Hi all,

I was debugging a script with which I was having a calculation issue, and traced the problem to some code that does some simple arithmetic. I boiled down the problem to a short test script:
#!/usr/bin/perl
use strict;

my @a;
$a[0] = (0.1 - 0.1) * 10;  #Should be 0
$a[1] = (0.2 - 0.1) * 10;  #Should be 1
$a[2] = (0.3 - 0.1) * 10;  #Should be 2
$a[3] = (0.4 - 0.1) * 10;  #Should be 3
$a[4] = (0.5 - 0.1) * 10;  #Should be 4

my $i;
for( $i = 0; $i < 5; $i++ ) {
  print "$i: ", $a[$i], " ", int($a[$i]), "\n";
}

print "\n";

for( $i = 0; $i < 5; $i++ ) {
  printf "%d: %s %d %f\n", $i, $a[$i], $a[$i], $a[$i];
}

[download]
And here is the output:
0: 0 0
1: 1 1
2: 2 1  *
3: 3 3
4: 4 4

0: 0 0 0.000000
1: 1 1 1.000000
2: 2 1 2.000000   *
3: 3 3 3.000000
4: 4 4 4.000000

[download]
On the lines with *s you would expect it to print '2 2' & '2 2 2.000000', but instead it prints '1's. I tested both 'print' & 'printf' here to see if it was an issue with one of those functions alone, or perhaps the data type the $a[]'s were assumed to be. This only seems to happen for the calculation of
$a[2] = (0.3 - 0.1) * 10;
(perhaps there are other instances). So it seems like $a[2] is not equal to int($a[2]), though they should be the same.

Does anyone know why this happens?

Please forgive if this is a known phenomenon. I'm not sure how to do a search for this type of problem.

BTW, I tested in Windows XP (Perl 5.8.7), OS X 10.4 (Perl 5.8.6), AIX (Perl 5.6.1), & Ubuntu (Perl 5.10.0) -- all the same result.

TIA

In reply to An arithmetic oddity by jxb

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