# [R] I know this is wrong, but why?

Bill.Venables@cmis.csiro.au Bill.Venables at cmis.csiro.au
Sat Aug 3 06:21:40 CEST 2002

```Others have already replied to this pointing out that if ... else ... is a
control statement whereas ifelse(...) is the vectorized function.

Just a minor follow-up point.  The way I would do this calculation is as
follows:

imprv1 <- ifelse(effects0[, 3]*effects1[, 3] > 0,
sign(effects0[, 3]) * (effects1[, 3] - effects0[, 3]),
NA)

I'm not suggesting that your way is any less efficient, but this way does
illustrate vectorization  a bit more explicitly as well as the "whole
object" view in a way which maximally contrasts with the SAS "line by line"
view.

Bill Venables.

>  -----Original Message-----
> From: 	Stuart Luppescu [mailto:s-luppescu at uchicago.edu]
> Sent:	Saturday, August 03, 2002 6:57 AM
> To:	R-Help List
> Subject:	[R] 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
> 才文と智奈美の父 -=-    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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```