[R] rollapply() to portions of a matrix

[rcoder]

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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