How do I create or test for NaN or infinity in Perl?

Question

How do I create or test for NaN or infinite values in Perl?

Answer 1

Succinct answer that works follows.

1: How to create a "NAN" variable for output (to printf, for example):

 {no strict 'subs'; $NAN="NAN"+1;}

2: How to test for "NAN" (looks like ascii art):

 sub isnan {!($_[0]<=0||$_[0]>=0)}

3: How to create an "INF" and INFN variables:

{$INF="INF"+1; $INFN=-"INF"+1}

4: How to test for "INF" (of any sign):

sub isinf {($_[0]==+"INF")||($_[0]==-"INF")}

Answer 2

Personally, I would use Math::BigFloat (or BigInt) for anything that is going to touch infinity of NaN.

Why reinvent the wheel with a hack solution when there are already modules that do the job?

Answer 3

Use Data::Float from CPAN. It exports the following functions:

• float_is_infinite()
• float_is_nan()
• ...

And contrary to the others half-working solutions posted here, it has a testsuite.

Answer 4

Here's a fairly reliable way:

my $inf    = 9**9**9;
my $neginf = -9**9**9;
my $nan    = -sin(9**9**9);

sub isinf { $_[0]==9**9**9 || $_[0]==-9**9**9 }
sub isnan { ! defined( $_[0] <=> 9**9**9 ) }
# useful for detecting negative zero
sub signbit { substr( sprintf( '%g', $_[0] ), 0, 1 ) eq '-' }

for my $num ( $inf, $neginf, $nan ) {
   printf("%s:\tisinf: %d,\tisnan: %d,\tsignbit: %d\n", $num, isinf($num), isnan($num), signbit($num));
}

Output is:

inf:    isinf: 1,   isnan: 0,   signbit: 0
-inf:   isinf: 1,   isnan: 0,   signbit: 1
nan:    isinf: 0,   isnan: 1,   signbit: 0

Answer 5

print "Is NaN\n" if $a eq 'nan';
print "Is Inf\n" if $a eq 'inf' or $a eq '-inf';

EDIT: Fixed for negative infinity.

Answer 6

When I searched I got this site (here) and http://www.learning-perl.com/2015/05/perls-special-not-a-numbers/

The linked article points out that "nan" == "nan" is never true, when the underlying c implementation supports NaN because Nan cannot match itself.

This is nicely illustrated with

die "This perl does not support NaN!\n" if "NaN" == "NaN";

I guess the risk of running you code in an environment where perl has degraded gracefully and your code has not might be low enough so that you don't worry too much.

And of course if you don't want perl to interpolate as a number, use 'eq' not '=='