From: Todd Benson <caduceass@...>
Date: 2007-07-17T10:17:18+09:00
Subject: Re: Force calculation result to BigDecimal

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.
 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.

class Float
  require 'bigdecimal'
  def big
    BigDecimal self.to_s
  end
end

p pi = Math::PI.big
#  prints out something like
# #<BigDecimal:2df3ffc,'0.3141592653 58979E1',20(20))
p "\n"

p pi * 2.0.big
# prints out something like
# #<BigDecimal:2deb35c,'0.6283185307 17958E1',20(36))
p "\n"

Todd