On Mar 30, 2012, at 12:50 PM, Julio Sergio wrote:
I'm trying to figure out about factors, however the on-line
documentation is
rather sparse. I guess, factors are intended for grouping arrays
members into
categories, which R names "Levels". And so we have:
* state <- c("tas", "sa", "qld", "nsw", "nsw", "nt", "wa", "wa",
"qld", "vic", "nsw", "vic", "qld", "qld", "sa",
"tas",
"sa", "nt", "wa", "vic", "qld", "nsw", "nsw",
"wa",
"sa", "act", "nsw", "vic", "vic", "act")
* statef <- factor(state)
* statef
[1] tas sa qld nsw nsw nt wa wa qld vic nsw vic qld qld sa tas
sa nt wa
[20] vic qld nsw nsw wa sa act nsw vic vic act
Levels: act nsw nt qld sa tas vic wa
With this, just visually, I know what the cateogries or Levels are.
Nonetheless,
two questions arise here: How can I have, computationally as opposed
to
visually, access to the names of these categories, and how do I get
the indexes
of the original array elements that belong to a particular category,
say, "act"?
This is, for instance, to select from another "parallel" array, the
corresponding elements, say
* incomes <- c(60, 49, 40, 61, 64, 60, 59, 54, 62, 69, 70, 42, 56,
61, 61, 61, 58, 51, 48, 65, 49, 49, 41, 48, 52, 46,
59, 46, 58, 43)
So to select, the corresponding elements to "act":
46 43
I think you need to understand indexing more than you need to
understand factors.
incomes [ which(statef == "act") ]
Do you have any comments on this?
If you want to understand how to programmatically access levels, then
you only need to follow the "See also" links on the ?factor page.
--
David Winsemius, MD
West Hartford, CT
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