[R] indexing lists

[Erik Iverson]

Chris,

Well, the 'answer' could be:

which(sapply(a, function(x) all(x == c(1,2))))

But I wonder how these elements of 'a' in your
actual application are coming to be?  If you're
constructing them, you can give the elements of
the list names, and then it doesn't matter what
numerical index they have, you can just reference
them by name.

a <- list(name1 = 1:2,
           name2 = 3:4)
a
a <- c(anothername = list(9:10), a)
a
a$name1

Chris Carleton wrote:
> Hi List,
> 
> I'm trying to work out how to use which(), or another function, to find the
> top-level index of a list item based on a condition. An example will clarify
> my question.
> 
> a <- list(c(1,2),c(3,4))
> a
> [[1]]
> [1] 1 2
> 
> [[2]]
> [1] 3 4
> 
> I want to find the top level index of c(1,2), which should return 1 since;
> 
> a[[1]]
> [1] 1 2
> 
> I can't seem to work out the syntax. I've tried;
> 
> which(a == c(1,2))
> 
> and an error about coercing to double is returned. I can find the index of
> elements of a particular item by
> 
> which(a[[1]]==c(1,2)) or which(a[[1]]==1) etc that return [1] 1 2 and [1] 1
> respectively as they should. Any thoughts?
> 
> C
> 
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> 
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