[Rd] Confused about NAMED

[Matthew Dowle]

>
> On Nov 24, 2011, at 12:34 , Matthew Dowle wrote:
>
>>>
>>> On Nov 24, 2011, at 11:13 , Matthew Dowle wrote:
>>>
>>>> Hi,
>>>>
>>>> I expected NAMED to be 1 in all these three cases. It is for one of
>>>> them,
>>>> but not the other two?
>>>>
>>>>> R --vanilla
>>>> R version 2.14.0 (2011-10-31)
>>>> Platform: i386-pc-mingw32/i386 (32-bit)
>>>>
>>>>> x = 1L
>>>>> .Internal(inspect(x))   # why NAM(2)? expected NAM(1)
>>>> @2514aa0 13 INTSXP g0c1 [NAM(2)] (len=1, tl=0) 1
>>>>
>>>>> y = 1:10
>>>>> .Internal(inspect(y))   # NAM(1) as expected but why different to x?
>>>> @272f788 13 INTSXP g0c4 [NAM(1)] (len=10, tl=0) 1,2,3,4,5,...
>>>>
>>>>> z = data.frame()
>>>>> .Internal(inspect(z))   # why NAM(2)? expected NAM(1)
>>>> @24fc28c 19 VECSXP g0c0 [OBJ,NAM(2),ATT] (len=0, tl=0)
>>>> ATTRIB:
>>>> @24fc270 02 LISTSXP g0c0 []
>>>>   TAG: @3f2120 01 SYMSXP g0c0 [MARK,gp=0x4000] "names"
>>>>   @24fc334 16 STRSXP g0c0 [] (len=0, tl=0)
>>>>   TAG: @3f2040 01 SYMSXP g0c0 [MARK,gp=0x4000] "row.names"
>>>>   @24fc318 13 INTSXP g0c0 [] (len=0, tl=0)
>>>>   TAG: @3f2388 01 SYMSXP g0c0 [MARK,gp=0x4000] "class"
>>>>   @25be500 16 STRSXP g0c1 [] (len=1, tl=0)
>>>>     @1d38af0 09 CHARSXP g0c2 [MARK,gp=0x21,ATT] "data.frame"
>>>>
>>>> It's a little difficult to search for the word "named" but I tried and
>>>> found this in R-ints :
>>>>
>>>>   "Note that optimizing NAMED = 1 is only effective within a primitive
>>>> (as the closure wrapper of a .Internal will set NAMED = 2 when the
>>>> promise to the argument is evaluated)"
>>>>
>>>> So might it be that just looking at NAMED using .Internal(inspect())
>>>> is
>>>> setting NAMED=2?  But if so, why does y have NAMED==1?
>>>
>>> This is tricky business... I'm not quite sure I'll get it right, but
>>> let's
>>> try
>>>
>>> When you are assigning a constant, the value you assign is already part
>>> of
>>> the assignment expression, so if you want to modify it, you must
>>> duplicate. So NAMED==2 on z <- 1 is basically to prevent you from
>>> accidentally "changing the value of 1". If it weren't, then you could
>>> get
>>> bitten by code like for(i in 1:2) {z <- 1; if(i==1) z[1] <- 2}.
>>>
>>> If you're assigning the result of a computation, then the object only
>>> exists once, so
>>> z <- 0+1  gets NAMED==1.
>>>
>>> However, if the computation is done by returning a named value from
>>> within
>>> a function, as in
>>>
>>>> f <- function(){v <- 1+0; v}
>>>> z <- f()
>>>
>>> then again NAMED==2. This is because the side effects of the function
>>> _might_ result in something having a hold on the function environment,
>>> e.g. if we had
>>>
>>> e <- NULL
>>> f <- function(){e <<-environment(); v <- 1+0; v}
>>> z <- f()
>>>
>>> then z[1] <- 5 would change e$v too. As it happens, there aren't any
>>> side
>>> effects in the forme case, but R loses track and assumes the worst.
>>>
>>
>> Thanks a lot, think I follow. That explains x vs y, but why is z
>> NAMED==2?
>> The result of data.frame() is an object that exists once (similar to
>> 1:10)
>> so shouldn't it be NAMED==1 too?  Or, R loses track and assumes the
>> worst
>> even on its own functions such as data.frame()?
>
> R loses track. I suspect that is really all it can do without actual
> reference counting. The function data.frame is more than 150 lines of
> code, and if any of those end up invoking user code, possibly via a class
> method, you can't tell definitively whether or not the evaluation
> environment dies at the return.

Ohhh, think I see now. After Duncan's reply I was going to ask if it was
possible to change data.frame() to be primitive so it could set NAMED=1.
But it seems primitive functions can't use R code so data.frame() would
need to be ported to C. Ok! - not quick or easy, and not without
consideable risk. And, data.frame() can invoke user code inside it anyway
then.

Since list() is primitive I tried to construct a data.frame starting with
list() [since structure() isn't primitive], but then merely adding an
attribute seems to set NAMED==2 too ?

> DF = list(a=1:3,b=4:6)
> .Internal(inspect(DF))     # so far so good: NAM(1)
@25149e0 19 VECSXP g0c1 [NAM(1),ATT] (len=2, tl=0)
  @263ea50 13 INTSXP g0c2 [] (len=3, tl=0) 1,2,3
  @263eaa0 13 INTSXP g0c2 [] (len=3, tl=0) 4,5,6
ATTRIB:
  @2457984 02 LISTSXP g0c0 []
    TAG: @3f2120 01 SYMSXP g0c0 [MARK,gp=0x4000] "names"
    @25149c0 16 STRSXP g0c1 [] (len=2, tl=0)
      @1e987d8 09 CHARSXP g0c1 [MARK,gp=0x21] "a"
      @1e56948 09 CHARSXP g0c1 [MARK,gp=0x21] "b"
>
> attr(DF,"foo") <- "bar"    # just adding an attribute sets NAM(2) ?
> .Internal(inspect(DF))
@25149e0 19 VECSXP g0c1 [NAM(2),ATT] (len=2, tl=0)
  @263ea50 13 INTSXP g0c2 [] (len=3, tl=0) 1,2,3
  @263eaa0 13 INTSXP g0c2 [] (len=3, tl=0) 4,5,6
ATTRIB:
  @2457984 02 LISTSXP g0c0 []
    TAG: @3f2120 01 SYMSXP g0c0 [MARK,gp=0x4000] "names"
    @25149c0 16 STRSXP g0c1 [] (len=2, tl=0)
      @1e987d8 09 CHARSXP g0c1 [MARK,gp=0x21] "a"
      @1e56948 09 CHARSXP g0c1 [MARK,gp=0x21] "b"
    TAG: @245732c 01 SYMSXP g0c0 [] "foo"
    @25148a0 16 STRSXP g0c1 [NAM(1)] (len=1, tl=0)
      @2514920 09 CHARSXP g0c1 [gp=0x20] "bar"


Matthew


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> Peter Dalgaard, Professor
> Center for Statistics, Copenhagen Business School
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