R help - Nov 2012 - 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

tmp0 <- list(a=1, b=2, c=3)
tmp0["b"] <- list(NULL) # single [, list(NULL), not double [[ and
bare NULL
str(tmp0)
# List of 3
#  $ a: num 1
#  $ b: NULL
#  $ c: num 3

Bill Dunlap
Spotfire, TIBCO Software
wdunlap tibco.com

> -----Original Message-----
> From: r-help-bounces at r-project.org [mailto:r-help-bounces at
r-project.org] On Behalf
> Of Gon?alo Ferraz
> Sent: Thursday, November 15, 2012 1:01 PM
> To: r-help at r-project.org
> 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
> 
> ______________________________________________
> R-help at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide
http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.

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: Gon?alo 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

______________________________________________
R-help at r-project.org mailing list
https://stat.ethz.ch/mailman/listinfo/r-help
PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
and provide commented, minimal, self-contained, reproducible code.