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fix nth and nth0 to accept unbound first argument.

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git-svn-id: https://yap.svn.sf.net/svnroot/yap/trunk@144 b08c6af1-5177-4d33-ba66-4b1c6b8b522a
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vsc 2001-08-27 15:27:29 +00:00
parent bef57eb26c
commit 94271789ef
2 changed files with 72 additions and 30 deletions
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22
docs/yap.tex
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@ -6655,7 +6655,7 @@ As @code{member/2}, but may only be used to test whether a known
@var{Element} occurs in a known Set. In return for this limited use, it
is more efficient when it is applicable.
@item nth0(+@var{N}, +@var{List}, ?@var{Elem})
@item nth0(?@var{N}, +@var{List}, ?@var{Elem})
@findex nth0/2
@syindex nth0/2
@cnindex nth0/2
@ -6665,14 +6665,14 @@ N elements and unify @var{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 @code{member/2}
@item nth(+@var{N}, +@var{List}, ?@var{Elem})
@item nth(?@var{N}, +@var{List}, ?@var{Elem})
@findex nth/2
@syindex nth/2
@cnindex nth/2
The same as @code{nth0/3}, except that it counts from
1, that is @code{nth(1, [H|_], H)}.
@item nth0(+@var{N}, ?@var{List}, ?@var{Elem}, ?@var{Rest})
@item nth0(?@var{N}, ?@var{List}, ?@var{Elem}, ?@var{Rest})
@findex nth0/4
@syindex nth0/4
@cnindex nth0/4
@ -6681,8 +6681,20 @@ counting from 0, and @var{Rest} with the other elements. It can be used
to select the Nth element of @var{List} (yielding @var{Elem} and @var{Rest}), or to
insert @var{Elem} before the Nth (counting from 1) element of @var{Rest}, when
it yields @var{List}, e.g. @code{nth0(2, List, c, [a,b,d,e])} unifies List with
@code{[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.
@code{[a,b,c,d,e]}. @code{nth/4} is the same except that it counts from 1. @code{nth0/4}
can be used to insert @var{Elem} after the Nth element of @var{Rest}.
@item nth(?@var{N}, ?@var{List}, ?@var{Elem}, ?@var{Rest})
@findex nth/4
@syindex nth/4
@cnindex nth/4
Unifies @var{Elem} with the Nth element of @var{List}, counting from 1,
and @var{Rest} with the other elements. It can be used to select the
Nth element of @var{List} (yielding @var{Elem} and @var{Rest}), or to
insert @var{Elem} before the Nth (counting from 1) element of
@var{Rest}, when it yields @var{List}, e.g. @code{nth(1, List, c,
[a,b,d,e])} unifies List with @code{[a,b,c,d,e]}. @code{nth/4}
can be used to insert @var{Elem} after the Nth element of @var{Rest}.
@item permutation(+@var{List},?@var{Perm})
@findex permutation/2

80
library/lists.yap
View File

@ -100,7 +100,7 @@ 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,
% 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
@ -108,29 +108,45 @@ nextto(X,Y, [_|List]) :-
% nth(+N, +List, ?Elem) is the same as nth0, except that it counts from
% 1, that is nth(1, [H|_], H).
nth0(V, In, Element) :- var(V), !,
generate_nth0(V, In, Element).
nth0(0, [Head|_], Head) :- !.
nth0(N, [_|Tail], Elem) :-
nonvar(N), !,
M is N-1,
nth0(M, Tail, Elem).
find_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.
find_nth0(0, [Head|_], Head) :- !.
find_nth0(N, [_|Tail], Elem) :-
M is N-1,
find_nth0(M, Tail, Elem).
generate_nth0(0, [Head|_], Head).
generate_nth0(I, [_|List], El) :-
generate_nth0(I1, List, El),
I is I1+1.
nth(V, In, Element) :- var(V), !,
generate_nth(V, In, Element).
nth(1, [Head|_], Head) :- !.
nth(N, [_|Tail], Elem) :-
nonvar(N), !,
M is N-1, % should be succ(M, N)
nth(M, Tail, Elem).
find_nth(M, Tail, Elem).
find_nth(1, [Head|_], Head) :- !.
find_nth(N, [_|Tail], Elem) :-
M is N-1,
find_nth(M, Tail, Elem).
generate_nth(1, [Head|_], Head).
generate_nth(I, [_|List], El) :-
generate_nth(I1, List, El),
I is I1+1.
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
@ -140,30 +156,44 @@ nth(N,[_|T],Item) :- % Clause added KJ 4-5-87 to allow mode
% [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(V, In, Element, Tail) :- var(V), !,
generate_nth0(V, In, Element, Tail).
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.
find_nth0(0, [Head|Tail], Head, Tail) :- !.
find_nth0(N, [Head|Tail], Elem, [Head|Rest]) :-
M is N-1,
find_nth0(M, Tail, Elem, Rest).
generate_nth0(0, [Head|Rest], Head, Rest).
generate_nth0(I, [Head|List], El, [Head|Rest]) :-
generate_nth0(I1, List, El, Rest),
I is I1+1.
nth(V, In, Element, Tail) :- var(V), !,
generate_nth(V, In, Element, Tail).
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.
find_nth(1, [Head|Tail], Head, Tail) :- !.
find_nth(N, [Head|Tail], Elem, [Head|Rest]) :-
M is N-1,
find_nth(M, Tail, Elem, Rest).
generate_nth(1, [Head|Rest], Head, Rest).
generate_nth(I, [Head|List], El, [Head|Rest]) :-
generate_nth(I1, List, El, Rest),
I is I1+1.
% permutation(List, Perm)