Rails - Apr 2006 - Problems doing addition/subtraction with floats

Ok, I am sure this is a simplistic problem that I am just missing right now
(maybe eyes are crossed??).
However, I am trying to do simply math here.

I have these variables initialized as follows:
@amount_due = @amount_paid = @total_due = 0.0

I get @total_due and it is set to 1739.6 from the database.
I then get @amount_paid and it is set to 1739.6 from the database.
I then calculate the @amount_due = @total_due - @amount_paid.

This value is 2.27373675443232e-013.

????

Sorry if this is on the list somewhere and I missed it.

Thanks in advance for any help.

~damon
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> Date: Wed, 12 Apr 2006 14:52:54 -0500
> From: "Damon Hill" <mdamonhill@gmail.com>
> Ok, I am sure this is a simplistic problem that I am just missing right now
> (maybe eyes are crossed??).
> However, I am trying to do simply math here.
> 
> I have these variables initialized as follows:
> @amount_due = @amount_paid = @total_due = 0.0
> 
> I get @total_due and it is set to 1739.6 from the database.
> I then get @amount_paid and it is set to 1739.6 from the database.
> I then calculate the @amount_due = @total_due - @amount_paid.
> 
> This value is 2.27373675443232e-013.
This isn''t a Rails issue: it''s a Ruby issue, and happens
because 1739.6
cannot be stored exactly as a float or double (nor can 0.1). Subtracting 
numbers that are close to each other can often introduce this kind of 
error. Changing the way numbers are stored would be a huge amount of work.

The easiest way to fix it is to round the result to whatever you''re 
working with.

(for two decimal places)
@amount_due = int(0.5 + 100.0 * (@total_due - @amount_paid)) / 100.0

or just print the result to two places... but that won''t help if
you''re
comparing to zero.

Chris

Chris--

I figured that it was a Ruby issue and I apologize for posting on this list.

I appreciate the reply.

Thanks,
~damon

On 4/12/06, Chris Schumann <chris@idlelion.net>
wrote:
>
> > Date: Wed, 12 Apr 2006 14:52:54 -0500
> > From: "Damon Hill" <mdamonhill@gmail.com>
>
> > Ok, I am sure this is a simplistic problem that I am just missing
right
> now
> > (maybe eyes are crossed??).
> > However, I am trying to do simply math here.
> >
> > I have these variables initialized as follows:
> > @amount_due = @amount_paid = @total_due = 0.0
> >
> > I get @total_due and it is set to 1739.6 from the database.
> > I then get @amount_paid and it is set to 1739.6 from the database.
> > I then calculate the @amount_due = @total_due - @amount_paid.
> >
> > This value is 2.27373675443232e-013.
>
> This isn''t a Rails issue: it''s a Ruby issue, and happens
because 1739.6
> cannot be stored exactly as a float or double (nor can 0.1). Subtracting
> numbers that are close to each other can often introduce this kind of
> error. Changing the way numbers are stored would be a huge amount of work.
>
> The easiest way to fix it is to round the result to whatever
you''re
> working with.
>
> (for two decimal places)
> @amount_due = int(0.5 + 100.0 * (@total_due - @amount_paid)) / 100.0
>
> or just print the result to two places... but that won''t help if
you''re
> comparing to zero.
>
> Chris
>
> _______________________________________________
> Rails mailing list
> Rails@lists.rubyonrails.org
> http://lists.rubyonrails.org/mailman/listinfo/rails
>
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