From simon.urbanek at r-project.org  Thu Nov 24 18:11:19 2011
From: simon.urbanek at r-project.org (Simon Urbanek)
Date: Thu, 24 Nov 2011 12:11:19 -0500
Subject: [Rd] Confused about NAMED
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On Nov 24, 2011, at 8:05 AM, Matthew Dowle wrote:

>> 
>> 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 ?
> 

Yes, because attr(x,y) <- z is the same as

`*tmp*` <- x
x <- `attr<-`(`*tmp*`, y, z)
rm(`*tmp*`)

so there are two references to the data frame: one in DF and one in `*tmp*`. It is the first line that causes the NAMED bump. And, yes, it's real:

> `f<-`=function(x,value) { print(ls(parent.frame())); x<-value }
> x=1
> f(x)=1
[1] "*tmp*" "f<-"   "x"    

You could skip that by using the function directly (I don't think it's recommended, though):

> .Internal(inspect(l <- list(a=1)))
@1028c82f8 19 VECSXP g0c1 [NAM(1),ATT] (len=1, tl=0)
  @1028c8268 14 REALSXP g0c1 [] (len=1, tl=0) 1
ATTRIB:
  @100b6e748 02 LISTSXP g0c0 [] 
    TAG: @100843878 01 SYMSXP g0c0 [MARK,gp=0x4000] "names"
    @1028c82c8 16 STRSXP g0c1 [] (len=1, tl=0)
      @1009cd388 09 CHARSXP g0c1 [MARK,gp=0x21] "a"
> .Internal(inspect(`names<-`(l, "b")))
@1028c82f8 19 VECSXP g0c1 [NAM(1),ATT] (len=1, tl=0)
  @1028c8268 14 REALSXP g0c1 [] (len=1, tl=0) 1
ATTRIB:
  @100b6e748 02 LISTSXP g0c0 [] 
    TAG: @100843878 01 SYMSXP g0c0 [MARK,gp=0x4000] "names"
    @1028c8178 16 STRSXP g0c1 [NAM(1)] (len=1, tl=0)
      @100967af8 09 CHARSXP g0c1 [MARK,gp=0x20] "b"
> .Internal(inspect(l))
@1028c82f8 19 VECSXP g0c1 [NAM(1),ATT] (len=1, tl=0)
  @1028c8268 14 REALSXP g0c1 [] (len=1, tl=0) 1
ATTRIB:
  @100b6e748 02 LISTSXP g0c0 [] 
    TAG: @100843878 01 SYMSXP g0c0 [MARK,gp=0x4000] "names"
    @1028c8178 16 STRSXP g0c1 [NAM(1)] (len=1, tl=0)
      @100967af8 09 CHARSXP g0c1 [MARK,gp=0x20] "b"

Cheers,
Simon



>> 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
> 
> 
>> --
>> Peter Dalgaard, Professor
>> Center for Statistics, Copenhagen Business School
>> Solbjerg Plads 3, 2000 Frederiksberg, Denmark
>> Phone: (+45)38153501
>> Email: pd.mes at cbs.dk  Priv: PDalgd at gmail.com
>> 
>> 
> 
> ______________________________________________
> R-devel at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-devel
> 
> 



[Rd] Confused about NAMED
Simon Urbanek