Prolog Recursion in lists, last but one element

Question

The question is to find the last but one character in a list, e.g.

?- last_but_one(X, [a,b,c,d]).
X = c.

My code is:

last         


        

Answer 1

Your original version is much simpler to read. In particular, the recursive rule reads - reading it right-to-left

last_but_one(X, [_|T]) :- last_but_one(X, T).
                          ^^^^^^^^^^
                              provided X is the lbo-element in T

                       ^^  then, it follows, that (that's an arrow!)
^^^^^^^^^^^^^^^^^^^^^^
      X is also the lbo-element of T with one more element

In other words: If you have already an lbo-element in a given list T, then you can construct new lists with any further elements in front that also have the very same lbo-element.

One might debate which version is preferable as to efficiency. If you are really into that, rather take:

last_but_one_f1(E, Es) :-
   Es = [_,_|Xs],
   xs_es_lbo(Xs, Es, E).

xs_es_lbo([], [E|_], E).
xs_es_lbo([_|Xs], [_|Es], E) :-
   xs_es_lbo(Xs, Es, E).

or even:

last_but_one_f2(E, [F,G|Es]) :-
    es_f_g(Es, F, G, E).

es_f_g([], E, _, E).
es_f_g([G|Es], _, F, E) :-
   es_f_g(Es, F, G, E).

Never forget general testing:

| ?- last_but_one(X, Es).
Es = [X,_A] ? ;
Es = [_A,X,_B] ? ;
Es = [_A,_B,X,_C] ? ;
Es = [_A,_B,_C,X,_D] ? ;
Es = [_A,_B,_C,_D,X,_E] ? ;
Es = [_A,_B,_C,_D,_E,X,_F] ? ...

And here are some benchmarks on my olde labtop:

          SICStus     SWI
          4.3.2     7.3.20-1
    --------------+----------+--------
    you   0.850s  |   3.616s |  4.25×
    they  0.900s  |  16.481s | 18.31×
    f1    0.160s  |   1.625s | 10.16×
    f2    0.090s  |   1.449s | 16.10×
    mat   0.880s  |   4.390s |  4.99×
    dcg   3.670s  |   7.896s |  2.15×
    dcgx  1.000s  |   7.885s |  7.89×
    ap    1.200s  |   4.669s |  3.89×

The reason for the big difference is that both f1 and f2 run purely determinate without any creation of a choicepoint.

Using

bench_last :-
   \+ ( length(Ls, 10000000),
        member(M, [you,they,f1,f2,mat,dcg,dcgx,ap]), write(M), write(' '),
        atom_concat(last_but_one_,M,P), \+ time(call(P,L,Ls))
   ).