Context added:
On Mar 2, 2012, at 3:51 PM, knavero wrote:
aggregate(z, identity, mean)
1 2 3 4 5
1.0 3.0 5.0 6.0 7.5
aggregate(z, mean)
Error: length(time(x)) == length(by[[1]]) is not TRUE
As generally happens when you call a function and fail to provide
enough arguments to fill up its formals list.
Can someone help me understand the error above and why "identity" is
necessary to satisfy the error
Well on my machine it throws an error, probably because you failed to
provide the requested code to create the objects you were working on.
Is 'z' so sort of special classed object for which there is an
aggregate method? Is 'identity' a list as expected by
aggregate.default or aggregate.data.frame? It would be an unfortunate
choice of an object name, since there is a function with that nam.
---------end context -----------------
On Mar 2, 2012, at 8:30 PM, knavero more recently wrote:
I've also searched "?identity" in the R shell and it doesn't seem to
be the
definition I'm looking for for this particular usage of "identity"
as an
argument in the aggregate function. I simply would appreciate a
conceptual
explanation of what it does here and how it relates to the error.
I see no relation of the function `identity` to the error. It was
simply an error caused by supplying 'mean` as the second arument to
aggregate.zoo. The second argument in all of the aggregate functions
is a grouping argument. In the case of aggregate.zoo that pattern has
been generalized to allow a function to be applied to the index, but
when aggregate.zoo attempted to apply `mean` to the index vector it
did not return a factor object of the same length as the time vector,
so there was no sensible grouping that could be formed formed.
As the help page (to which Dr Dalgaard referred you) says: `identity`
got applied to the zoo-index. So its use in the first instance
succeeded in creating a vector of the proper length so that when
coerced to a factor provided a grouping. You could have supplied any
function that created a vector length 8.
> index(z) < 3
[1] TRUE TRUE TRUE TRUE FALSE FALSE FALSE FALSE
> aggregate(z, function(x){ x < 3}, mean)
FALSE TRUE
6.5 2.5
Now. ... Will you PLEASE read the Posting Guide and stop sending
emails to Rhelp with no context. The posting behavior of new Nabble
users is a constant annoyance to the list members who view this list
with their mail clients. This is NOT Nabble and most of us do not want
to use a web interface to read Rhelp.
--
David Winsemius, MD
West Hartford, CT
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