On Jan 22, 2010, at 2:07 PM, Fabrice DELENTE wrote:
Here's another R-way:
lets<-factor(c( 'A', 'B', 'A', 'C', 'B', 'D', 'B'))
# you did say they were factors, right?
nums <- factor(c('1', '2', '2', '3', '2', '2', '3'))
lets=="B"
[1] FALSE TRUE FALSE FALSE TRUE FALSE TRUE
sum(lets=="B" & nums=="2")
[1] 2
Thanks very much, it will save me :^)
As I am a beginner in R, I have a little trouble understanding
factors. Can
they be used interchangeably with lists, or are they a different
data-type?
Can I do on factors everything that I can do on lists?
Not at all. Lists can be arbitrarily complex structures. Factors are
one-dimensional. The main use of factors is to represent discrete
groupings of one dimension. Internally elements within factors are
represented by integers and the levels of the factors are usually
converted to string-valued "labels" or "'levels". To be frank, I'm not
clear about how levels and labels could be used in parallel. I
generally apply functions that work with levels. So factors are much
more like vectors than they are like lists. To work with R you need to
have a clear idea of the distinctions between:
lists: tree structures, do have a length property which is the number
of elements at the top level.
dataframes: (a specialized type of a two-dimensional list) lengths
are the number of "columns" rather than the number of rows, can hold
columns of different types
matrices: 2-dimensional arrays, element are of only one type
arrays: n-dimensional
vectors 1-dim, elements all of the same type: logical, integer,
numeric or character
factors 1-dim, essentially integers with character labels
Thanks again!
--
Fabrice DELENTE
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David Winsemius, MD
Heritage Laboratories
West Hartford, CT
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PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
and provide commented, minimal, self-contained, reproducible code.
