[m-users.] Why does list.length use an auxiliary function?
zoltan.somogyi at runbox.com
Sun Dec 23 21:16:58 AEDT 2018
On Sun, 23 Dec 2018 01:10:36 -0500, Jeffrey Brown <jeffbrown.the at gmail.com> wrote:
> simple_length( , 0 ).
> simple_length( [_|Rest], N ) :-
> simple_length( Rest, N0 ),
> N = 1 + N0.
> Why is it instead this more complex one?
> length_acc(, N, N).
> length_acc([_ | L1], N0, N) :-
> N1 = N0 + 1,
> list.length_acc(L1, N1, N).
Because length_acc is tail recursive (i.e. the recursive clause does nothing
after the recursive call, while simple_length is NOT tail recursive (since
it increments the result of the recursive call). This makes the accumulator version
more efficient. See the wikipedia page on tail recursion.
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