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Re^2: is_numeric

by syphilis (Bishop)
on Apr 12, 2007 at 07:09 UTC ( #609583=note: print w/replies, xml ) Need Help??

in reply to Re: is_numeric
in thread is_numeric

Scalar::Util provides access to the underlying Perl API looks_like_number routine which Perl itself uses internally

Perl will evaluate the string (eg) '5621hello' in numeric context as 5621 ... yet looks_like_number() will tell you that the string '5621hello' does not look like a number. That has always struck me as an anomaly .... I tend to think that a a string that "does not look like a number" should evaluate to zero in numeric context, but that (obviously) doesn't always happen.
use warnings; no warnings 'numeric'; use Scalar::Util; $string = '12.3hello'; print Scalar::Util::looks_like_number($string), "\n"; $string *= 2; print $string, "\n"; __END__ Outputs: 0 24.6
I don't think this is a big deal, btw - it's just that it doesn't DWIM (for me, anyway).

Update: I meant to add that this might even have some siginificance for the op - because we can't assume anything about the numeric value of a variable, based on the fact that Scalar::Util says it doesn't look like a number.

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Re^3: is_numeric
by Fletch (Chancellor) on Apr 12, 2007 at 14:26 UTC

    But you've turned off the warning telling you that it doesn't looks_like_number. If you don't have warnings on, Perl assumes you know what you're doing and tries to do what you've asked it to with the (crummy) numberish data you've given it. If you have enabled warnings, it lets you know about it's misgivings but still goes ahead and does what you've asked.

    I think the expectation mismatch is because Perl uses the (POSIX) C strtod(3) which by design (or at least by the manual page description :) follows this heuristic (from the start of the char* passed parse off as many digits followed by an optional "." and more digits followed by an optional exponent notation, and stop when you hit a not-digit-not-plusminus-not-exponent-marker character) rather than using if-looks_like_number-number-else-it's-zero. This is mentioned in perldata in passing, but if you're not familiar with the C API I'll grant that it's certainly open to causing confusion.

      It used to use strtod, but doesn't any more. The C99 standard pulled the rug out from under everyone by changing e.g. "0x10" from being 0 (in C89) to being 16.

        Ooop, you're quite right. Perhaps perldata shouldn't mention strtod as a way to check for numerical-ness then. Of course if you're getting hex numbers as strings you're going to need to convert them (through POSIX::strtod or hex) anyhow before they're interpreted as the correct value so maybe it's kind of a corner case.

        Good point though.

      You are The Man.

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