Question

I have a file with a specific format that I would like to parse. In this file I have a number on a line which specifies the number of lines to follow it.

example excerpt from the file:

3 // number of lines to follow = 3
1 3 // 1st line
3 5 // 2nd line
2 7 // 3rd line

I want to read the number (3 in this example) and then read the following 3 lines only. If there are less or more lines I want to generate an error. How can I do this in javacc?

Thank you

Answer 1

Here’s a way:

/* file: Lines.jj */
options {
    STATIC = false ;
}

PARSER_BEGIN(Lines)

class Lines {
    public static void main(String[] args) throws ParseException, TokenMgrError {
        Lines parser = new Lines(System.in);
        System.out.println("diff="+parser.parse());        
    }
}

PARSER_END(Lines)

SKIP  : { " " }
TOKEN : { <NL : ("\r\n" | "\n" | "\r")> }
TOKEN : { <NUMBER : (["0"-"9"])+> }

int parse() :
{
    int linesToRead = 0;
    int linesRead = 0;
    Token n = null;
}
{
    n = <NUMBER> <NL> {linesToRead = Integer.parseInt(n.image);}
    (
        (<NUMBER>)+ <NL> {linesRead++;} 
    )*
    <EOF>
    {return linesToRead - linesRead;}
}

The Lines.parse() : int method returns the difference between the number of lines it is supposed to read, and the actual number of lines that are read.

Generate the parser like this:

javacc Lines.jj

compile all classes:

java *.java

And feed it your test data (test.txt):

3
1 3
3 5
2 7

like this:

java Lines < test.txt

which produces the output:

diff=0