I know this is wrong, but why?

My mind has become corrupted by having to use SAS too much. I wanted to
calculate the difference of elements of two vectors if the signs are the
same. I tried it the corrupted way first:

if (effects1[,3] < 0 & effects0[,3] < 0) {
  imprv1 <- effects0[,3] - effects1[,3]
} else  if  (effects1[,3] > 0 & effects0[,3] > 0) {
  imprv1 <- effects1[,3] - effects0[,3]
} else {
  imprv1 <- NA

Then I realized it was giving me wrong results, (for one thing, I was
getting no NAs) and I should be doing it ``The R Way'' like this:

imprv1 <- rep(NA, 267)
imprv1[effects1[,3] < 0 & effects0[,3] < 0] <-  effects0[,3] - \
effects1[,3]
imprv1[effects1[,3] > 0 & effects0[,3] > 0] <-  effects1[,3] - \
effects0[,3]

But what I don't understand is why the first method doesn't work, or at
least why it didn't give me any warning or error messages. Can someone
enlighten me?

(I'm running R 1.5.1 on RedHat Linux 7.3 (intel).)

Stuart Luppescu -=- s-luppescu at uchicago.edu        
University of Chicago -=- CCSR 
$B:MJ8$HCRF`H~$NIc(B -=-    Kernel 2.4.19-pre10-xf                
People don't usually make the same mistake twice --
 they make it three times, four time, five times...
 

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