I Tried to write a simple code in Prolog which translate a list to another list. for instance, if we call listtrans([a,b,c],L), L will become [1,2,3]. (a,b,c is replaced with 1,2,3). But i faced with a syntax error in last line. what is the problem? here is my code:
trans(a,1).
trans(b,2).
trans(c,3).
listtrans([],L).
listtrans([H|T],L1):-
trans(H,B),
append(B,L,L2),
listtrans(T,L2).
There are no syntactic errors, only semantic ones. This is it, with corrections:
listtrans([],[]).
listtrans([H|T],L1):-
trans(H,B),
append([B],L2,L1),
listtrans(T,L2).
But this is not Prolog style. One writes rather:
listtrans([],[]).
listtrans([A|As],[I|Is]):-
trans(A,I),
listtrans(As,Is).
Note that in Prolog explicitly appending elements is much rarer than in languages supporting functions. As an extra bonus, you can now use the relation in both directions:
?- listtrans([a,b,c],Is).
Is = [1,2,3].
?- listtrans(As, [1,2,3]).
As = [a,b,c].
And you can write this more compactly:
listtrans(As, Is) :-
maplist(trans, As, Is).
See this for more.
The error is very likely because in your code:
listtrans([H|T],L1):-
trans(H,B),
append(B,L,L2),
listtrans(T,L2).
the variable L1 is declared in the head, but not referenced anywhere: you mispelled something?
Anyway, your code is not going to work.
Moreover, using append/3 for this kind of tasks which are easily defined by recursion is considered terrible (also because of the bad performance you get out of it).
Applying a function to a list is straightforward. You already know that in prolog you don't write Y = f(X) but rather declare the functional relation between X and Y as: f(X, Y).. (That's basically what you did with trans(X,Y)).
Now the (easy) recursive formulation:
[X|Xs] is [Y|Ys] if trans(X,Y) and we recursively transform Xs into Ysor expressed in prolog:
listtrans([],[]).
listtrans([X|Xs],[Y|Ys]) :- trans(X,Y), listtrans(Xs,Ys).
I recommend you reading the first 4 chapters of Learn Prolog Now to better understand these concepts.
