[R] assigning NULL to a list element without changing the length of the list

[arun]

HI,
You could try these:
tmp1<-list(a=1,b=NA,c=3,d=NA)
 lapply(tmp1,function(x) if(is.na(x)) NULL else x)  #changing NA to NULL

#$a
#[1] 1
#
#$b
#NULL
#
#$c
#[1] 3
#
#$d
#NULL

#other case;
tmp2<-list(a=1,b=3,c=4,d=5)  #want to change list elements "a", "c" to NULL

tmp3<-lapply(tmp2,function(x) {if(names(tmp2)[match.call()[[2]][[3]]]%in% c("a","c")) NULL else x})

 tmp3
#$a
#NULL
#
#$b
#[1] 3
#
#$c
#NULL
#
#$d
#[1] 5


A.K.




----- Original Message -----
From: Gonçalo Ferraz <gferraz29 at gmail.com>
To: r-help at r-project.org
Cc: 
Sent: Thursday, November 15, 2012 4:01 PM
Subject: [R] assigning NULL to a list element without changing the length of the list

Hi,

I have a list:

tmp0 <- list(a=1, b=2, c=3)

And I realize that I can append a NULL element to the end of this list, just by writing:

length(tmp0) <- 4

Now, the question is, how can I assign NULL to one of the existing list elements without changing the length of the list?

Please note I am working from inside a for loop that is working on one list element at a time and in some circumstances I want to 
set one element to NULL. So, specifying the whole list again as in:

tmp0 <- list(a=1,b=NULL,c=3)

is not an option.

But writing, say:

tmp0 [[2]] <- NULL

is not an option either, because it leaves me with a list of length 2.

Is there a solution for this?

Thank you

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