Does it look like a number?

[broquaint]

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);
}

[princepawn]

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

[broquaint]

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

[merlyn]

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.

[xdg]

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).