[R] subtotal, submean, aggregate

[Roger Bivand]

On Sun, 26 Feb 2006, Patrick Giraudoux wrote:

> Dear All,
> 
> I would like to make partial sums (or means or any other function) of 
> the values in intervals along a sequence (spatial transect) where groups 
> are defined.
> 
> For instance:
> 
> habitats<-rep(c("meadow","forest","meadow","pasture"),c(10,5,12,6))
> observations<-rpois(length(habitats),2)
> transect<-data.frame(observations=observations,habitats=habitats)
> 
> aggregate() is not suitable for my purpose because I want a result 
> respecting the order of the habitats encountered although they may have 
> the same name (and not pooling each group on each level of the factor 
> created). For instance, the output of the ideal function 
> mynicefunction() would be something as:
> 
> mynicefunction(transect$observations, by=list(transect$habitats),sum)
> meadow     16
> forest      9
> meadow     21
> pasture    17
> 
> and not
> 
> aggregate(transect$observations,by=list(transect$habitats),sum)
>   Group.1  x
> 1  forest  9
> 2  meadow 37
> 3 pasture 17
> 
> Did anybody hear about such a function already written in R? If no, any 
> idea to make it simple and elegant to write?

I got as far as:

rle.habs <- rle(habitats)
habitats1 <- rep(make.names(rle.habs$values, unique=TRUE), rle.habs$lengths)
aggregate(observations,by=list(habitats1),sum)

making an extra habitats vector with a unique label for each run. 

Since I don't know your seed, the results are not the same, but rle() is 
quite good for runs.

Roger

> 
> Cheers,
> 
> Patrick Giraudoux
> 
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-- 
Roger Bivand
Economic Geography Section, Department of Economics, Norwegian School of
Economics and Business Administration, Helleveien 30, N-5045 Bergen,
Norway. voice: +47 55 95 93 55; fax +47 55 95 95 43
e-mail: Roger.Bivand at nhh.no