CODEWITHSUNDEEP
prolog
Itai
Itai

Reputation: 23

Prolog - simple predicate - filter a list of pairs

I have a list of pairs (represent some locations in a matrix) and I like to filter the list to contain only locations at raw i. this is what I wrote:

getRaw(_,[],[]).
getRaw(i,[(i,j)|LocationsTail],[(i,j)|result]) :-   
    getRaw(i,LocationsTail,result).
getRaw(i,[(k,t)|LocationsTail],result) :-
    i\=k,
    getRaw(i,LocationsTail,result).

This is what I wrote at the console:

1 ?- getRaw(1,[(1,2)],R).

false.

Whats wrong with it?

Upvotes: 1

Views: 159

Answers (2)

Itai
Itai

Reputation: 23

the problem was the lower case of the variables. here is the fixed code:

getRaw(_,[],[]).
getRaw(Raw,[(Raw,Col)|LocationsTail],[(Raw,Col)|Result]) :- 
    getRaw(Raw,LocationsTail,Result).
getRaw(Raw,[(K,_)|LocationsTail],Result) :-
    Raw\=K,
    getRaw(Raw,LocationsTail,Result).

Upvotes: 0

false
false

Reputation: 10152

What's wrong: Variables start with a capital letter, you are using an incorrect version of negation. Here is a clean approach:

First, define the precise condition you are after in a separate predicate:

fst_pair(X,(X,_)).

Then, try to define a more general predicate, that has its truth value as a separate argument:

fst_pair_truth(X,(X,_),true).
fst_pair_truth(X,(Y,_),false) :-
   dif(X,Y).

Note that the negative case is not simply the negation of the positive case: Both expect pairs.

Now, your definition is, maybe reconsider the name...

getRaw(I, Xs, Ys) :-
   tfilter(fst_pair_truth(I), Xs, Ys).

See this answer for a definition of tfilter/3.

Upvotes: 1

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