R help - Mar 2009 - detecting NULL in recursive lists

Dear R-users,

How can I detect a NULL in a recursive list?

For a regular list I could use lapply:
> lapply(list(x=NULL), is.null)
$x
[1] TRUE


However that doesn't work for structures like list(list(x=NULL)). I tried
rapply but it treats NULL as a list and discards them:
> rapply(list(a=1, b=list(x=NULL)), is.null)
    a
FALSE

Any suggestion?

Thank you for your help,
Vadim

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On Mon, 9 Mar 2009, Vadim Ogranovich wrote:
> Dear R-users,
>
> How can I detect a NULL in a recursive list?
>
> For a regular list I could use lapply:
>
>> lapply(list(x=NULL), is.null)
> $x
> [1] TRUE
>
>
> However that doesn't work for structures like list(list(x=NULL)). I
tried rapply but it treats NULL as a list and discards them:
>
>> rapply(list(a=1, b=list(x=NULL)), is.null)
>    a
> FALSE
>
> Any suggestion?
Is this what you want?
> foo <- function(x) sapply(x, function(x) if (is.list(x)) foo(x) else
length(x)==0,simplify=FALSE)
> foo(list(a=list(A=1:2,B=NULL), b=list(C=NULL,D=1)))
$a
$a$A
[1] FALSE

$a$B
[1] TRUE


$b
$b$C
[1] TRUE

$b$D
[1] FALSE

> 
>
HTH,

Chuck
>
> Thank you for your help,
> Vadim
>
> Note: This email is for the confidential use of the named addressee(s) only
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and does not constitute a recommendation, offer, request or solicitation of any
kind to buy, sell, subscribe, redeem or perform any type of transaction of a
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>
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