Question of variable interpolation in regex

[SparkeyG]

I'm working on the regular Perl Quiz of the Week, and while I've got it solved, I don't understand why I had to do something to dwiw. The included code shows the pertinant problem.

I take a line of the box to be put into the output. $putNum is a boolean variable. The problem is that instead of updating $boxLine with the current word number, it continues to put in the number 1.

The problem went away if I instead use a seperate variable in the foreach line and then later assign its value to $boxLine.

Please help me understand why this is happening.

my $wordNum = 0;
foreach my $boxLine ( @empty_square) {
    
    if ($putNum) {
        $wordNum ++;
        my $space = $boxLine =~ s/\./\./g;
        my $string = sprintf "%-*d", $space, $wordNum;
        $boxLine =~ s/\.+/$string/o;
    } else {
        $boxLine =~ s/\./ /g;
    }
}

[tommyw]

This line $boxLine =~ s/\.+/$string/o; uses the /o modifier, so the regular expression is only compiled once. That is, even if the value of $string changes, the regular expression won't reflect that. So the first time through, it compiles as s/\.+/    1/o and never changes again.

updated: Apparently, I'm talking rubbish. Sorry!

--
Tommy
Too stupid to live.
Too stubborn to die.

[thelenm]

Actually, the left-hand side of the substitution (the regex) is the part that is only compiled once. The right-hand side will change as $string changes. Try this:

my $foo = 'a';
for my $string ('b'..'z') {
    $foo =~ s/./$string/o;
    print "$foo\n";
}

-- Mike

--
just,my${.02}

[SparkeyG]

A few people commented that I wasn't clear in my question and also asked for example input and output. The input in this loop is:

my @empty_square = qw(+----+
                      |....|
                      |....|
                      |....|
                      +----+                    
                     );

We iterate over several of these boxes, and need to replace . with spaces and a number in the upper left hand corner. I have some logic, that isn't included hear, to determine when to add the number.


The desired output of this loop would be if $putNum is true, the replace |....| with |1   | the first time, |2   | the second, etc. However, all it does now, if $putNum is true, it replaces all |....| with |1   |.

I can fix this if I replace the following:

foreach my $boxLine ( @empty_square) {

with

foreach my $line ( @empty_square) {
    my $boxLine = $line;

I hope this makes it clearer. --Sparkey

[l2kashe]

Ok I saw a couple of things..

1) In your code you incremented your value even though the first line doesnt match "/\./". So you always start at 2 instead of 1.
2) you were using sprintf for your output when perl already had what you wanted to do built in via the nifty 'x' operator...

I also changed the var names, just cause I could :)
So here is what I came up with

#!/usr/local/bin/perl -wT

use strict;

my($putNum,$line,$s_count,$num,$n_line, @square);

$putNum = 1;

@square = (
   '+----+',
   '|....|',
   '|....|',
   '|....|',
   '+----+'
);

foreach $line (@square) {

   if (!$putNum) {
      $line =~ s/\./ /g;

   } else {

      if ($line =~ /\./) {
         $s_count = $line =~ s/\./\./g;

         $num++ if ($s_count);

         $n_line = "$num" . ' ' x ($s_count - length($num));

         $line =~ s/\.+/$n_line/o;
      }
   }

   print "$line\n";
}

# OUTPUT

+----+
|1   |
|2   |
|3   |
+----+

Is this what you were looking for? The same number of spaces that were previously '.'s on the line?

Happy Hackin :)

/* And the Creator, against his better judgement, wrote man.c */

[SparkeyG]

I solved the 1st problem you rose when $putNum is decided. If the line being considered is either the top or bottom line, $putNum is false.

As to your second point, tmtowtdi. ;-)

But I see I still haven't been clear. :-(
the output I'm seeing is

+----+----+----+----+
|1   |1   |1   |1   |
|    |    |    |    |
|    |    |    |    |
+----+----+----+----+

When what I want is

+----+----+----+----+
|1   |2   |3   |4   |
|    |    |    |    |
|    |    |    |    |
+----+----+----+----+

Update:
Take a look at my scratch pad for the full code

[Thelonius]

Dunno, it works for me on perl 5.6.0, 5.6.1, and 5.8.0. Which version are you using?

print join("\n", @empty_square), "\n";

+----+
|2   |
|3   |
|4   |
+----+

[Thelonius]

When what I want is 
+----+----+----+----+
|1   |2   |3   |4   |
|    |    |    |    |
|    |    |    |    |
+----+----+----+----+

Well, you'll need somthing like:

 while ($boxLine =~ s/(\.+)/sprintf("%-*d", length($1), $wordNum)/e) {
   $wordNum++
 }

But then you'll have to skip over the other lines which have blanks (you have multiple lines per square).