R help - Apr 2010 - replace elements in a list

Dear all, I have a list like this:  l <- list(list(a=1,b=NULL),
list(a=2,b=2))
I want to find out the elements with value of NULL and replace them with NA.
The actual case has a very long list, so manually find out and replace
them is not an option.
I can use for loop to do this, but I want to know if there is
vectorized way (or other ways) to do it?

Thanks
-- 
Wincent Rong-gui HUANG
Doctoral Candidate
Dept of Public and Social Administration
City University of Hong Kong
http://asrr.r-forge.r-project.org/rghuang.html

Wincent wrote:
> Dear all, I have a list like this:  l <- list(list(a=1,b=NULL),
list(a=2,b=2))
> I want to find out the elements with value of NULL and replace them with
NA.
> The actual case has a very long list, so manually find out and replace
> them is not an option.
> I can use for loop to do this, but I want to know if there is
> vectorized way (or other ways) to do it?
>
Note:

You have provided a list *of lists*.  Is that what you actually have?  If so,

Solution 1:

Change whatever is generating the NULL values to NA in the first place

Solution 2:

lapply(l, function(x) lapply(x, function(x) ifelse(is.null(x), NA, x)))

Thanks.

On 30 April 2010 12:14, Erik Iverson <eriki at ccbr.umn.edu>
wrote:
> Wincent wrote:
>>
>> Dear all, I have a list like this: ?l <- list(list(a=1,b=NULL),
>> list(a=2,b=2))
>> I want to find out the elements with value of NULL and replace them
with
>> NA.
>> The actual case has a very long list, so manually find out and replace
>> them is not an option.
>> I can use for loop to do this, but I want to know if there is
>> vectorized way (or other ways) to do it?
>>
> Note:
>
> You have provided a list *of lists*. ?Is that what you actually have? ?If
> so,
>
> Solution 1:
>
> Change whatever is generating the NULL values to NA in the first place
Yes, it is a list of lists. I realized the returned value might have
NULL after I have the data. I just need to deal with it.
You are right, when I knew that, I will generate NA in the first place.
> Solution 2:
>
> lapply(l, function(x) lapply(x, function(x) ifelse(is.null(x), NA, x)))
Cool. that is what I want.

Thanks gain.
>


-- 
Wincent Rong-gui HUANG
Doctoral Candidate
Dept of Public and Social Administration
City University of Hong Kong
http://asrr.r-forge.r-project.org/rghuang.html

> -----Original Message-----
> From: r-help-bounces at r-project.org 
> [mailto:r-help-bounces at r-project.org] On Behalf Of Wincent
> Sent: Thursday, April 29, 2010 8:40 PM
> To: r help
> Subject: [R] replace elements in a list
> 
> Dear all, I have a list like this:  l <- 
> list(list(a=1,b=NULL), list(a=2,b=2))
> I want to find out the elements with value of NULL and 
> replace them with NA.
I think that rapply(how="replace") ought to be able to do this
but it doesn't do the right thing when the list has a NULL in
it.  E.g., see how the following returns a list the shape of
the input list, changing all the non-NULL entries as requested,
but the NULL entries are replaced by empty lists instead of 0's:
  > rapply(l, function(x)
  +   if(is.null(x)) 0 else paste("*", deparse(x), "*",
collapse="/",
sep=""),  how="replace")
  [[1]]
  [[1]]$a
  [1] "*1*"

  [[1]]$b
  list()
  
  
  [[2]]
  [[2]]$a
  [1] "*2*"

  [[2]]$b
  [1] "*2*"

The following function may do what you want:
  replaceInList <- function (x, FUN, ...) 
  {
      if (is.list(x)) {
          for (i in seq_along(x)) {
              x[i] <- list(replaceInList(x[[i]], FUN, ...))
          }
          x
      }
      else FUN(x, ...)
  }
E.g.,
  > replaceInList(l, function(x)if(is.null(x))0 else x)
  [[1]]
  [[1]]$a
  [1] 1

  [[1]]$b
  [1] 0
  
  
  [[2]]
  [[2]]$a
  [1] 2
  
  [[2]]$b
  [1] 2

Bill Dunlap
Spotfire, TIBCO Software
wdunlap tibco.com 
> The actual case has a very long list, so manually find out and replace
> them is not an option.
> I can use for loop to do this, but I want to know if there is
> vectorized way (or other ways) to do it?
> 
> Thanks
> -- 
> Wincent Rong-gui HUANG
> Doctoral Candidate
> Dept of Public and Social Administration
> City University of Hong Kong
> http://asrr.r-forge.r-project.org/rghuang.html
> 
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