NULL assignment will change the expression's class into list

On Fri, Oct 8, 2010 at 7:49 PM, Duncan Murdoch <murdoch.duncan at gmail.com>wrote:

  

 On 08/10/2010 12:24 PM, Vitalie Spinu wrote:

    

On Fri, Oct 8, 2010 at 12:14 PM, Duncan Murdoch<murdoch.duncan at gmail.com
      

wrote:
        
 Vitalie Spinu wrote:

        

 Hello Everyone!

 NULL replacement will change expression object into list:



          

 te<- expression(a=23*4, b=33-2)
 te


            

 expression(a = 23 * 4, b = 33 - 2)



          

 te[["a"]]<- quote(blabla) #ok
 te


            

 expression(a = blabla, b = 33 - 2)



          

 te[["a"]]<- NULL #change to list
 te


            

 $b
 33 - 2

 I am on w32, version 2.11.1 (2010-05-31)


          

 That's certainly an inconsistency, still present in a recent R-devel
        

(but I
      

 haven't checked the latest beta).  I don't know if it's a bug:  NULL
 assignments are handled specially in other situations (e.g. if te was a
        

list
      

 to start, the NULL assignment would remove the "a" entry).

 A simple workaround is to use

 te["a"]<- expression(NULL)

 or te<- te[-1]

 instead, depending on what you expected to happen.

        

As ussual with NULL assignment in recursive structures, I would expect to
remove the elements altogether. And this is exactly what I need.

I would say it's a bug, because NULL assignment in data.frames would not
convert them to lists, for example.

      

I think you're probably right.

 Thanks for looking into it. It's quite inconvenient when you have to
    

manipulate named expression. Have to use constructs like
et<-et[!names(et)%in%"a"].

      

Or simply follow te["a"] <- NULL

with

te <- as.expression(te)

This is a pretty fast operation if te is an expression or a list formed by
mistaken conversion from one.

D
    

But is this a reliable way to do it? I have been struggling some time ago
with this type of conversion.
Can not thing of an example now. But as far as I could remember, the
conversion to list of an expression is not always reversible with
as.expression. Am I wrong?
  

uncan Murdoch


 Vitally.
    

 Duncan Murdoch

  Regards,
        

 Vitally.

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