[R] detecting NULL in recursive lists

[Vadim Ogranovich]

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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