How to find out whether a variable is a factor or continuous in R

Question

I have a table with a bunch of variables. What statement I can use to find out whether these variables are considered as a factor or continuous?

Answer 1

Assuming foo is the name of your object and it is a data frame,

f <- sapply(foo, is.factor)

will apply the is.factor() function to each component (column) of the data frame. is.factor() checks if the supplied vector is a factor as far as R is concerned.

Then

which(f)

will tell you the index of the factor columns. f contains a logical vector too, so you could select the factor columns via

foo[, f]

or select all but them

foo[, !f]

Here is an example:

> ## some dummy data
> foo <- data.frame(a = factor(1:10), b = 1:10, c = factor(letters[1:10]))
> foo
    a  b c
1   1  1 a
2   2  2 b
3   3  3 c
4   4  4 d
5   5  5 e
6   6  6 f
7   7  7 g
8   8  8 h
9   9  9 i
10 10 10 j
> ## apply is.factor
> f <- sapply(foo, is.factor)
> f
   a     b     c 
TRUE FALSE  TRUE
> ## which are factors
> which(f)
a c 
1 3
> ## select those
> foo[, f]
    a c
1   1 a
2   2 b
3   3 c
4   4 d
5   5 e
6   6 f
7   7 g
8   8 h
9   9 i
10 10 j

There are equivalent checks for numeric and integer too, amongst others: is.numeric() and is.integer(), but you only need is.numeric() if you don't care about the type of numbers:

> is.numeric(1L)
[1] TRUE

(Also is.character(), is.logical(), ...)

Answer 2

You have to use is.factor and is.numeric.