[R] rollapply() to portions of a matrix

[Gabor Grothendieck]

rollapply over an index.  Modify this appropriately:

> z <- zoo(101:109, 11:19)
> f <- function(ix) if (ix[1] < 5) min(z[ix]) else max(z[ix])
> rollapply(zoo(seq_along(z)), 3, by = 3, f)
  2   5   8
101 104 109

On Thu, Jul 31, 2008 at 3:36 PM, rcoder <mpdotbook at gmail.com> wrote:
>
> Hi everyone,
>
> I have a rollapply statement that applies a function, in steps, over a data
> matrix as follows:
>
> #Code start
> testm<-rollapply(mat, 100, by=100, min, na.rm=F)
> #Code end
>
> This moves down matrix 'mat' and calculates the minimum value over a 100 row
> range, every 100 rows (i.e. no overlaps). NAs are not removed.
>
> I want to modify this statement somehow so that the rollapply() stops at a
> certain row in the matrix: i.e. it applies from the top, as instructed by
> the statement, but only to a certain row in the matrix. Can anyone suggest a
> way to do this using the additional arguments in the parentheses? One way
> would be to introduce an if statement, but is this the only way?
>
> Thanks,
>
> rcoder
> --
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