[R] ifelse and "&&" vs "&"

[Christos Argyropoulos]

Hi,

I noticed whether some one could explain why "&" and "&&" behave differently in data frame transformations.

Consider the following :

a<-data.frame(r=c(0,0,2,3),g=c(0,2,0,2.1))

Then:

> transform(a,R=ifelse(r>0 && g> 0,log(r/g),NA))

  r   g  R
1 0 0.0 NA
2 0 2.0 NA
3 2 0.0 NA
4 3 2.1 NA

but 

> transform(a,R=ifelse(r>0 & g> 0,log(r/g),NA))
  r   g         R
1 0 0.0        NA
2 0 2.0        NA
3 2 0.0        NA
4 3 2.1 0.3566749


If my understanding of the differences between "&" and "&&" and how 'transform' works are accurate, both statements should produce the same output.


I got the same behaviour in Windows XP Pro 32-bit (running R v 2.7) and Ubuntu Hardy (running the same version of R).


Thanks

Christos Argyropoulos

University of Pittsburgh Medical Center
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