest], Rest).
select(Element, [Head|Tail], [Head|Rest]) :-
select(Element, Tail, Rest).
% sublist(Sublist, List)
% is true when both append(_,Sublist,S) and append(S,_,List) hold.
sublist(Sublist, List) :-
prefix(Sublist, List).
sublist(Sublist, [_|List]) :-
sublist(Sublist, List).
% substitute(X, XList, Y, YList)
% is true when XList and YList only differ in that the elements X in XList
% are replaced by elements Y in the YList.
substitute(X, XList, Y, YList) :-
'$substitute'(XList, X, Y, YList).
'$substitute'([], _, _, []).
'$substitute'([X0|XList], X, Y, [Y|YList]) :-
X == X0, !,
'$substitute'(XList, X, Y, YList).
'$substitute'([X0|XList], X, Y, [X0|YList]) :-
'$substitute'(XList, X, Y, YList).
% suffix(Suffix, List)
% holds when append(_,Suffix,List) holds.
suffix(Suffix, Suffix).
suffix(Suffix, [_|List]) :-
suffix(Suffix,List).
% sumlist(Numbers, Total)
% is true when Numbers is a list of integers, and Total is their sum.
sumlist(Numbers, Total) :-
sumlist(Numbers, 0, Total).
sum_list(Numbers, Total) :-
sumlist(Numbers, 0, Total).
sumlist([], Total, Total).
sumlist([Head|Tail], Sofar, Total) :-
Next is Sofar+Head,
sumlist(Tail, Next, Total).
% list_concat(Lists, List)
% is true when Lists is a list of lists, and List is the
% concatenation of these lists.
list_concat(Lists, List) :-
list_concat(Lists, [], List).
list_concat([], []).
list_concat([H|T], L) :-
list_concat(H, L, Li),
list_concat(T, Li).
list_concat([], L, L).
list_concat([H|T], [H|Lf], Li) :-
list_concat(T, Lf, Li).