[Rd] names<- bug, and incompatibility with c() (PR#2358)

[ripley@stats.ox.ac.uk]

This is S-compatible, though, and it I think it is quite legitimately
an error.  I will fix it in R-devel, but I am little concerned that
advanced users may be surprised (but they probably would never do this).

> there is inconsistent behaviour depending on whether the element being named
> is the last one:
>
> test> mm <- 1:3
> test> names( mm)[1] <- 'y'
> Error in "names<-.default"(*tmp*, value = "y") :
>         names attribute must be the same length as the vector
> test> names( mm)[2] <- 'y'
> Error in "names<-.default"(*tmp*, value = "y") :
>         names attribute must be the same length as the vector
> test> names( mm)[3] <- 'y'
> test> mm
> <NA> <NA>    y
>    1    2    3
>
> As long as the last element of mm gets mentioned, R will allow the
> operation, filling in any blanks with NA-strings. But if the last element of
> mm isn't specified somewhere in the vector of things-to-be-named, R won't
> allow the operation.
>
> If mm already has names, there's no problem.
>
> The usage above is sometimes valuable, and it should be easy to fix it, by
> supplying default names in all other positions. But there is also some
> inconsistency with the use of "c": should default names be NA-strings, or
> empty strings?

They should now be NA-strings.

> test> mm <- 1:2
> test> names( mm)[2] <- 'y'
> test> mm
> <NA>    y
>    1    2
> test> mm <- c( mm, 3)
> test> mm
> NA  y
>  1  2  3
> test> names( mm)[1]
> [1] "NA"
> test>
>
>  Note that "c" has supplied an empty string for the unspecified name, and
> has also taken the liberty of changing the special NA-string into a "NA"
> (i.e. a 2-character string, first letter N, second letter A)!

That's a bug.

> An easy way to achieve consistency, would be to make unsupplied names should
> always default to an empty string, rather than a special NA-string.

But what's the point of having a special string then?

-- 
Brian D. Ripley,                  ripley@stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272860 (secr)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595