On Wed, Jun 18, 2008 at 3:10 PM, Christos Argyropoulos <[EMAIL PROTECTED]> wrote: > > 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 > _________________________________________________________________
from ?"&" : " The shorter form performs elementwise comparisons in much the same way as arithmetic operators. The longer form evaluates left to right examining only the first element of each vector. " Thus, > a$r & a$g [1] FALSE FALSE FALSE TRUE > a$r && a$g [1] FALSE ifelse takes a vector as argument. isince && only gives a single value, ifelse(r>0 && g> 0,log(r/g),NA) will only return NA, which then is recycled by transform. When using &, ifelse returns a vector, and this vector is appended to the data frame. /Gustaf -- Gustaf Rydevik, M.Sci. tel: +46(0)703 051 451 address:Essingetorget 40,112 66 Stockholm, SE skype:gustaf_rydevik ______________________________________________ R-help@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.
