From: Gabor Grothendieck <ggrothendieck_at_gmail.com>

Date: Mon, 7 Jan 2008 13:49:13 -0500

R-help_at_r-project.org mailing list

https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code. Received on Mon 07 Jan 2008 - 18:54:36 GMT

Date: Mon, 7 Jan 2008 13:49:13 -0500

Try testing the performance of transforming your series to one in which the values of each partition are larger than all prior partitions and the untransforming back:

# test data

myseq <- c(1, 4, 2, 6, 7, 5)

part <- c(1, 4, 5)

M <- max(myseq)

# transform

myseq2 <- myseq + M * cumsum(replace(0 * myseq, part, 1))

# calcuate on transformed version

tmp <- partiCmax(myseq2, part)

# untransform

tmp - M * seq_along(tmp) # c(4, 6, 7)

Also you might check how it compares to the simpler

tapply(myseq, cumsum(replace(0 * myseq, part, 1)), max)

On Jan 7, 2008 11:18 AM, Talbot Katz <topkatz_at_msn.com> wrote:

*>
**> 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
**> [[alternative HTML version deleted]]
**>
**> ______________________________________________
**> R-help_at_r-project.org mailing list
**> https://stat.ethz.ch/mailman/listinfo/r-help
**> PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
**> and provide commented, minimal, self-contained, reproducible code.
**>
*

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https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code. Received on Mon 07 Jan 2008 - 18:54:36 GMT

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