[R] The correct way to set an element in a list to NULL? (FAQ is not clear)

[Steve Lianoglou]

Hi,

On Dec 11, 2009, at 12:07 PM, Peng Yu wrote:

> http://cran.r-project.org/doc/FAQ/R-FAQ.html#How-can-I-set-components-of-a-list-to-NULL_003f
> 
> The explanation on this FAQ entry is not clear. It says '... similarly
> for named components...'. What I understood was x[i]<-list(NULL) is
> the same as x$a_name<-list(NULL). But, they are not.

You're right, x[i] is not the same as x$a_name:

R> x <- list(a=1:3, b='hello')
R> identical(x[1], x$a)
[1] FALSE

R> identical(x[1], x['a'])
[1] TRUE

I think that's what it means by "similarly for named components." You see, x$a_name is really x[[1]], which does the destructive-null-assignment-thing:

R> identical(x[[1]], x$a)
[1] TRUE

-steve


> As the example
> below shows, x$a_name<-list(NULL) is the same as x[[i]]<-list(NULL).
> 
>> x=list(a=1:3,b=NULL,c=2:5,d=NULL)
>> x
> $a
> [1] 1 2 3
> 
> $b
> NULL
> 
> $c
> [1] 2 3 4 5
> 
> $d
> NULL
> 
>> x[[3]]=list(NULL)
>> x
> $a
> [1] 1 2 3
> 
> $b
> NULL
> 
> $c
> $c[[1]]
> NULL
> 
> 
> $d
> NULL
> 
>> x$c=list(NULL)
>> x
> $a
> [1] 1 2 3
> 
> $b
> NULL
> 
> $c
> $c[[1]]
> NULL
> 
> 
> $d
> NULL
> 
>> 
>> x[[3]]=7:8
>> x
> $a
> [1] 1 2 3
> 
> $b
> NULL
> 
> $c
> [1] 7 8
> 
> $d
> NULL
> 
>> x$c=7:8
>> x
> $a
> [1] 1 2 3
> 
> $b
> NULL
> 
> $c
> [1] 7 8
> 
> $d
> NULL
> 
> 
> What seems confusing to me is:
> even 'x[i]<-list(NULL)' and 'x[[i]]<-list(NULL)' are different, why
> x[i]<-NULL and x[[i]]<-NULL are the same?
> 
> Shouldn't the meaning of 'x[[i]]<-NULL' be defined as the set the i'th
> element NULL, rather than deleting the i'th element?
> 
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--
Steve Lianoglou
Graduate Student: Computational Systems Biology
  |  Memorial Sloan-Kettering Cancer Center
  |  Weill Medical College of Cornell University
Contact Info: http://cbio.mskcc.org/~lianos/contact