[R] rounding in calculation

[Prof Brian Ripley]

On Fri, 21 Mar 2008, John Lande wrote:

> dear all,
>
> I report a problem very simple, that I does non know how to handle.
>
> look at the following code:
>
>> a = rep(16.256, 5)
>> sum(a[1:5]^2) - (sum(a[1:5])^2/5)
> [1] 2.273737e-13
>
> as you can see i retrieve a non 0 value, when i am expected to. what can I
> do?

Use mean() when you want a mean:

> sum(a[1:5]^2) - 5*mean(a[1:5])^2
[1] 0

Your number is not exactly representable so some rounding error would be 
acceptable, but there are more accurate ways to do this, e.g.

sum((a-mean(a))^2)

or make use of var(), which is written by someone knowledgable about these 
things.

>
>> sessionInfo()
> R version 2.6.2 (2008-02-08)
> i386-pc-mingw32
>
> locale:
> LC_COLLATE=Italian_Italy.1252;LC_CTYPE=Italian_Italy.1252;LC_MONETARY=Italian_Italy.1252;LC_NUMERIC=C;LC_TIME=Italian_Italy.1252
>
> attached base packages:
> [1] stats     graphics  grDevices utils     datasets  methods   base
>
> loaded via a namespace (and not attached):
> [1] rcompgen_0.1-17
>
> 	[[alternative HTML version deleted]]
>
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> and provide commented, minimal, self-contained, reproducible code.
>

-- 
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)
1 South Parks Road,                     +44 1865 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595