CODEWITHSUNDEEP
prolog
jc2000
jc2000

Reputation: 13

Looping through and modifying a separate list in Prolog

I have a list called X and I am trying to iterate through the list, I have a separate list Y where the elements of Y are (U,V) where U is X+1 and V is X-1. I have been trying to append U and V to Y with no success. SO it should work such that if X is [4,5,6] Y will be [(3,5),(4,6),(5,7)]

/**
list_looping([], _).
list_looping([H|T],A) :-
   x is H-1,
   x is H+1,
   append([(x,y),T],A).

pairs([],[]).
pairs([H|T],[(U,V),L]):-
   list_looping([H|T],[(U,V),L]).
*/
pairs(X,Y) :-
   forall(member(X,[1,2,3]),((U is X+1,V is X-1),writeln((U,V)))).

I have tried two different implementations, in the first I am trying to recursively search through the list, with little success. In the second I am trying to use Prolog's inbuilt forall function, this obviously doesn't work either.

Upvotes: 1

Views: 67

Answers (1)

false
false

Reputation: 10102

list_pairs([], []).
list_pairs([X|Xs], [(U,V)|UVs]) :-
   U is X-1,
   V is X+1,
   list_pairs(Xs, UVs).

or more compactly

list_pairs2(Xs, UVs) :-
   maplist(e_pair, Xs, UVs).

e_pair(X, (U,V)) :-
   U is X-1,
   V is X+1.

and most compactly using library(lambda) for SICStus|SWI

list_pairs3(Xs, UVs) :-
   maplist(\X^(U,V)^( U is X-1, V is X+1 ), Xs, UVs).

Most frequently, one does not even name these predicates but uses the maplist-goal directly.

Upvotes: 3

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