Anonymous Monk has asked for the wisdom of the Perl Monks concerning the following question:

Hi,

I am facing an issue with using the %item hash for subrules which have a "(?)" repetition specifier (i.e. match one or zero times). As an example, please have a look at the code below: For the below code, I am expecting that I get following output input 

2
0
a

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instead, I get 

input
a

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The documentation states that when we use (s), the value associated with the rule is not a scalar but a reference to a list of scalars. Does this hold for the (?) construct too? If yes, then the below code should work as I dereference the value "$item{range}->[0]".

Thanks and Regards,

Aditya 

use Parse::RecDescent;

my $text = "input [2:0] a";

my $grammar = q {
    rule : 'input' range(?) identifier 
        {
            print $item[1]."\n";
            if (defined @{$item{range}->[0]}) {
                my @arr = @{$item{range}->[0]};
                foreach my $i (@arr) {
                    print $i."\n";
                }
            }
            print $item{identifier};
        }
    range : "[" from_num ":" to_num "]" {$return = [$item{from_num}, $
+item{to_num}]}
    from_num : number {$return = $item{number}}
    to_num : number {$return = $item{number}}
    number : /[0-9][0-9_]*/ 
    identifier : /[a-zA-Z][a-zA-Z_0-9\.]*/
};

my $parser = new Parse::RecDescent($grammar) or die "Bad Grammar";

$parser->rule($text);

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Code tags added by GrandFather

Replies are listed 'Best First'.
Re: RecDescent Issue: using item hash with (?) repetition specifier
by ikegami (Patriarch) on Apr 02, 2009 at 21:47 UTC

    Please don't put use <pre>. Use <c> instead. It has numerous benefits.

    Please don't put your entire post in <pre> or <c> tags. It hurts readability. Just place <p> at the start of every paragraph instead.

    Being new is no excuse. Previewing your node would have revealed a problem.

    Finally, the second item is range(?), not range. Use the right name as the hash key (range(?)), and your code works. (Using $item[2] would also work.)

    Tip: 

    my $grammar = q {
...
};

    [download]
    will get you into problems. Use 
    my $grammar = <<'__EOI__';
...
__EOI__

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    instead.

    Tip: "." is not special in character classes, so it doesn't need to be escaped then.

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Re: RecDescent Issue: using item hash with (?) repetition specifier
by almut (Canon) on Apr 02, 2009 at 23:23 UTC

    Your key should be 'range(?)' — as the docs say:

    "The results of named subrules are stored in the hash under each subrule's name (including the repetition specifier, if any)"

    With that change, i.e. 

         if (defined @{$item{'range(?)'}->[0]}) {
             my @arr = @{$item{'range(?)'}->[0]};
             foreach my $i (@arr) {
                     print $i."\n";
             }
     }

    [download]

    I do get the expected output: 

    input
2
0
a

    [download]
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[select]
Re: RecDescent Issue: using item hash with (?) repetition specifier
by jethro (Monsignor) on Apr 02, 2009 at 21:41 UTC

    Please don't use pre tags. Use p-tags for text and c- or code-tags for code. I promise I'll take a closer look after you have edited your post

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      On-site CSS can be used to work round the horrible page-widening effect:

      pre { white-space: pre-wrap }

      I suppose there's a good reason why <pre> is permitted at all?

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