NULL assignment will change the expression's class into list

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)
  

Regards,
Vitally.

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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.

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"].

Vitally.

 Duncan Murdoch

  Regards,

 Vitally.

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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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