From gilal@md.huji.ac.il

Hi,
I sent this message by using perlbug, but I'm not sure if it was
properly sent from my account. If you're the wrong address, please let
me know.

I have been having trouble with the perl numerical comparison operators
(e.g. ==).

For example, if I run the following program (enclosed between the
lines), the answer I get is​:
6 is not equal to 6

#!/usr/bin/perl -w

my $ten = 10;
my @​a = (0.4,0.4,0.4);
my $point4 = mean(@​a);

tmpSub($ten,$point4);

sub tmpSub {
  ($num1,$num2) = @​_;
  my $num3 = $num1 * (1-$num2);
  if ($num3 == 6) {
  print "$num3 is equal to 6\n";
  }
  else {
  print "$num3 is not equal to 6\n";
  }
}

sub mean {
  my @​arr = @​_;
  my $mean = 0;
  foreach my $val (@​arr) {
  $mean += $val;
  }
  if (@​arr > 0) {
  $mean /= @​arr;
  }
  else {
  $mean = undef;
  }
  return $mean;
}

I have tried running this for​:
perl, v5.8.0 built for i386-linux-thread-multi
and
perl, v5.6.1 built for irix-n32

I haven't quite managed to pinpoint the source of the bug, which is why
the example is a little funny. I think it's the result of using a value
returned from a subroutine, since when I replace the call to the
subroutine with actual numbers (tmpSub(10,0.4)), I get the answer I'd
expect​: 6 is equal to 6. I have also encountered a similar problem with
the < operator.

Thanks,

Gila

From @smpeters

[gilal@​md.huji.ac.il - Thu Oct 28 00​:58​:12 2004]​:

Hi,
I sent this message by using perlbug, but I'm not sure if it was
properly sent from my account. If you're the wrong address, please let
me know.

I have been having trouble with the perl numerical comparison operators
(e.g. ==).

For example, if I run the following program (enclosed between the
lines), the answer I get is​:
6 is not equal to 6

---------------------------------------------------------------
#!/usr/bin/perl -w

my $ten = 10;
my @​a = (0.4,0.4,0.4);
my $point4 = mean(@​a);

tmpSub($ten,$point4);

sub tmpSub {
($num1,$num2) = @​_;
my $num3 = $num1 * (1-$num2);
if ($num3 == 6) {
print "$num3 is equal to 6\n";
}
else {
print "$num3 is not equal to 6\n";
}
}

sub mean {
my @​arr = @​_;
my $mean = 0;
foreach my $val (@​arr) {
$mean += $val;
}
if (@​arr > 0) {
$mean /= @​arr;
}
else {
$mean = undef;
}
return $mean;
}
--------------------------------------------------------------------

I have tried running this for​:
perl, v5.8.0 built for i386-linux-thread-multi
and
perl, v5.6.1 built for irix-n32

I haven't quite managed to pinpoint the source of the bug, which is why
the example is a little funny. I think it's the result of using a value
returned from a subroutine, since when I replace the call to the
subroutine with actual numbers (tmpSub(10,0.4)), I get the answer I'd
expect​: 6 is equal to 6. I have also encountered a similar problem with
the < operator.

Thanks,

Gila

I made a slight change to your program above to find out what $num3 is
equal to if it isn't 6. I also took a look at $point4, $num1, and $num2
as well.

$ perl comp.pl
point4​: 0.40000000000000007772
num1​: 10
num2​: 0.40000000000000007772
6 is not equal to 6
num3​: 5.99999999999999822364

The problem is not that "==" is not working. Its that floats are not
always exact. This is not a bug in Perl, but simply the facts of life
when dealing with floating point numbers.

The RT System itself - Status changed from 'new' to 'open'

@smpeters - Status changed from 'open' to 'resolved'

From @schwern

On Thu, Nov 04, 2004 at 04​:37​:10PM -0000, Steve Peters via RT wrote​:

The problem is not that "==" is not working. Its that floats are not
always exact. This is not a bug in Perl, but simply the facts of life
when dealing with floating point numbers.

See perlfaq4 for more details.

--
Michael G Schwern schwern@​pobox.com http​://www.pobox.com/~schwern/
Moosy doom.