[R] Apply functions along "layers" of a data matrix
David Winsemius
dwinsemius at comcast.net
Fri Nov 18 15:32:53 CET 2011
On Nov 18, 2011, at 8:05 AM, <saschaview at gmail.com> wrote:
> Hello
>
> How can I apply functions along "layers" of a data matrix?
>
> Example:
>
> daf <- data.frame(
> 'id' = rep(1:5, 3),
> matrix(1:60, nrow=15, dimnames=list( NULL, paste('v', 1:4,
> sep='') )),
> rep = rep(1:3, each=5)
> )
>
> The data frame "daf" contains 3 repetitions/layers (rep) of 4
> variables of 5 persons (id). For some reason, I want to calculate
> various statistics (e.g., mean, median) *along* the repetitions. The
> "mean" calculation, for example, would produce the means of daf[1,
> 'v1'] *along* the 3 repetition:
>
> (daf[1, 'v1'] + daf[6, 'v1'] + daf[11, 'v1']) / 3
>
> That is to say, each of the calculations would result in a data
> frame with 4 variables (and the id) of the 5 persons:
>
> id v1 v2 v3 v4
> 1 1 6 21 36 51
> 2 2 7 22 37 52
> 3 3 8 23 38 53
> 4 4 9 24 39 54
> 5 5 10 25 40 55
I see you have gotten acouple of plyr solutions but this is really
easy in base R:
> aggregate(daf[-c(1,6)], list(daf$id), mean)
Group.1 v1 v2 v3 v4
1 1 6 21 36 51
2 2 7 22 37 52
3 3 8 23 38 53
4 4 9 24 39 54
5 5 10 25 40 55
You read this as "use the mean function within categories defined by
the "id" INDEX to aggregate the columns except tof the first and 6th
columns"
>
> Currently, I do this in a loop, but I was wondering about a quick
> and ressource-friendly way to achieve this?
>
> Thanks
> *S*
>
> --
> Sascha Vieweg, saschaview at gmail.com
>
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> and provide commented, minimal, self-contained, reproducible code.
David Winsemius, MD
West Hartford, CT
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