From: Robert Klemme <shortcutter@...>
Date: 2007-07-17T16:30:00+09:00
Subject: Re: Force calculation result to BigDecimal

2007/7/17, Todd Benson <caduceass@gmail.com>:
> On 7/16/07, Robert Brown <rob99brown@yahoo.com> wrote:
> > Hi all,
> >
> > I'm doing some complex math which I need to be based on BigDecimal
> > accuracy (not Float).  It seems that ruby defaults to Float when there's
> > a mixture of type in there, e.g.
> >
> > Code :
> >
> >    1. irb(main):001:0> require 'bigdecimal'
> >    2. => true
> >    3. irb(main):002:0> a = 5.123
> >    4. => 5.123
> >    5. irb(main):003:0> b = BigDecimal("6.789")
> >    6. => #<BigDecimal:8b4c0,'0.6789E1',8(8)>
> >    7. irb(main):004:0> c = a * b
> >    8. => 34.780047
> >    9. irb(main):005:0> c.class
> >   10. => Float
> >
> > Is there a way I can force a BigDecimal outcome.
> >
> > One way I guess would work is to make sure all args in the calculation
> > are BigDecimal, but that's a bit tiresome.  For a start, I'd have to
> > type check each arg and convert to BigDecimal if it was not BigDecimal -
> > that's gotta be slow...
> >
> > On a related note, how do I initalize a BigDecimal from a Float (without
> > converting to string first).  How could I extend BigDecimal for this?
> >
> >
> > Thanks,
> >
> >
> >
> > Rob
>
> Here's a quick fix.  There may be a better way to handle this, though.

Hm....  I don't think this is a fix because once you have a float you
lost all the precision.  A better fix is probably to change #coerce to
work properly. Another solution is to make sure all values are
converted to BD before starting the calculation.

Note, that BigDecimal is returned when working with integers.

>  Again, I'll give the usual warning about modifying core classes: you
> shouldn't normally do it unless you can be certain it won't break
> yours or anyone else's code.

Very good point!

Kind regards

robert