Re: Can scalar representation cause infinite loops?

length($a) lt 2 should be length($a) < 2. You are converting the returned length to a string and doing a lexical (not a numerical) compare.

$a = "abcdefghij";
length($a) lt 2  ==>  "10" lt "2"  ==>  true
$a = "0$a";
length($a) lt 2  ==>  "11" lt "2"  ==>  true
...
$a = "0$a";
length($a) lt 2  ==>  "20" lt "2"  ==>  false
[download]

If length($a) starts in [0..1], the final length($a) will be 2.
If length($a) starts in [2..9], $a won't change.
If length($a) starts in [10..19], the final length($a) will be 20.
If length($a) starts in [20..99], $a won't change.
If length($a) starts in [100..199], the final length($a) will be 200.
If length($a) starts in [200..999], $a won't change.
If length($a) starts in [1000..1999], the final length($a) will be 2000.
If length($a) starts in [2000..9999], $a won't change.
If length($a) starts in [10000..19999], the final length($a) will be 20000.
etc.

This could cause thrashing with very long strings (which localtime doesn't return), but I don't see this ever creating an infinite loop.

Try using

my $min = (localtime)[1];
$min = sprintf('%02d', $min);
[download]

$a (and $b) are reserved for sort. List::Util and List::MoreUtils also use these variables. They're somewhat magical and should be avoided.
A nearby declared lexical will avoid the possibility of the variable used being tied or other special.
Using sprintf will change the code to something quite different, possibly avoiding a compiler bug.

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Re^2: Can scalar representation cause infinite loops? by Anonymous Monk on Jul 19, 2007 at 22:20 UTC
Aha! Thanks for pointing out the use of `lt` vs. `<`. I've noticed it was odd before, but didn't pause to think about what the consequences are. There is another similar loop earlier in the script that looks like this: `while (length($extnum) lt $numlength) { $extnum = "0" . $extnum; }` [download] where `$numlength` can potentially be set to 4. Again, `$extnum` is unlikely to be a very long string, so this doesn't seem to be the culprit either, but I'll have to comb through the script again to see if other abuses of lt could have caused the bug. Thanks for the prompt reply!	[reply] [d/l] [select]
Re^2: Can scalar representation cause infinite loops? by Anonymous Monk on Jul 19, 2007 at 22:22 UTC
Yeah, I told the author of the script to use `sprintf` instead, because it's more efficient, but also because it is actually correct. :-)	[reply] [d/l]