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Editorial Team
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Asked: 2026-05-24T01:40:20+00:00

I’ve been trying to parse a file containing lines of integers using phrase_from_file with

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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)
                     ) , !.
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1 Answer

  1. Editorial Team

    I guess there is a conceptional problem here. Although I don’t know the details of phrase_from_file/2, i.e. which Prolog system you are using, I nevertheless assume that it will produce character codes. So for an integer 123 in the file you will get the character codes 0’1, 0’2 and 0’3. This is probably not what you want.

    If you would like to process the characters, you would need to use a non-terminal instead of a bare bone variable I, to fetch them. And instead of the integer test, you would need a character test, and you can do the test earlier:

    line --> [I], {0'0=<I, I=<0'9}, line.

    Best Regards

    P.S.: Instead of going the DCG way, you could also use term read operations. See also:
    read numbers from file in prolog and sorting

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