CODEWITHSUNDEEP
perl
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Reputation: 8969

Strange Perl code bug, exit on 0

I am writing a Perl code, using substr to extract characters one by one but encountered a very strange problem.

I am trying to do the following Scan character one by one, if it is # go to end of line, if it is ' or " then find the next matching one. Also added HTML color tag to highlight them. Everything else just print.

Here is the block of code

while ($char = (substr $src, $off_set, 1)) {

  if ($char eq '#') {
    $end_index = index $src, "\n", $off_set+ 1;
    my $c = substr($src, $off_set, $end_index-$off_set+1);
    print $comment_color.$c.$color_end;
  } elsif (($char eq '"') || ($char eq "'")) {
    $end_index = index ($src, $char, $off_set+1);
    my $char_before = substr $src, $end_index-1, 1;

    while ($end_index > 0 && $char_before eq '\\') {
      $end_index = index $src, $char, $end_index + 1;
      $char_before = substr $src, $end_index-1, 1;
    }
    my $s = substr($src, $off_set, $end_index-$off_set+1);
    print $string_color.$s.$color_end;
  } else {
    print $char;
    $end_index++;
  }
    $off_set = $end_index + 1;
}

When I use the following testing code, the script will just exit on first 0, if I remove all the 0 then it runs ok. If I remove first 0, it will exit on 2nd. I really have no idea why this happens.

# Comment 1

my $zero = 0;
my @array = (0xdead_beef, 0377, 0b011011);
# xor
sub sample2
{
  print "true or false";
  return 3 + 4 eq " 7"; # true or false
}
   #now write input to STDOUT
print $time . "\n";
my $four = "4";

Upvotes: 1

Views: 94

Answers (3)

Ask and Learn
Ask and Learn

Reputation: 8969

Finally, figured out it is the while loop. It exit the loop when it sees a 0.

Updated the while loop condition to 

while (($char = (substr $src, $off_set, 1)) || ($off_set < (length $src))) {

and it is working now.

Upvotes: 0

amon
amon

Reputation: 57656

This is your loop condition:

while ($char = (substr $src, $off_set, 1)) {
    ...

So what happens when $char = "0"? As Perl considers that to be a false value, the loop will terminate. Instead, loop as long as characters are left:

while ($off_set < length $src) {
    my $char = substr $src, $off_set, 1;
    ...

Anyway, your code is convoluted and hard to read. Consider using regular expressions instead:

use re '/xsm';

my $src = ...;
pos($src) = 0;
my $out = '';
while (pos($src) < length $src) {
    if ($src =~ m/\G ([#][^\n]*)/gc) {
        $out .= colored(comment => $1);
    }
    elsif ($src =~ m/\G (["] (?:[^"\\]++|[\\].)* ["])/gc) {
        $out .= colored(string => $1);
    }
    elsif ($src =~ m/\G (['] (?:[^'\\]++|[\\]['\\])* ['])/gc) {
        $out .= colored(string => $1);
    }
    elsif ($src =~ m/\G ([^"'#]+)/gc) {
        $out .= $1;
    }
    else {
        die "illegal state";
    }
}

where colored is some helper function.

Upvotes: 3

Miller
Miller

Reputation: 35218

Check for defined in your while loop:

while (defined(my $char = substr $src, $off_set, 1)) {

The reason why your code was exiting early is because '0' is a false value, and therefore the while would end. Instead, this will check if any value is pulled from the substr call.

Upvotes: 4

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