Re: [R] strange behaviour with equality after simple subtraction

From: Mike Prager <mike.prager_at_noaa.gov>
Date: Fri 26 Jan 2007 - 16:40:34 GMT

"martin sikora" <martin.sikora@upf.edu> wrote:

> today while trying to extract data from a list for subsequent analysis, i
> stumbled upon this funny behavior on my system:
>
> > x<-c(0.1,0.9)
>
> > 1-x[2]
>
> [1] 0.1
>
> > x[1]
>
> [1] 0.1
>
> > x[1]==1-x[2]
>
> [1] FALSE
>
> > x[1]>1-x[2]
>
> [1] TRUE
>

Not at all strange, an expected property of floating-point arithmetic and one of the most frequently asked questions here.

> print(0.1, digits=17)

[1] 0.1
> print(1 - 0.9, digits=17)

[1] 0.09999999999999998
>

A simple description of the issue is at

http://docs.python.org/tut/node16.html

In most cases, it suffices to test for approximate difference or relative difference. The former would look like this

if (abs(x[1] - x[2]) < eps)) ...  

with "eps" set to something you think is an insignificant difference, say 1.0e-10.

-- 
Mike Prager, NOAA, Beaufort, NC
* Opinions expressed are personal and not represented otherwise.
* Any use of tradenames does not constitute a NOAA endorsement.

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Received on Sat Jan 27 03:44:47 2007

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