On Nov 15, 2010, at 4:24 PM, 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))
It's a bit more involved than that (since which is expecting a vector
of logicals and mapply is expecting a set of list arguments.) I needed
to send mapply a MoreArgs list that would remain constant from test to
test when using identical.
> which(mapply(identical, a, MoreArgs=list(c(1,2))))
[1] 1
(Admittedly very similar to Iverson's solution, but it is more
readable to my eyes. On the other hand my method may stumble on more
complex object with attributes. You should read hte identical help
page at any rate.)
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.
But neither was testing the "overall equality" of a[1]'s contents with
c(1,2) which appears to be your goal. Iverson's "all" and my
"identical" accomplish that goal.
Any thoughts?
C
--
David Winsemius, MD
West Hartford, CT
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