[R] how to efficiently extract elements of a list?

[Duncan Murdoch]

On 07/02/2011 10:13 AM, Sean Zhang wrote:
> Dear R helper,
>
> I wonder whether there is a quick way to extract some elements for a list.
>
> for a vector we can do the following
>
> vec<- seq(3)
> names(vec)<- LETTERS[1:3]
>
> vec[c(1,3)]
> vec[c('A','C')]
>
>
> But for a list,
> test.l<- list(c(1,3),array(NA,c(1,2)),array(0,c(2,3)))
> names(test.l)<-LETTERS[1:3]
>
> The following does not work. is there some command (I was thinking of
> do.call) that can do the job?
>
> test.l[[c('A','B')]]
> test.l[[c(1,3)]]

Use single brackets, i.e.

test.l[c('A', 'B')]
test.l[c(1,3)]

The single bracket is the "subsetting" operator, which is what you're 
doing here, since you want a list as the result.  Double brackets 
extract single elements and return whatever type the element is.

Duncan Murdoch


> do.call('[',c(test.l,c(1,3)))
> do.call('[[',c(test.l,c(1,3)))
> do.call('[',c(test.l,c('A','C')))
> do.call('[[',c(test.l,c('A','C')))
>
> Thanks in advance.
>
> -Sean
>
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>
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