[R] ifelse() fill order and recycling rules [Sec=Unclassified]

[John McKinlay]

Hi all,

Using R v2.7.1, platform i386-pc-mingw32

Can someone please shed some light on the behaviour of ifelse() for me?
My intent is to calc relative proportions of z$b, at the same time
subsetting z$b based on z$a. I could attack the problem other ways
(suggestions welcome) but I am also intrigued by the _order_ in which
ifelse seems to assign values, and how recycling works. For instance, 
	
z <- data.frame(a = c(1,2,3,4,1), b = 6:10)
z$c <- ifelse(z$a > 1, z$b[z$a > 1]/sum(z$b[z$a > 1]), NA)
z # seems to have filled z$c by row 4,2,3
  a  b         c
1 1  6        NA
2 2  7 0.3333333
3 3  8 0.3750000
4 4  9 0.2916667
5 1 10        NA

z$b[z$a > 1]/sum(z$b[z$a > 1]) # order looks fine here
[1] 0.2916667 0.3333333 0.3750000


z$d <- ifelse(z$a > 1, 6:8, NA) # fills by row 4,2,3 (as above) z$e <-
ifelse(z$a > 1, 6:7, NA) # recycling, but why values 7,6,7 instead of
6,7,6 ? 

z
  a  b         c  d  e
1 1  6        NA NA NA
2 2  7 0.3333333  7  7
3 3  8 0.3750000  8  6
4 4  9 0.2916667  6  7
5 1 10        NA NA NA

TIA for any insight into how ifelse is filling/recycling in this
instance. I know there's not much code in ifelse, but it's still not
immediately apparent to me why I'm getting what I'm seeing.

John.mckinlay at aad.gov.au

Applied Statistics
Southern Ocean Ecosystems
Australian Antarctic Division, Tasmania

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