In my code I need some specified precision decimal operations, so I decided to use Math::BigFloat and I run into problems. Consider simple test case below:

use Test::More tests => 10;

use_ok 'Math::BigFloat';

note 'Math tests';

for( my $i = 1; $i < 1000000000; $i *= 10 ) {
    my $bf = Math::BigFloat->new( $i );
    $bf->precision(-2);
    my $xpect = 
       ($bf > 1 ? '3' x (scalar($bf->length) - 1) : 0)
       . ".33";
    is $bf->bdiv(3), $xpect, $i . ' / 3 = ' . $xpect;
}

[download]

At some level it looses its precision, every result should end with .33(3), which after rounding should give .33 - but it doesn't. At 100`000 level you get 33`333.30 as a result, at 100`000`000 result is even less precise 33`333`300.00.

I use my own Decimal "class", so I've implemented some walk-around - before issuing division change precision to much more "precise", issue 'bdiv', chage precision back as it was - and this helps, but it is very uncool ;) solution.

And as for this test case, doing similar as in my walk-around, changing code in loop to this helps:

for( my $i = 1; $i < 1000000000; $i *= 10 ) {
    my $bf = Math::BigFloat->new( $i );
    my $xpect = 
       ($bf > 1 ? '3' x (scalar($bf->length) - 1) : 0)
       . ".33";
    is $bf->bdiv(3)->bfround(-2), $xpect, $i . ' / 3 = ' . $xpect;
}

[download]

In reply to Another Math::BigFloat precision question by Seba

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