Using phrase_from_file to read a file's lines

Question

I've been trying to parse a file containing lines of integers using phrase_from_file with the grammar rules

line --> I,line,{integer(I)}.
line --> ['\n'].

thusly: phrase_from_file(line,'input.txt').

It fails, and I got lost very quickly trying to trace it. I've even tried to print I, but it doesn't even get there.

EDIT:: As none of the solutions below really fit my needs (using read/1 assumes you're reading terms, and sometimes writing that DCG might just take too long), I cannibalized this code I googled, the main changes being the addition of:

read_rest(-1,[]):-!.

read_word(C,[],C) :- ( C=32 ;
                       C=(-1)
                     ) , !.

As none of the solutions below really fit my needs (using read/1 assumes you're reading terms, and sometimes writing that DCG might just take too long), I cannibalized this code I googled, the main changes being the addition of: ` read_rest(-1,[]):-!. read_word(C,[],C) :- ( C=32 ; C=(-1) ) , !. ` — moshewe
You are mixing characters and codes. Either stick to characters with set_prolog_flag(double_quotes, chars) or stick to codes (less recommended) — false

false false · Accepted Answer · 2011-07-26T12:11:53

If you are using phrase_from_file/2 there is a very simple way to test your programs prior to reading actual files. Simply call the very same non-terminal with phrase/2. Thus, a goal

phrase(line,"1\n2").

is the same as calling

phrase_from_file(line,fichier)

when fichier is a file containing above 3 characters. So you can test and experiment in a very compact manner with phrase/2.

There are further issues @Jan Burse already mentioned. SWI reads in character codes. So you have to write

newline --> "\n".

for a newline. And then you still have to parse integers yourself. But all that is tested much easier with phrase/2. The nice thing is that you can then switch to reading files without changing the actual DCG code.

Using phrase_from_file to read a file's lines

2 Answers