CODEWITHSUNDEEP
listindexingprologelement
Wirsing
Wirsing

Reputation: 45

Prolog - IndexofElement

I have to find all Indexes of a given Element in a List and return it as a list. So for example:

?- indexList(a,[a,b,c,a,b],Rs).
Rs = [0,3]

My Idea is:

positions( E, L, P ) :- positions( E, L, 0, Rs ).
positions( E, [], _, [] ).
positions( E, [E|Xs], P, [P|PT] ) :- P1 is P + 1, positions( E, Xs, P1, PT ).
positions( E, [X|Xs], P, PT ) :-  P1 is P+1, positions( E, Xs, P1, PT ).

When I trace this I get a List in which every Position is in (in the step before the last step). But in the last steps he somehow changes it and returns just true.

Upvotes: 2

Views: 255

Answers (2)

gusbro
gusbro

Reputation: 22585

Your problem is that instead of using Rs to return the list of positions you are using P. You should have seen a lint warning (singleton variables P and Rs) in positions/3 that should have warned you of this.

Also, in the third clause of positions/4 you should check that E is different than X:

positions( E, L, Rs ) :- positions( E, L, 0, Rs ).
positions( E, [], _, [] ).
positions( E, [E|Xs], P, [P|PT] ) :- P1 is P + 1, positions( E, Xs, P1, PT ).
positions( E, [X|Xs], P, PT ) :-  E\=X, P1 is P+1, positions( E, Xs, P1, PT ).

Upvotes: 2

Mihai
Mihai

Reputation: 149

You made a small mistake in the first line. You are invoking positions(E, L, 0, Rs), but the last parameter should be P, in order to make a binding with the last parameter with the same name from the head of the clause. So the first line should look like this:

positions(E, L, P) :- positions(E, L, 0, P).

Upvotes: 2

Related Questions