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Does it look like a number?

by broquaint (Abbot)
on Nov 11, 2002 at 19:42 UTC ( #212058=snippet: print w/ replies, xml ) Need Help??

Description: Ask perl if a given scalar looks like a number (requires Inline::C).

Inspired by Slaven Rezic's "Include Perl_looks_like_number in Scalar::Util?" post to p5p.

Hopefully this function, or something very similar to it, should be making it into Scalar::Util per the p5p thread.

update: changed the title (was Is it a number?) as it was a little ambiguous in relation to the code

use Inline C;

my $var = shift;

print +(isnum($var) ? "is a number" : "not a number"), ": $var", $/;

__END__
__C__

int isnum(SV* val) {
  return Perl_looks_like_number(val);
}
Comment on Does it look like a number?
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Re: Is it a number?
by princepawn (Parson) on Nov 11, 2002 at 20:48 UTC
    I remember in "Effective Perl Programming" merlyn did something like this:
    $_ && bit_flip $_ eq "0"
    Where bit_flip is something I forgot how to do in Perl.

    Carter's compass: I know I'm on the right track when by deleting something, I'm adding functionality

      Ah yes, that would be
      sub is_numeric { ($_[0] & ~ $_[0]) eq "0"; }
      Which tests whether a scalar is a numeric value or a string value e.g 42 vs "42". Whereas the snippet discerns whether a given scalar looks like a number or not e.g
      print +(isnum($_) ? "is a number" : "not a number"), ": $_", $/ for qw( 1 2.2 3e3 4. .5); __output__ is a number: 1 is a number: 2.2 is a number: 3e3 is a number: 4. is a number: .5

      HTH

      _________
      broquaint

        Yeah, the difference is the difference between "could this string be converted into a number without getting the warnings if -w is on?" vs "is this value always and only a number?". Everything in the latter case is also true for the former, but the inverse is not true. For example, all external data read from filehandles is always a string initially, so it could never pass this "is_numeric" test.

        -- Randal L. Schwartz, Perl hacker
        Be sure to read my standard disclaimer if this is a reply.

Re: Does it look like a number?
by xdg (Monsignor) on Dec 15, 2004 at 17:16 UTC

    Lots more on this topic came up recently. See Detecting if a scalar has a number or string, which includes a lot of discussion about how to treat objects that overload like numbers. I personally like Re^3: Detecting if a scalar has a number or string by davido, which ducks a lot of the issues and tests to see if a given input would generate a warning if used in arithmetic -- which is what I usually find myself wanting to check when I'm being given user input (as opposed to whether something is represented internally as a number or string).

    -xdg

    Code posted by xdg on PerlMonks is public domain. It has no warranties, express or implied. Posted code may not have been tested. Use at your own risk.

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