CODEWITHSUNDEEP
prolog
PSchmi
PSchmi

Reputation: 3

How can I pair all elements in a list in Prolog?

The goal is to create pairs/triplets/quartets from short lists, since these lists occur in a list of lists that I flatten. Since I want these elements to stay connected, I need a way to flatten the lists without losing the connection between the items in these particular lists.

In short, [a, b, c] needs to be converted to a-b-c. In theory long lists need to be handled too, but in reality only short lists will be relevant.

What I tried so far (which I know is horribly wrong):

create_pair([], Pair).

create_pair([H, H1|T], Pair):-
    NPair = H-H1, 
    create_pair(T, NPair).

This is just for the case of where the list has 2 elements.

Upvotes: 0

Views: 513

Answers (2)

Nicholas Carey
Nicholas Carey

Reputation: 74267

If the data structure you want to convert the short lists to doesn't really matter, you can just use =../2 to convert the list to a term. Something like:

list_term(L,T) :- T =.. [ listlet | L ].

So evaluating list_to_term( [a,b,c], T) binds T as listlet(a,b,c) and evaluating list_to_term( L , listlet(a,b,c,d) ) binds L as [a,b,c,d].

See https://swish.swi-prolog.org/p/list-to-term.pl for a runnable playground.

Upvotes: 0

gusbro
gusbro

Reputation: 22585

You can build your pair/triplet/quartet/... by joining the two first items of the list and replacing it with your connection term until the whole list is processed:

create_ntets([H], H).
create_ntets([H,H1|T], NTet):-
  create_ntets([H-H1|T], NTet).

This procedure assumes there is no 0-tet. Sample runs

?- create_ntets([a,b,c], Triplet).
Triplet = a-b-c

?- create_ntets([a,b,c,d], Quartet).
Quartet = a-b-c-d

Upvotes: 1

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