[R] Unexpected behavior from which.max (or possibly max)

[Duncan Murdoch]

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]]
>
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