# [R] Seeking a more efficient way to find partition maxima

jim holtman jholtman at gmail.com
Tue Jan 8 01:02:22 CET 2008

```Does this do what you want?

> x <- c(1,4,2,6,7,5)
> x.i <- c(1,4,5)
> partiMax <- function(vec, index){
+     # create a vector to identify the subvectors
+     x.b <- diff(c(index, length(vec) + 1))
+     # split up the vector
+     x.s <- split(vec, rep(seq_along(index), times=x.b))
+     unlist(lapply(x.s, max))
+ }
>
> partiMax(x, x.i)
1 2 3
4 6 7
> partiMax(6:1, x.i)
1 2 3
6 3 2
>
>

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]]
>
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>

--
Jim Holtman
Cincinnati, OH
+1 513 646 9390

What is the problem you are trying to solve?

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