[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????

[William Dunlap]

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-bounces at r-project.org 
> [mailto:r-help-bounces at r-project.org] On Behalf Of Thomas Lumley
> Sent: Tuesday, September 15, 2009 10:12 AM
> To: Corrado
> Cc: r-help at r-project.org; gavin.simpson at 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
> tlumley at u.washington.edu	University of Washington, Seattle
> 
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