Skip to content

I know this is wrong, but why?

3 messages · Stuart Luppescu, Thomas Lumley, Peter Dalgaard

Original
Stuart Luppescu
# 
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).)
Thomas Lumley
#
On 2 Aug 2002, Stuart Luppescu wrote:

            

      
      
      
    

        
<snip>

            

      
      
      
    

        
Because if() isn't vectorised.  Eg

            

      
      
      
    

        
[1] "a"

Only the first element is used.

I would have thought there was a good case for a warning here, and I
remember seeing one in the past (though it may have been in S-PLUS 3.x in
the pre-R past).

	-thomas

-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-
r-help mailing list -- Read http://www.ci.tuwien.ac.at/~hornik/R/R-FAQ.html
Send "info", "help", or "[un]subscribe"
(in the "body", not the subject !)  To: r-help-request at stat.math.ethz.ch
_._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._

  
  


  


      

    

    

      
      

        


  

        

      Peter Dalgaard
    

    

      
      #
    

  


  

      
Stuart Luppescu <s-luppescu at uchicago.edu> writes:

            

      
      
      
    

        
The main thing is that R generally deals in entire vectors and that
if-statements will only make one decision, never do one thing for some
elements and something else for the rest. In  if (effects1[,3] < 0 &
effects0[,3] < 0)  the condition is a vector. but only the first
element is used, as in

            

      
      
      
    

        
[1]  TRUE  TRUE FALSE

            

      
      
      
    

        
[1]  TRUE FALSE  TRUE

for by-element decisions consider ifelse(), or, as you did, indexing.