Prolog character stream error
My code is supposed to stop when it finds stop in the file its reading from, but its not. I keep getting an error:
% reads in a character and then checks whether this character is a blank,
% a carriage return or the end of the stream. In any of these cases a
% complete word has been read otherwise the next character is read.
calculate([stop],_) :- !.
calculate([],_):-!.
calculate([Word|Rest],X) :-
word_to_number(Word,Symbol),
concat(X,Symbol,NewX),
calculate(Rest,NewX),
atom_to_term(NewX,Eq,[]),
print('Calculating '),print(NewX),print(' The result is: '),
Result is Eq,
print(Result),nl,
execute.
Any help wo开发者_如何学Gould be greatly appreciated!
Declare "plus", "minus" and "times" as operators, and you can use read/1 to read Prolog terms directly, since the input is then valid Prolog syntax.
The problem is that calculate is recursive. At some point, calculate([one], '03+')
called, which in turn calls calculate([], '03+1')
, which gives a result (4). It then invokes execute and processes the rest of the input.
Then, the calling calculate succeeds, and now goes on to applying atom_to_term
to '03+'
, which gives the error.
You can fix this by moving the conversion to an atom into a separate predicate:
to_atom([Word], Symbol) :- word_to_number(Word, Symbol).
to_atom([Word|Rest], Term) :-
word_to_number(Word,Symbol),
to_atom(Rest, Symbol2),
concat(Symbol,Symbol2,Term).
...
calculate(List) :-
to_atom(List, NewX),
atom_to_term(NewX,Eq,[]),
...
Then you won't need the dummy 0 in the beginning, either.
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