Should the help file for identical() say that it mainly useful for testing code (as part of a QA program) and it is not recommended for general use? Currently help(identical) gives, IMO, inappropriate advice:
A call to identical is the way to test exact equality in if and while statements, as well as in logical expressions that use && or ||. In all these applications you need to be assured of getting a single logical value. Users often use the comparison operators, such as == or !=, in these situations. It looks natural, but it is not what these operators are designed to do in R. They return an object like the arguments. If you expected x and y to be of length 1, but it happened that one of them wasn't, you will not get a single FALSE. Similarly, if one of the arguments is NA, the result is also NA. In either case, the expression if(x == y).... won't work as expected. That advice encourages folks to ignore logic or input errors, in addition to this user's problem of identical() being too picky about equality for his purposes. I think that the user should be encouraged to use all(x==y) or any(x==y), perhaps with na.rm=TRUE, in conditionals that might involve non-scalars, not identical(x,y). If you expect a scalar then leave off the any or all and learn that the warning from if that the condition is not a scalar means real trouble (should it be a fatal error?). || and && should give such a warning or error but do not. Bill Dunlap TIBCO Software Inc - Spotfire Division wdunlap tibco.com > -----Original Message----- > From: r-help-boun...@r-project.org > [mailto:r-help-boun...@r-project.org] On Behalf Of Thomas Lumley > Sent: Tuesday, September 15, 2009 10:12 AM > To: Corrado > Cc: r-help@r-project.org; gavin.simp...@ucl.ac.uk > Subject: Re: [R] identical(length(x), 1) returns FALSE, but > print(length(x)) is 1, length(x)==1 is TRUE, and > is.integer(lenght(x)) is TRUE???? > > On Tue, 15 Sep 2009, Corrado wrote: > > > On Tuesday 15 September 2009 17:28:02 Gavin Simpson wrote: > >> [note you don't give us your x so I'm making this up - This is what > >> Duncan was going on about in an earlier thread, give us > something we can > >> just paste into R and it works] > > > > Dear Gavin, > > > > I do not understand what more information! Take any vector > of length 1, for > > example x<-1. Plus all the command that where in my > previous email .... > > > > What is the logic behind > > > > identical(length(x),1) > > > > being false? > > > > length(x) is an integer, and 1 is a floating point number. > They aren't identical, just equal, so > identical(length(x), 1) is FALSE and length(x)==1 is TRUE. > > identical(length(x), 1L) should be TRUE, since 1L is the way > to specify an integer value of 1. > > > -thomas > > Thomas Lumley Assoc. Professor, Biostatistics > tlum...@u.washington.edu University of Washington, Seattle > > ______________________________________________ > 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. > ______________________________________________ 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.
