Hi.
Suppose I have a vector that I partition into disjoint, contiguous subvectors.
For example, let v = c(1,4,2,6,7,5), partition it into three subvectors, v1 =
v[1:3], v2 = v[4], v3 = v[5:6]. I want to find the maximum element of each
subvector. In this example, max(v1) is 4, max(v2) is 6, max(v3) is 7. If I
knew that the successive subvector maxima would never decrease, as in the
example, I could do the following:
partiCmax <- function( values, seriesIdx ) {
# assume seriesIdx is increasing integer sequence beginning with 1, ending at
less than or equal to length(values)
parti <- cbind( seriesIdx, c( ( seriesIdx[ -1 ] - 1 ), length( values ) ) )
return( cummax( values )[ parti[ , 2 ] ] )
}
The use of cummax makes that pretty efficient, but if the subvector maxima are
not non-decreasing, it doesn't work. The following function works (at least it
did on the examples I tried):
partiMax <- function( values, seriesIdx ) {
# assume seriesIdx is increasing integer sequence beginning with 1, ending at
less than or equal to length(values)
parti <- cbind( seriesIdx, c( ( seriesIdx[ -1 ] - 1 ), length( values ) ) )
return( sapply( ( 1:length(seriesIdx) ), function ( i ) {return( max( values[
parti[ i, 1 ]:parti[ i, 2 ] ] ) ) } ) )
}
but I figured someone out there could come up with something cleverer. Thanks!
-- TMK --212-460-5430 home917-656-5351 cellt o p k a t z @ m s n . c o m
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