Extracting element from nested lists

Question

I know that to extract the third element from a simple list containing four elements such as [1,2,3,4] I would do so as below.

third([_,_,E,_], E).

I would like to extract the third element from a group of nested lists. For example if I had [[1,2,3,4],[5,6,7,8],[9,10,11,12]], I would want an output of [3,7,11].

Any help would be much appreciated.

score 2 · Answer 1 · edited May 23 '17 at 12:10

2

Use the widely available meta-predicate maplist/3 like in the following sample query:

?- maplist(third, [[1,2,3,4],[5,6,7,8],[9,10,11,12]], Xs).
Xs = [3, 7, 11].

edited May 23 '17 at 12:10

Community

1
1

answered Mar 13 '16 at 06:35

repeat

18,496
4
54
166

Arĥimedeς ℳontegasppα ℭacilhας · Accepted Answer · 2016-03-14T16:34:04.050

0

My guess:

third([], []) :- !.
third([[_, _, E|_]|Rest], [E|R]) :- third(Rest, R).

I know some people dislike cut (!), but it sounds enough to me.

[update 2016-03-14]

A robuster version:

third([], []).
third([Xs|Rest], R) :-
    length(Xs, Len),
    (Len < 3 -> third(Rest, R);
                (Xs = [_, _, E|_],
                 R = [E|R1],
                 third(Rest, R1))).

[/update]

edited Mar 14 '16 at 16:34

answered Mar 13 '16 at 02:52

Arĥimedeς ℳontegasppα ℭacilhας

1,073
9
18

1

Why use [tag:prolog-cut] here? Is there even a single benefit of doing so? – repeat Mar 13 '16 at 06:31
Nop. I did it just to make clear the stop clause. You can ignore it. – Arĥimedeς ℳontegasppα ℭacilhας Mar 13 '16 at 13:56
Thanks, this is exactly what I needed :) – Mateusz Wesoly Mar 13 '16 at 14:29
1

@Arĥimedeςℳontegasppαℭacilhας. What is the advantage of that? All I see right now is that it makes the most general use `third(Xs,Ys)` incomplete. – repeat Mar 13 '16 at 21:20
@MateuszWesoly Just be careful, ’cause this rule doesn’t support lists with less then three elements. – Arĥimedeς ℳontegasppα ℭacilhας Mar 14 '16 at 00:23
@repeat There’s a step clause, and a stop one. I put the cut to mark the stop. What makes this rule incomplete is it doesn’t handle lists with less then three elements. – Arĥimedeς ℳontegasppα ℭacilhας Mar 14 '16 at 00:25
2

`?- third(Xs,_), Xs = [_].` fails, `?- Xs = [_], third(Xs,_).` succeeds! *That's* incomplete. – repeat Mar 14 '16 at 10:52

Ans Piter · Answer 3 · 2016-03-14T21:28:46.683

certainly, the solution of @repeat (maplist predicat) is the elegant answer however, this is a simplest answer :

version 1 :

third_([], []).
third_([List|Rest], [R|Res]) :-
        third(List, R),
        third_(Rest, Res).

version 2

third_([], []).
third_([[_,_,R,_]|Rest], [R|Res]) :-
        third_(Rest, Res).

test :

| ?- third_(Xs,Res).
Xs = [],
Res = [] ? ;
Xs = [[_A,_B,_C,_D]],
Res = [_C] ? ;
Xs = [[_A,_B,_C,_D],[_E,_F,_G,_H]],
Res = [_C,_G] ? ;
Xs = [[_A,_B,_C,_D],[_E,_F,_G,_H],[_I,_J,_K,_L]],
Res = [_C,_G,_K] ?

Extracting element from nested lists

3 Answers3