[R] More efficient way to use ifelse()?

[David Winsemius]

On May 26, 2010, at 1:25 AM, Ian Dworkin wrote:

> # This is more about trying to find a more effecient way to code some
> simple vectorized computations using ifelse().
>
> # Say you have some vector representing a factor with a number of
> levels (6 in this case), representing the location that samples were
> collected.
>
> Population <- gl( n=6, k=5,length=120, labels =c("CO", "CN","Ga","KO",
> "Mw", "Ng"))
>
>
> # You would like to assign a particular value to each level of
> population (in this case the elevation at which they were collected).
> In a vectorized approach (for speed... pretend this was a big data
> set..)
>
> elevation <-  ifelse(Population=="CO", 2169,
> ifelse(Population=="CN", 1121,
>  ifelse(Population=="Ga", 500,
>    ifelse(Population=="KO", 2500,
>    	ifelse(Population=="Mw", 625,
>    	  ifelse(Population=="Ng", 300, NA ))))))
    	
elevation <-  (Population=="CO")* 2169+
               (Population=="CN")* 1121+
               (Population=="Ga")* 500+
               (Population=="KO")* 2500+
    	      (Population=="Mw")* 625+
    	      (Population=="Ng")* 300
is.na(elevation) <- elevation==0

> # Which is fine, but is a pain to write...

Also approaching the nesting limits (seven levels if I remember) of  
the ifelse function.

>
> # So I was trying to think about how to vectorize directly. i.e use
> vectors within the test, and for return values for T and F
>
> elevation.take.2 <- ifelse(Population==c("CO",  "CN", "Ga", "KO",
> "Mw", "Ng"), c(2169, 1121, 500, 2500, 625, 300), c(NA, NA, NA, NA, NA,
> NA))
>
> # It makes sense to me why this does not work (elevation.take.2), but
> I am not sure how to get it to work. Any suggestions? I suspect it
> involves a trick using "any" or "II" or something, but I can't seem to
> work it out.
>
>
> # Thanks in advance!
>
> # Ian Dworkin
> # idworkin at msu.edu
>
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David Winsemius, MD
West Hartford, CT