R help - Sep 2011 - If NA Problem!

Hi All!

Please find code and the respective lists below.  My problem: I specify the
case that lilwin[[p]] is not an NA and want the code found in iwish to be
returned ONLY for that case.  Why do I get a list of length 2 (and why is
NULL the first element)? I understand that the code below is quite
senseless.  I have run into a problem while working on a large project and
wanted to simplify it in order for it to be more understandable and
accessible.  If I should not be using the if function, please let me know
what I should be doing instead.  I know that I must use the for function for
my project.  The thing I most want to understand is how, after specifying a
certain condition, one may save certain data that occurs when that condition
is met.   I hope I have been clear enough!

Thank you very much for your help!
Anna

biglist<-list(a=1:4,b=2:6)
lilwin<-list(x=NA,y=2)
lilloss<-list(m=1,n=3)


> biglist$a
[1] 1 2 3 4

$b
[1] 2 3 4 5 6
> lilwin$x
[1] NA

$y
[1] 2
> lilloss$m
[1] 1

$n
[1] 3


iwish<-list()
for(p in 1:length(biglist)){
if(is.na(lilwin[[p]])==F) iwish[p]<-list(biglist[[p]][lilwin[[p]]])
}

> iwish[[1]]
NULL

[[2]]
[1] 3

	[[alternative HTML version deleted]]

Anna Dunietz wrote on 09/02/2011 07:16:45 AM:
> 
> Hi All!
> 
> Please find code and the respective lists below.  My problem: I specify 
the
> case that lilwin[[p]] is not an NA and want the code found in iwish to 
be
> returned ONLY for that case.  Why do I get a list of length 2 (and why 
is
> NULL the first element)? 

The list is length 2 because when p=2 in your for loop you save the output 
to iwish[2].  Since nothing was saved to iwish[1] (when lilwin[[1]] was 
NA) that gives a NULL value for iwish[1].  If you want to allow the length 
of iwish to be shorter, you need to change the way you save your output. 
For example,

iwish <- list() 
count <- 0
for(p in 1:length(biglist)){ 
        if(is.na(lilwin[[p]])==F) {
                count <- count + 1
                iwish[count] <- list(biglist[[p]][lilwin[[p]]]) 
                }
} 

Jean

> I understand that the code below is quite
> senseless.  I have run into a problem while working on a large project 
and
> wanted to simplify it in order for it to be more understandable and
> accessible.  If I should not be using the if function, please let me 
know
> what I should be doing instead.  I know that I must use the for function 
for
> my project.  The thing I most want to understand is how, after 
specifying a
> certain condition, one may save certain data that occurs when that 
condition
> is met.   I hope I have been clear enough!
> 
> Thank you very much for your help!
> Anna
> 
> biglist<-list(a=1:4,b=2:6)
> lilwin<-list(x=NA,y=2)
> lilloss<-list(m=1,n=3)
> 
> 
> 
> > biglist$a
> [1] 1 2 3 4
> 
> $b
> [1] 2 3 4 5 6
> > lilwin$x
> [1] NA
> 
> $y
> [1] 2
> > lilloss$m
> [1] 1
> 
> $n
> [1] 3
> 
> 
> iwish<-list()
> for(p in 1:length(biglist)){
> if(is.na(lilwin[[p]])==F) iwish[p]<-list(biglist[[p]][lilwin[[p]]])
> }
> 
> 
> > iwish[[1]]
> NULL
> 
> [[2]]
> [1] 3
	[[alternative HTML version deleted]]

On 2011-09-02 05:16, Anna Dunietz wrote:
> Hi All!
>
> Please find code and the respective lists below.  My problem: I specify the
> case that lilwin[[p]] is not an NA and want the code found in iwish to be
> returned ONLY for that case.  Why do I get a list of length 2 (and why is
> NULL the first element)? I understand that the code below is quite
> senseless.  I have run into a problem while working on a large project and
> wanted to simplify it in order for it to be more understandable and
> accessible.  If I should not be using the if function, please let me know
> what I should be doing instead.  I know that I must use the for function
for
> my project.  The thing I most want to understand is how, after specifying a
> certain condition, one may save certain data that occurs when that
condition
> is met.   I hope I have been clear enough!
>
> Thank you very much for your help!
> Anna
>
> biglist<-list(a=1:4,b=2:6)
> lilwin<-list(x=NA,y=2)
> lilloss<-list(m=1,n=3)
>
>
>
>> biglist$a
> [1] 1 2 3 4
>
> $b
> [1] 2 3 4 5 6
>> lilwin$x
> [1] NA
>
> $y
> [1] 2
>> lilloss$m
> [1] 1
>
> $n
> [1] 3
>
>
> iwish<-list()
> for(p in 1:length(biglist)){
> if(is.na(lilwin[[p]])==F) iwish[p]<-list(biglist[[p]][lilwin[[p]]])
> }
>
>
>> iwish[[1]]
> NULL
>
> [[2]]
> [1] 3
Jean has given you one fix. Here's another (see ?'c'):

  iwish<-list()
  for(p in seq_along(biglist)){
    if(!is.na(lilwin[[p]]))
        iwish <- c(iwish, biglist[[p]][lilwin[[p]]])
  }

BTW, it's not a good idea to use 'F' instead of FALSE and the
negation operator is usually a better way to test.

Peter Ehlers
>
> 	[[alternative HTML version deleted]]
>
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