Question
I am trying to do a word calculator .. read words from a file .. translate them into numbers and then calculate the result .. i managed to do all of that but i think i have two bugs in my program ..
I mainly have two functions ...
extract(Words), calculate( Words,0).
extract will read from the file .. and then return a list of Words .. ex: [one,plus,three] .. now calculate will translate the value for these words into numbers and calculate .. i managed to do that also .. now the bugs are : i must stop reading and terminate if i encounter stop in the file .. so if Words was [stop] End. i tried the following ...
execute :-
extract(Words),
Words = [stop],nl,print('Terminating ...'),!.
execute :-
extract(Words),
calculate( Words,0).
it successfully terminates .. but it skips lines as i extract more than once .. i have tried to do ..
execute :-
extract(Words),
Words \= [stop],execute(Words).
execute(Words) :-
calculate( Words,0).
if the Words is not stop .. then go and calculate .. but its not working !!
i appreciate the help .. Thank You
Solution
Side-effects (here: reading from a file and moving on to the next term) are not undone on backtracking. You can read once and then make the choice based on the read term with if/then/else or an auxiliary predicate, for example:
execute :-
extract(Words),
( Words == [stop] -> nl, write('Terminating ...')
; calculate(Words, 0)
).
In SWI-Prolog, consider using library(pio)
for pure file reading described via dcg, which handles backtracking as expected.