Perl substr goes crazy when presented with zero's

nodger · 01-21-2007, 06:51 AM

Im relatively new to Perl, and well heres my problem:
If I run the following program

Code:

#!/usr/bin/perl

@lines=("<table width=811>\r\n","</table>\r\n");

foreach(@lines)
{
	$x=0;
	while($c=substr($_,$x,1))
	{
		print $c;
		$x++;
	}
}

I get the correct output:

Code:

<table width=811>
</table>

cool. but if I change it to:

Code:

#!/usr/bin/perl

@lines=("<table width=800>\r\n","</table>\r\n");

foreach(@lines)
{
	$x=0;
	while($c=substr($_,$x,1))
	{
		print $c;
		$x++;
	}
}

I get this output:

Code:

<table width=8</table>

Seems like when it encounters a zero it just bails out completely and moves on to the next iteration of the loop.

makyo · 01-21-2007, 08:47 AM

Hi.

Yes, that's how while knows when to stop:

Quote:

The number 0, the strings '0' and '', the empty list "()", and "undef" are all false in a boolean context. All other values are true.
...
"while" repeats the statement while the condition is true.
-- man perlsyn

I'm not sure what you are trying to do, but function index, or defined might be of some use ... cheers, makyo

tbutttbutt · 01-21-2007, 08:54 AM

You could print the strings as they are:

print "<table width=800>\r\n","</table>\r\n";

The '\n's and '\r's inside will get interpreted and you should get the result you want.

nodger · 01-21-2007, 08:55 AM

thanks

I should have figured that out.

matthewg42 · 01-21-2007, 09:49 AM

I can't imagine why you'd want to go through a string in this manner, but assuming it's just some experimentation, you could do it by adding a defined(...):

Code:

#!/usr/bin/perl

@lines=("<table width=800>\r\n","</table>\r\n");

foreach(@lines)
{
	$x=0;
	while(defined($c=substr($_,$x,1)))
	{
		print $c;
		$x++;
	}
}

A [probably slightly less weird] way to iterate over each character in a string might be like this:

Code:

my $str = "J. R. Bob Dobbs";
for (my $i=0; $i<length($str); $i++) { 
    print "got char: " . substr($str,$i,1) . "\n";
}

Or maybe if you're a smartass:

Code:

my $str = "Hello"; 
$str=reverse $str; 
while(length($str) > 0) { 
    print "gimme a " . chop($str) . "\n";
}