[R] strange behaviour with equality after simple subtraction

[Benilton Carvalho]

In addition to Mike's comment:

 > x<-c(0.1,0.9)
 > 1-x[2]
[1] 0.1
 > x[1]==1-x[2]
[1] FALSE
 > all.equal(x[1], 1-x[2])
[1] TRUE

b

On Jan 26, 2007, at 11:40 AM, Mike Prager wrote:

> 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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