[R] Suggestion about "R equivalent of Splus peaks() function"

[Petr Pikal]

Hi

On 8 Feb 2007 at 16:34, Earl F. Glynn wrote:

To:             	r-help at stat.math.ethz.ch
From:           	"Earl F. Glynn" <efg at stowers-institute.org>
Date sent:      	Thu, 8 Feb 2007 16:34:31 -0600
Subject:        	[R] Suggestion about "R equivalent of Splus peaks() function"

> In 2004 there was this R-Help posting from Jan 2004:
> 
>     http://finzi.psych.upenn.edu/R/Rhelp02a/archive/33097.html
>     R equivalent of Splus peaks() function?
> 
> The peaks function there has worked well for me on a couple of
> projects, but some code using "peaks" failed today, which had worked
> fine in the past.
> 
> I was looking for a peak in a test case that was a sine curve over one
> cycle, so there should have been only one peak.  My unexpected
> surprise was to sometimes get one peak, or two adjoining peaks (a
> tie), but the no peaks case cause subsequent code to fail.  I wanted
> to eliminate this "no peak" case when there was an obvious peak.
> 
> I thought it was odd that the peak failure could be controlled by the
> random number seed.
> 
> # R equivalent of Splus peaks() function
> # http://finzi.psych.upenn.edu/R/Rhelp02a/archive/33097.html
> 
> peaks <- 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
> }
> 
> > set.seed(19)
> > peaks(c(1,4,4,1,6,1,5,1,1),3)
> [1] FALSE  TRUE FALSE FALSE  TRUE FALSE  TRUE
> > peaks(c(1,4,4,1,6,1,5,1,1),3)
> [1] FALSE  TRUE FALSE FALSE  TRUE FALSE  TRUE
> > peaks(c(1,4,4,1,6,1,5,1,1),3)
> [1] FALSE  TRUE  TRUE FALSE  TRUE FALSE  TRUE
> > peaks(c(1,4,4,1,6,1,5,1,1),3)
> [1] FALSE FALSE FALSE FALSE  TRUE FALSE  TRUE
> > peaks(c(1,4,4,1,6,1,5,1,1),3)
> [1] FALSE FALSE  TRUE FALSE  TRUE FALSE  TRUE
> 
> 
> Above, the "4" peak at positions 2 and 3 is shown by the TRUE and
> FALSE in positions 2 and 3 above.  Case 4 of FALSE, FALSE was most
> unexpected -- no peak.
> 
> 
> I studied the peaks code and found the problem seems to be in max.col:
> > z
>      [,1] [,2] [,3]
> [1,]    4    4    1
> [2,]    1    4    4
> [3,]    6    1    4
> [4,]    1    6    1
> [5,]    5    1    6
> [6,]    1    5    1
> [7,]    1    1    5
> 
> > max.col(z)
> [1] 2 3 1 2 3 2 3
> > max.col(z)
> [1] 2 2 1 2 3 2 3
> > max.col(z)
> [1] 1 2 1 2 3 2 3
> > max.col(z)
> [1] 2 2 1 2 3 2 3
> > max.col(z)
> [1] 1 3 1 2 3 2 3
> > max.col(z)
> [1] 2 2 1 2 3 2 3
> 
> The ?max.col help shows that it has a ties.method that defaults to
> "random". I want a peak, any peak if there is a tie, but I don't want
> the case that a tie is treated as "no peak".  For now, I added a
> "first" parameter to max.col in peaks:
> 
> # Break ties by using "first"
> 
> peaks <- function(series,span=3)
> {
>   z <- embed(series, span)
>   s <- span%/%2
>   v <- max.col(z, "first") == 1 + s
>   result <- c(rep(FALSE,s),v)
>   result <- result[1:(length(result)-s)]
>   result
> }

Here is a little bit different version of peaks which I currently 
use. It uses ties method for peaks function, controls odd span and 
works differently with adding FALSE values to keep resulting vector 
the same length as the input one.

peaks<-function(series,span=3, ties.method = "first")
{
if((span <- as.integer(span)) %% 2 != 1) stop("'span' must be odd")
z <- embed(series, span)
s <- span%/%2
v <- max.col(z, ties.method=ties.method) == 1 + s
pad <- rep(FALSE, s)
result <- c(pad, v, pad)
result
}

Petr

> 
> A better solution might be a ties.method parameter to peaks, which can
> be passed to max.col.
> 
> I did all of this in R 2.4.1, but the problem seems to be in earlier
> versions too.
> 
> Just in case anyone else is using this "peaks" function.
> 
> efg
> 
> Earl F. Glynn
> Stowers Institute for Medical Research
> 
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Petr Pikal
petr.pikal at precheza.cz