On 28/08/2008, at 11:30 AM, John McKinlay wrote:
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.
(a) Don't cloud the issue by dragging in data frames. As was
recently pointed
out in a discussion on how to learn R, not all data structures need
be rectangular.
(b) The function ifelse() takes 3 vector arguments: test, yes, and no.
The help (RTFM) for ifelse() says quite explicitly that if 'yes' and
'no' are too
short, their elements are recycled.
(c) So if test is a > 1, yes is 6:7 and no is NA you get (the following
needs to be read in fixed width font and no mucking around by an
arrogant
mailer that thinks it knows your mind better than your do):
test yes no
---- --- --
FALSE 6 NA
TRUE 7 NA
TRUE 6 NA
TRUE 7 NA
FALSE 6 NA
Consequently the result of ifelse(a>1,6:7,NA) is c(NA,7,6,7,NA).
What's the problem?
cheers,
Rolf Turner
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