Path: utzoo!attcan!uunet!mcsun!hp4nl!swi.psy.uva.nl!anjo
From: anjo@swi.psy.uva.nl (Anjo Anjewierden)
Newsgroups: comp.lang.prolog
Subject: Definition of list_length/2
Message-ID: <2733@swi.swi.psy.uva.nl>
Date: 13 Feb 90 15:27:17 GMT
Reply-To: anjo@swi.psy.uva.nl ()
Organization: SWI, UvA, Amsterdam
Lines: 38

Below is a definition of list_length/2 (length/2 in Edinburgh
compatibles).  I would like to know whether there is a better/shorter
solution that handles all cases and is deterministic given certain
calling patterns (see definition below).

-- Anjo.


%   list_length(?List, ?Length)
%
%   List is a list of length Length.
%
%   Note: list_length/2 is deterministic if the arguments are
%   sufficiently instantiated (i.e. if Length is given and/or
%   List is a proper list), otherwise it generates alternative
%   solutions on backtracking.

list_length(List, Length) :-
        integer(Length), !, 
        list_length_det(List, Length).	% Always deterministic
list_length(List, Length) :- 
	var(Length),
        list_length_non(List, Length).	% Possibly non-deterministic
         
/* 
list_length_non(List, N) :-		% This clause only if the Prolog
        List == [], !,			%  does not perform clause indexing.
        N = 0.
*/         
list_length_non([], 0). 
list_length_non([_|Xs], N) :-
        list_length_non(Xs, M),
        N is M+1.
         
list_length_det([], 0) :- !.
list_length_det([_|Xs], N) :-
        M is N-1,
        list_length_det(Xs, M).