[R] precision in seq

[Prof Brian Ripley]

FAQ Q7.31 and ?"==" should enlighten you.

None of 0.05, 0.1 ... 0.9, 0.95 are exactly representable on a binary 
computer.

On Mon, 4 Feb 2008, Eric Elguero wrote:

> Hi everybody,
>
> this is a warning more than a question.
>
> I noticed that seq produces approximate results:
>
>> seq(0,1,0.05)[19]==0.9
> [1] TRUE
>> seq(0,1,0.05)[20]==0.95
> [1] FALSE
>> seq(0,1,0.05)[21]==1
> [1] TRUE
>
>> seq(0,1,0.05)[20]-0.95
> [1] 1.110223024625157e-16
>
> I do not understand why 0.9 and 1 are correct (within some
> tolerance or strictly exact?)  and 0.95 is not.
>
> this one works:
>
>> ((0:20)/20)[20]==0.95
> [1] TRUE
>
> Eric Elguero
>
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>

-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
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