Opposite letter program.

[CaptainSafety]

Hello. I am new to prolog. I am trying switch letters in a string to the opposite letter in the alphabet. This is my program, I can swap one letter but not change an entire String. Help me please.

letter(a, z).
letter(b, y).
letter(c, x).
letter(d, w).
letter(e, v).
letter(f, u).
letter(g, t).
letter(h, s).
letter(i, r).
letter(j, q).
letter(k, p).
letter(l, o).
letter(m, n).
letter(n, m).
letter(o, l).
letter(p, k).
letter(q, j).
letter(r, i).
letter(s, h).
letter(t, g).
letter(u, f).
letter(v, e).
letter(w, d).
letter(x, c).
letter(y, b).
letter(z, a).

swap(X) :- letter(Y, X), write(Y).

 

[joel76]

You can use ASCII codes so letter will be written :

letter(X, Y) :-
	char_code(X, C),
	C1 is  219 - C,
	char_code(Y, C1).

Be aware that it works only for lower cases letters.
Now for a string it's quite different because string are list of codes

string(X, Y) :-
	Y is 219 - X.

change_letters_in_string(S1, S2) :-
	maplist(string, S1, ST),
	string_to_list(S2, ST).

And you get

?- change_letters_in_string("azbt", S2).
S2 = "zayg".