Hi:
> is there a difference between the "as.factor" and "factor" commands
> and also between "as.data.frame" and "data.frame"?
The as.* construct coerces an object from one class to another, so
as.factor() can be used to, for example, coerce a character string to a
factor object. The factor() function is used to *[re]define* a factor
variable. [Sometimes it is useful or necessary to redefine a factor: for
example, a subset of a data frame may contain a subset of the levels of a
particular factor. Without a redefinition, all of the levels in the original
data frame are retained by default, which may not be desirable for some
reason.]
Similarly, as.data.frame() coerces an object of another type (e.g., table,
matrix) to a data frame, whereas data.frame() is used to define a data
frame, where you have the freedom to use whatever variables and types that
you wish, subject to the constraint that the number of rows is the same in
each variable. Usually, the variable types are atomic, such as character,
numeric, integer or factor, but it is possible to include other types of
objects in a data frame.
HTH,
Dennis
On Sat, Jan 22, 2011 at 7:11 AM, analys...@hotmail.com <
analys...@hotmail.com> wrote:
>
>
> On Jan 22, 9:50 am, Berwin A Turlach <ber...@maths.uwa.edu.au> wrote:
> > On Sat, 22 Jan 2011 06:16:43 -0800 (PST)
> >
> > "analys...@hotmail.com" <analys...@hotmail.com> wrote:
> > > (1)
> >
> > > > a = c("a","b")
> > > > mode(a)
> > > [1] "character"
> > > > b = c(1,2)
> > > > mode(b)
> > > [1] "numeric"
> > > > c = data.frame(a,b)
> > > > mode(c$a)
> > > [1] "numeric"
> >
> > R> str(c)
> > 'data.frame': 2 obs. of 2 variables:
> > $ a: Factor w/ 2 levels "a","b": 1 2
> > $ b: num 1 2
> >
> > Character vectors are turned into factors by default by data.frame().
> >
> > OTOH:
> >
> > R> c = data.frame(a,b, stringsAsFactors=FALSE)
> > R> mode(c$a)
> > [1] "character"
> >
> > > (2)
> >
> > > > a = c("a","a","b","b","c")
> > > > levels(as.factor(a))
> > > [1] "a" "b" "c"
> > > > levels(as.factor(a[1:3]))
> > > [1] "a" "b"
> > > > a = as.factor(a)
> > > > levels(a)
> > > [1] "a" "b" "c"
> > > > levels(a[1:3])
> > > [1] "a" "b" "c"
> >
> > Subsetting factors does not get rid of no-longer used levels by default.
> >
> > OTOH:
> >
> > R> levels(a[1:3, drop=TRUE])
> > [1] "a" "b"
> >
> > or
> >
> > R> levels(factor(a[1:3]))
> > [1] "a" "b"
> >
> > HTH.
> >
> > Cheers,
> >
> > Berwin
> >
>
> Thanks for both responses.
>
> is there a difference between the "as.factor" and "factor" commands
> and also between "as.data.frame" and "data.frame"?
>
> ______________________________________________
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>
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