R help - Sep 2011 - Unexpected behavior from which.max (or possibly max)

Hi all,

I was recently writing a script to identify the value and id of the maximum
observation in a sliding window when I ran into some unexpected behavior.  I
have included an example.
> test <- c()
> test$elev <- c(1:200)
> test$i <- 1
> test$window <- 10
The following works for me:
> check.max <- function(x){obs.max <- x$elev[x$i:x$i+x$window];
obs.max}
> check.max(test)
[1] 11

But then which.max doesn't seem to honor x$i+x$window:
> check.max <- function(x){obs.max <-
which.max(x$elev[x$i:x$i+x$window]);
obs.max}
> check.max(test)
[1] 1

If I enclose x$i+x$window in parentheses it works:
> check.max <- function(x){obs.max <-
which.max(x$elev[x$i:(x$i+x$window)]);
obs.max}
> check.max(test)
[1] 11


What am I missing?

Here is sessionInfo (I know I need to upgrade; I am behind the state's
McAfee firewall thingy which makes it very difficult for me):
> sessionInfo()
R version 2.11.1 (2010-05-31)
x86_64-pc-linux-gnu

locale:
 [1] LC_CTYPE=en_US.utf8       LC_NUMERIC=C
 [3] LC_TIME=en_US.utf8        LC_COLLATE=en_US.utf8
 [5] LC_MONETARY=C             LC_MESSAGES=en_US.utf8
 [7] LC_PAPER=en_US.utf8       LC_NAME=C
 [9] LC_ADDRESS=C              LC_TELEPHONE=C
[11] LC_MEASUREMENT=en_US.utf8 LC_IDENTIFICATION=C

attached base packages:
[1] stats     graphics  grDevices utils     datasets  methods   base

loaded via a namespace (and not attached):
[1] tools_2.11.1

Thanks, Jason

	[[alternative HTML version deleted]]

If I enclose x$i+x$window in parentheses it works:
>> check.max<- function(x){obs.max<-
which.max(x$elev[x$i:(x$i+x$window)]);
> obs.max}
x$elev[x$i:x$i+x$window] means:
x$elev[(x$i:x$i)+x$window]

Simple.
		mario
>> check.max(test)
> [1] 11
>
>
> What am I missing?
>
> Here is sessionInfo (I know I need to upgrade; I am behind the state's
> McAfee firewall thingy which makes it very difficult for me):
>
>> sessionInfo()
> R version 2.11.1 (2010-05-31)
> x86_64-pc-linux-gnu
>
> locale:
>   [1] LC_CTYPE=en_US.utf8       LC_NUMERIC=C
>   [3] LC_TIME=en_US.utf8        LC_COLLATE=en_US.utf8
>   [5] LC_MONETARY=C             LC_MESSAGES=en_US.utf8
>   [7] LC_PAPER=en_US.utf8       LC_NAME=C
>   [9] LC_ADDRESS=C              LC_TELEPHONE=C
> [11] LC_MEASUREMENT=en_US.utf8 LC_IDENTIFICATION=C
>
> attached base packages:
> [1] stats     graphics  grDevices utils     datasets  methods   base
>
> loaded via a namespace (and not attached):
> [1] tools_2.11.1
>
> Thanks, Jason
>
> 	[[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.
-- 
Ing. Mario Valle
Data Analysis and Visualization Group            | http://www.cscs.ch/~mvalle
Swiss National Supercomputing Centre (CSCS)      | Tel:  +41 (91) 610.82.60
v. Cantonale Galleria 2, 6928 Manno, Switzerland | Fax:  +41 (91) 610.82.82

On 15/09/2011 10:40 AM, Jason Curole wrote:
> Hi all,
>
> I was recently writing a script to identify the value and id of the maximum
> observation in a sliding window when I ran into some unexpected behavior. 
I
> have included an example.
>
> >  test<- c()
> >  test$elev<- c(1:200)
> >  test$i<- 1
> >  test$window<- 10
>
> The following works for me:
>
> >  check.max<- function(x){obs.max<- x$elev[x$i:x$i+x$window];
obs.max}
> >  check.max(test)
> [1] 11
>
> But then which.max doesn't seem to honor x$i+x$window:
>
> >  check.max<- function(x){obs.max<-
which.max(x$elev[x$i:x$i+x$window]);
> obs.max}
> >  check.max(test)
> [1] 1
>
> If I enclose x$i+x$window in parentheses it works:
>
> >  check.max<- function(x){obs.max<-
which.max(x$elev[x$i:(x$i+x$window)]);
> obs.max}
> >  check.max(test)
> [1] 11
>
>
> What am I missing?
Operator precedence.  You had

x$i:x$i+x$window

but consider this:

1:1+5

The : has higher precedence than the +, so that gives 6.  Yours gives

x$i+x$window

Duncan Murdoch
> Here is sessionInfo (I know I need to upgrade; I am behind the state's
> McAfee firewall thingy which makes it very difficult for me):
>
> >  sessionInfo()
> R version 2.11.1 (2010-05-31)
> x86_64-pc-linux-gnu
>
> locale:
>   [1] LC_CTYPE=en_US.utf8       LC_NUMERIC=C
>   [3] LC_TIME=en_US.utf8        LC_COLLATE=en_US.utf8
>   [5] LC_MONETARY=C             LC_MESSAGES=en_US.utf8
>   [7] LC_PAPER=en_US.utf8       LC_NAME=C
>   [9] LC_ADDRESS=C              LC_TELEPHONE=C
> [11] LC_MEASUREMENT=en_US.utf8 LC_IDENTIFICATION=C
>
> attached base packages:
> [1] stats     graphics  grDevices utils     datasets  methods   base
>
> loaded via a namespace (and not attached):
> [1] tools_2.11.1
>
> Thanks, Jason
>
> 	[[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.