Reading and storing all words of file in prolog

Question

I am a newbie to prolog, till now I am able to read all words of file, displayed them one by one, now I want to store them in a list(one by one, as I soon as I am displaying them). All logic for append given everywhere, append content of two lists in an empty list. For example append(new_word,word_list,word_List), intially my word_list is empty, so everything fine, but afterwards it says no, and stop at that point. Need help to be able to store element in list one by one.

Answer 1

You can use difference lists :

file_to_list(W, L) :-
   read_word(Word),
   append_dl(W, [Word|U]-U, Ws),
   !, file_to_list(Ws, L).

file_to_list_1(Ws, Ws).


append_dl(X-Y, Y-Z, X-Z).

You call file_to_list(UU, L-[]) to get the list of words. There is no slowdown but takes more inferences than CapelliC's code (one per word).

Answer 2

Let say you have a predicate read_word(W) that fetch W and fails when there are no more. Then with this definition

file_to_list_rev([W|Ws]) :-
   read_word(W),
   !, file_to_list_rev(Ws).
file_to_list_rev([]).

a query file_to_list_rev(R),reverse(R, L) , will store all words (in reverse order) in R, and in L the ordered list.

edit I must still be sleeping... That code dosn't build a reverse list... please consider

file_to_list([W|Ws]) :-
   read_word(W),
   !, file_to_list(Ws).
file_to_list([]).

and drop altogether the reverse/2 at end.

edit done

Note that appending each word to the end of the list would result in big slowdown, turning a 'linear size' problem into a 'quadratic' one. Anyway, this could be accomplished in this ugly way (note we need a service variable):

file_to_list(L) :-
    file_to_list([], L).

file_to_list(SoFar, L) :-
   read_word(W),
   append(SoFar, [W], Temp),
   !, file_to_list(Temp, L).
file_to_list(L, L).