> > I wonder if we shouldn't polish that a bit and add to R's > standard 'utils' package. >
Hm, I figured out there are (at least) two versions out there, one being
the "original" idea and a modification.
=== Petr Pikal in 2001 (based on Brian Ripley's idea)==
peaks <- function(series, span=3) {
z <- embed(series, span)
result <- max.col(z) == 1 + span %/% 2
result
}
versus
=== Petr Pikal in 2004 ==
peaks2<-function(series,span=3) {
z <- embed(series, span)
s <- span%/%2
v<- max.col(z) == 1 + s
result <- c(rep(FALSE,s),v)
result <- result[1:(length(result)-s)]
result
}
Comparison shows
> peaks(c(1,4,1,1,6,1,5,1,1),3)
[1] TRUE FALSE FALSE TRUE FALSE TRUE FALSE
which is a logical vector for elements 2:N-1 and
> peaks2(c(1,4,1,1,6,1,5,1,1),3)
[1] FALSE TRUE FALSE FALSE TRUE FALSE TRUE
which is a logical vector for elements 1:N-2.
As I would expect to "lose" (span-1)/2 elements on each side
of the vector, to me the 2001 version feels more natural.
Also, both "suffer" from being non-deterministic in the
multiple-maxima-case (the two 4s here)
> peaks(c(1,4,4,1,6,1,5,1,1),3)
[1] FALSE TRUE FALSE TRUE FALSE TRUE FALSE
> peaks(c(1,4,4,1,6,1,5,1,1),3)
[1] TRUE TRUE FALSE TRUE FALSE TRUE FALSE
> peaks(c(1,4,4,1,6,1,5,1,1),3)
[1] FALSE FALSE FALSE TRUE FALSE TRUE FALSE
> peaks(c(1,4,4,1,6,1,5,1,1),3)
[1] FALSE TRUE FALSE TRUE FALSE TRUE FALSE
which also persits for span > 3 (without the 6 then, of course):
> peaks(c(1,4,4,1,1,1,5,1,1),5)
[1] TRUE FALSE FALSE FALSE TRUE
> peaks(c(1,4,4,1,1,1,5,1,1),5)
[1] FALSE FALSE FALSE FALSE TRUE
> peaks(c(1,4,4,1,1,1,5,1,1),5)
[1] TRUE FALSE FALSE FALSE TRUE
This could (should?) be fixed by modifying the call to max.col()
result <- max.col(z, "first") == 1 + span %/% 2;
Just my two cents,
Marc
--
========================================================
Dipl. Inform. Med. Marc Kirchner
Interdisciplinary Centre for Scientific Computing (IWR)
Multidimensional Image Processing
INF 368
University of Heidelberg
D-69120 Heidelberg
Tel: ++49-6221-54 87 97
Fax: ++49-6221-54 88 50
[EMAIL PROTECTED]
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