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Postings from April 2010
Hi Rob,
On Monday 26 Apr 2010 13:38:07 Rob Coops wrote:
> Hi list,
>
> I am just wondering if I sumbled upon an error in perl an error in my logic
> or somehtign else see the below perl one liners
>
> $ perl -e '$n = 0.945; $r = sprintf("%.2f", $n); print "$r\n";'
> 0.94
>
> $ perl -e '$n = 0.9451; $r = sprintf("%.2f", $n); print "$r\n";'
> 0.95
>
> $ perl -e '$n = 0.9450; $r = sprintf("%.2f", $n); print "$r\n";'
> 0.94
>
> $ perl -e '$n = 0.94500; $r = sprintf("%.2f", $n); print "$r\n";'
> 0.94
>
> $ perl -e '$n = 0.945001; $r = sprintf("%.2f", $n); print "$r\n";'
> 0.95
>
> Now I expected perl to round the number: 0.945 to 0.95 and not to 0.94 like
> it is doing. I have no idea why it is doing this but I sure don't like it.
> The problem is I have to do some VAT calculations on thousands of invoices
> and invoice elements. I found that in rare cases I'm of by 1 cent or even 3
> cents if there are lots of elements. Going over the logic I ran into this
> problem.
>
> Cany anyone advise me how to deal with this as I need to find a way to
> round correctly in all cases not just in cases where Perl decides it is a
> good idea to do so. :-)
>
The problem is likely caused by the fact that floating-point numbers in most
modern computers are represented by using binary digits (and binary fractions)
instead of decimal ones. So there may be rounding errors. As a result, it is
not advisable to use the regular floating-point numbers for dealing with
monetary values. Some of the options you may wish to consider instead are:
1. Use a rational library for computation such as Math::BigRat.
2. Use a binary-coded-decimal (BCD) system. Last time I investigated I
couldn't find one for Perl, though , and I've misplaced the link to the
downloadable software library at IBM for that. (Though Python has something
like that in pure-Python)
3. Work always in the smallest unit - cents or whatever.
Regards,
Shlomi Fish
> Regards,
>
> Rob
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