Question

I already have the following code:

"atom_length(Var, Len) :- length(Var, Len)."

I wanna construct a predicate atom_lengths/2 that does the same thing with a list of atoms:

?-  atom_lengths([one, two, three, four], [3, 3, 5, 4]).
true.
?-  atom_lengths([one, two, three, four], LS).
LS = [3, 3, 5, 4].
?-  atom_lengths([], LS).
LS = [].

How to write “atom_lengths”??
Thanks in advance!

Answer 1

You cannot use length/2 to compute the length of an atom.
You can however first convert each atom to a list of characters with atom_chars/2 and then use length/2 to get its length:

atom_lengths([], []).
atom_lengths([Atom|Atoms], [Length|LAtoms]):-
  atom_chars(Atom, L),
  length(L, Length),
  atom_lengths(Atoms, LAtoms).

Test:

?- atom_lengths([one, two, three, four], LS).

LS = [3,3,5,4]

Instead of using the pair atom_chars/2-length/2 you could also use ISO builtin predicate atom_length/2:

atom_lengths([], []).
atom_lengths([Atom|Atoms], [Length|LAtoms]):-
  atom_length(Atom, Length),
  atom_lengths(Atoms, LAtoms).

or using findall/3:

atom_lengths(Atoms, LAtoms):-
  findall(Length, (member(Atom, Atoms), atom_length(Atom, Length)), LAtoms).

As suggested by commenter, a better idiom would be to use maplist/3:

atom_lengths(Atoms, LAtoms):-
  maplist(atom_length, Atoms LAtoms).