[R] apply (or similar preferred) for multiple columns

[Gabor Grothendieck]

On Tue, Jul 12, 2011 at 5:27 AM, Dimitris.Kapetanakis
<dimitrios.kapetanakis at gmail.com> wrote:
> Dear all,
>
> I would like to use the apply or a similar function belonging to this
> family, but applying for each column (or row) but let say for each q
> columns. For example I would like to apply a function FUN for the first q
> columns of matrix X then for q+1:2*q and so on. If I do apply (X, 2, FUN) it
> applies for each column and not for every q columns. Is that possible with
> any similar function?
>
> Thank you
>
> Dimitris

Here is an example of using rollapply to sum all elements in the first
4 columns, then the next 4 columns, etc.   anscombe is a data set that
comes with R.  Be sure you are using zoo 1.7-0 which was just released
today.

> library(zoo)
> packageVersion("zoo")
[1] '1.7.0'
> rollapply(t(anscombe), 4, by = 4, sum, by.column = FALSE)
[1] 396.00 330.03
> c(sum(anscombe[, 1:4]), sum(anscombe[, 5:8]))
[1] 396.00 330.03


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