Question

I’m learning Prolog and I need idea how to convert a list of strings:

['f(a,45)', 'f(b,13)', 'f(c,12)']

into a list of pairs that looks like this:

[[45,'a'],[13,'b'],[12,'c']]

Answer 1

That’s a list of atoms, not strings. Strings in Prolog are usually list of character codes, expressed like

["f(a,45)", "f(b,13)", "f(c,12)"]

Anyway, apply a conversion to each element with a recursive function:

convert([], []).
convert([Atom|Atoms], [Pair|Pairs]) :-
  convert_element(Atom, Pair),
  convert(Atoms, Pairs).

Instead of recursion you could use maplist/3 in this way:

convert(As, Ps) :- maplist(convert_element, As, Ps).

To convert an element, you need a parser. DCGs are convenient:

convert_element(Atom, [N, A]) :-
  atom_codes(Atom, Codes),
  phrase(("f(", atomc(Ac), ",", numc(Nc), ")"), Codes, []),
  atom_codes(A, Ac),
  number_codes(N, Nc).

atomc(A) --> ([C], {is_lowerc(C)}, atomc(Cs), {A = [C|Cs]}) ; {A = []}.
numc(N) --> ([C], {is_numc(C)}, numc(Cs), {N = [C|Cs]}) ; {N = []}.

is_lowerc(C) :- C @>= 0'a, C @=< 0'z.
is_numc(C) :- C @>= 0'0, C @=< 0'9.

test:

?- convert(['f(a,45)', 'f(b,13)', 'f(c,12)'],L).
L = [[45, a], [13, b], [12, c]] .

atomc//1 & numc//1 are expressed in a compact way, but are very simple recursive pattern matching procedures, i.e. atomc//1 could be

atomc([C|Cs]) --> [C], {is_lowerc(C)}, !, atomc(Cs).
atomc([]) --> [].