[Rd] Subscripting fails if name of element is "" (PR#8161)

[Duncan Murdoch]

I haven't been following this conversation in order, but I think there's 
another bug here besides the one(s?) you identified:

Jens had this example:

 > x <- 1:4
 > names(x) <- c(NA, "NA", "a", "")
 > x[names(x)]
<NA> <NA>    a <NA>
    1    1    3   NA

Shouldn't the second entry in the result be 2, with name "NA"?  It seems 
the string "NA" has been converted to <NA> here.

Duncan Murdoch


Prof Brian Ripley wrote:
> On Thu, 6 Oct 2005, "Jens Oehlschlägel" wrote:
> 
> 
>>Dear Thomas,
>>
>>
>>>This looks deliberate (there is a function NonNullStringMatch that does
>>>the matching).  I assume this is because there is no other way to
>>>indicate that an element has no name.
>>
>>>If so, it is a documentation bug -- help(names) and FAQ 7.14 should
>>>specify this behaviour.  Too late for 2.2.0, unfortunately.
>>
>>I respectfully disagree: the element has a name, its an empty string. Of
>>course "" is a doubtful name for an element, but as long as we allow this
>>name when assigning names()<- we also should handle it like a name in
>>subscripting. The alternative would be to disallow "" in names at all.
>>However, both alternatives rather look like code changes, not only
>>documentation.
> 
> 
> I think Thomas is right as to how S interprets this: "" is no name on 
> assignment, wheread NA as a name is a different thing (there probably is a 
> name, we just do not know what it is).
> 
> Here is the crux of the example.
> 
> p <- c(a=1, 2)
> 
>>p <- c(a=1, 2)
>>names(p)
> 
> [1] "a" ""
> 
>>p
> 
> a
> 1 2
> 
>>p2 <- c(1,2)
>>names(p2) <- c("a", "")
>>identical(p, p2)
> 
> [1] TRUE
> 
> so giving the name is "" really is the same as giving no name.
> 
> `Error 1' is said to be
> 
> 
>>p[""]
> 
> <NA>
>    NA
> 
> You haven't given a name, so I think that is right.  S (which has no 
> character NAs) uses "" as the name, but here there may be a name or not.
> 
> 
>>P <- list(a=1, 2)
> 
> 
> I think Jens then meant as `error 2' that
> 
> 
>>P
> 
> $a
> [1] 1
> 
> [[2]]
> [1] 2
> 
> shows no name for the second element, and that seems right to me (although 
> S shows "" here).
> 
> Finally (`error 3')
> 
> 
>>P[""]
> 
> $"NA"
> NULL
> 
> is a length-one list with name character-NA.  (S has no name here.)  That 
> seems the right answer but if so is printed inconsistently.
> 
> I would say that
> 
> 
>>Q <- list(1, 2)
>>names(Q) <- c("a", NA)
>>Q
> 
> $a
> [1] 1
> 
> $"NA"
> [1] 2
> 
> was the only bug here (the name should be printed as <NA>).  Now that
> comes from this bit of code
> 
>  		    if( isValidName(CHAR(PRINTNAME(TAG(s)))) )
>  			sprintf(ptag, "$%s", CHAR(PRINTNAME(TAG(s))));
>  		    else
>  			sprintf(ptag, "$\"%s\"", CHAR(PRINTNAME(TAG(s))));
> 
> so non-syntactic names are printed surrounded by "".  Nowadays I think we 
> would prefer ``, as in
> 
> 
>>A <- list("a+b"=1)
>>A
> 
> $"a+b"
> [1] 1
> 
> 
>>A$"a+b"
> 
> [1] 1
> 
>>A$`a+b`
> 
> [1] 1
> 
> but NA needs to be a special case as in
> 
> 
>>A <- list(1, 2)
>>names(A) <- c("NA", NA)
>>A
> 
> $"NA"
> [1] 1
> 
> $"NA"
> [1] 2
> 
> 
>>is.na(names(A))
> 
> [1] FALSE  TRUE
> 
> 
> 
> 
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