R help - Jul 2014 - access an element of a list without looping

Hi,
Is there any way to access an element of a list without looping over the list
nor using unlist? Just to avoid parsing a very long list.


For ex, how to find a vector of a length 2 in a list without using a loop?

l = list (c(1), c(2,3), c(1,2,3))
for (i in 1:length(l))
    if(length(l[[i]]==2){
        print (i)
        break
    }

Thanks

Carol

	[[alternative HTML version deleted]]

You could use

which( sapply(l, length) == 2 )

but that still uses a loop internally.

On Thu, Jul 3, 2014 at 1:35 PM, carol white <wht_crl at yahoo.com>
wrote:
> Hi,
> Is there any way to access an element of a list without looping over the
list nor using unlist? Just to avoid parsing a very long list.
>
>
> For ex, how to find a vector of a length 2 in a list without using a loop?
>
> l = list (c(1), c(2,3), c(1,2,3))
> for (i in 1:length(l))
>     if(length(l[[i]]==2){
>         print (i)
>         break
>     }
>
> Thanks
>
> Carol
>
>         [[alternative HTML version deleted]]
>
>
> ______________________________________________
> 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.
>


-- 
Gregory (Greg) L. Snow Ph.D.
538280 at gmail.com

On Jul 3, 2014, at 2:35 PM, carol white <wht_crl at yahoo.com> wrote:
> Hi,
> Is there any way to access an element of a list without looping over the
list nor using unlist? Just to avoid parsing a very long list.
> 
> 
> For ex, how to find a vector of a length 2 in a list without using a loop?
> 
> l = list (c(1), c(2,3), c(1,2,3))
> for (i in 1:length(l))
>     if(length(l[[i]]==2){
>         print (i)
>         break
>     }
> 
> Thanks
> 
> Carol

You can use one of the *apply() functions, albeit, it is still effectively
looping through the list. It may or may not be faster in some cases than an
explicit for() loop, but it can be easier to read, depending upon the complexity
of the function being utilized within the call.

For example:
> which(sapply(l, function(x) length(x) == 2))
[1] 2

This presumes that you only have a single level of list elements to scan. If you
have "sub-levels" within the list, you might want to look at ?rapply,
which is a recursive version.

Regards,

Marc Schwartz