Hi,

A NaN (Not a Number) evaluates as "true" in both Perl and C. That seems a bit counter-intuitive to me. Are there sound, well thought out reasons for having a NaN deemed to be true, or is it just done that way for historical reasons ?

Here's a simple demo: 
use warnings;
use strict;

my $inf = 99**99**99;
my $nan = $inf / $inf;

if($nan != $nan) {print "\$nan is a nan\n"}
else {die "$nan is not a nan\n"}

if($nan) {print "nans are true\n"}
else {print "nans are false\n"}

__END__

Outputs:
$nan is a nan
nans are true

[download]
In both Math::MPFR and Math::MPC, I've overloaded 'bool' so that a NaN is false ... mainly because it struck me as being the logical thing to do.
Now I'm wondering whether that was a mistake. (I couldn't google up any information that helped clarify things for me.)
For a Math::MPFR demo: 
use warnings;
use strict;
use Math::MPFR;

my $nan = Math::MPFR->new();

if($nan != $nan) {print "\$nan is a nan\n"}
else {die "$nan is not a nan\n"}

if($nan) {print "nans are true\n"}
else {print "nans are false\n"}

__END__
Outputs:
$nan is a nan
nans are false

[download]
Any thoughts ?

Cheers,
Rob

In reply to NaNs are true by syphilis

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