[R] list indexing

[David Meyer]

David Orme wrote:
> 
> Hello
> 
> I've got a question about  list indexing. If anyone can help or point
> in the direction of some documentation, that'd be great. I've checked
> the Introduction to R and R language definition.
> 
> Say you have a list of vectors:
> 
> >
> x_list(c("a","b","c"),c("a","b","c","d"),c("a","b"),c("a","b","c","d","e"))
> > x
> [[1]]
> [1] "a" "b" "c"
> 
> [[2]]
> [1] "a" "b" "c" "d"
> 
> [[3]]
> [1] "a" "b"
> 
> [[4]]
> [1] "a" "b" "c" "d" "e"
> Indexing single list components is fine using [[i]] and elements
> within using [[i]][j] but what if you want to get some information,
> for instance vector length or attributes of the components, across (or
> should that be down) the top level components. You can do it by
> looping across 1:length(list):
> 
> > store <- numeric(0)
> > for(components in 1:length(x)) store <-
> c(store,length(x[[components]]))
> > store
> [1] 3 4 2 5
> I know it's only two lines of code. I  guess I'm wondering whether
> there is something that does what I'd expect from length(x[[1:4]]) if
> [[i]] allowed multiple indices.

sapply(components,length)

-d

> 
> Thanks,
> David Orme.

-- 
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