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#1
April 9th, 2008, 10:45 AM
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Other Language - PROLOG Problem
am trying to print the elements of a list.... one per line using a predicate of arity 1 .... which I am able to do with the code:
Code:
display_list([]) :- !, write('\nThe list is empty'). % handles empty list
display_list([H|T]) :- write(' '), write(H), nl, display_list
The problem is: I want to have the sequence number of the list element in front of it: For example, 1 North 2 South 3 East 4 West I am not able to able to do that. The code I am writing is: Code:
increment(X,Y) :- Y is X+1.
display_list([]) :- !, write('\nThe list is empty').
display_list([H|T]) :- X = 0, increment(X,Y), write(Y),write(' '), write(H), nl, display_list(T).increment(X,Y) :- Y is X+1.
The output I get as expected is: 11 ?- display_list([north, south, east]). 1 north 1 south 1 east The list is empty true. Also I keep on getting this 'The list is empty' everytime the elements are exhausted and not only when the input list is empty ..... Please advice ...... Thanks
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#2
April 11th, 2008, 04:40 AM
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I've never really worked with Prolog before, so I may be off here (especially regarding where to put the cut), but from what I read here I think you need a rule that matches a one-element list between the empty list match and the match for a mist of more than one element:
Code:
display_list([H]) :- X = 0, increment(X,Y), write(Y),write(' '), write(H), nl, !
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#3
June 1st, 2008, 09:58 PM
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Unless there's a particular reason that you're limiting yourself to a single predicate then try this:
Code:
display_list(List) :- display_list(List,1).
display_list([Head|Tail],Count) :-
Number is Count + 1,
write(Count), write(' '), write(Head), nl,
display_list(Tail,Number).
Providing a simplified interface to a predicate like this is a very common idiom in Prolog. It also allows access to the raw predicate should you need it. The output is as expected. Code:
?- display_list(['North','South','East','West']). 1 North 2 South 3 East 4 West fail. ?- display_list(['North','South','East','West'], 0). 0 North 1 South 2 East 3 West fail. ?- I hope this helps, Mark.
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