[R] numeric equality

[David M Smith]

On Wed, Mar 18, 2009 at 8:58 AM, Yu, Changhong <YuC at ccf.org> wrote:
> Dear all,
> I am totally confused by the following R output, but don't have a clue
> for it.
>
>> a <- 1 - 0.2
>
>> a == 0.8
>
> [1] TRUE
>
>> a <- 1 - 0.8
>
>> a == 0.2
>
> [1] FALSE

This is expected behaviour.  The bottom line is that you shouldn't
rely on the equality operator when dealing with floating point values
in any numerical software, including R.

I write about this in detail at:

http://blog.revolution-computing.com/2009/03/when-is-a-zero-not-a-zero.html

which will hopefully clear up your questions.

# David Smith

-- 
David M Smith <david at revolution-computing.com>
Director of Community, REvolution Computing www.revolution-computing.com
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