Read a file line by line in Prolog

Question

I'd like to read a plain text file and apply a predicate to each line (the predicates contain write which does the output). How would I do that?

Answer 1

You can use read to read the stream. Remember to invoke at_end_of_stream to ensure no syntax errors.

Example:

readFile.pl

main :-
    open('myFile.txt', read, Str),
    read_file(Str,Lines),
    close(Str),
    write(Lines), nl.

read_file(Stream,[]) :-
    at_end_of_stream(Stream).

read_file(Stream,[X|L]) :-
    \+ at_end_of_stream(Stream),
    read(Stream,X),
    read_file(Stream,L).

myFile.txt

'line 0'.
'line 1'.
'line 2'.
'line 3'.
'line 4'.
'line 5'.
'line 6'.
'line 7'.
'line 8'.
'line 9'.

Thus by invoking main you will recieve the output:

?- main.
[line 0,line 1,line 2,line 3,line 4,line 5,line 6,line 7,line 8,line 9]
true 

Just configure main. The output here is an example by using write, of course. Configure to match your request.

I assume that this principle can be applied to answer your question. Good luck.