[R] Why can't "apply" be used with "as.factor" on a data.frame ?

[hadley wickham]

The basic reason because apply works with matrices - it first turns
the input into a matrix, processes each column and then returns a
matrix.  See colwise in the plyr package for a function that works
column wise on a data frame, returning a data frame.

Hadley

On Sun, Mar 7, 2010 at 11:07 AM, Tal Galili <tal.galili at gmail.com> wrote:
> Hi all,
>
> Let's say I have a data.frame and wants to turn each of it's columns into a
> factor.
> My instinct would be to use as.factor with apply. But this won't work, and
> result with a data.frame of characters.
> I found another solution for how to achieve this, but I would also like to
> understand - *WHY* does it work this way?
>
> Here is an example script:
> a <- data.frame(x1 = rnorm(100), x2 = sample(c("a","b"), 100, replace = T),
> x3 = factor(c(rep("a",50) , rep("b",50))))
> apply(a2, 2,class) # why is column 3 not a factor ?
> a[,3]  # since it IS a factor.
> a2 <- apply(a, 2,as.factor) # won't work - why not ?
> a2[,3]  # Why was this just turned into a character ???
> # A solution
> a2 <- lapply(a, as.factor)
> a3 <- as.data.frame(a2)
> str(a3)
>
>
> Thanks,
> Tal
>
>
>
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-- 
Assistant Professor / Dobelman Family Junior Chair
Department of Statistics / Rice University
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