Why do we have to turn factors into characters for various functions?

Anyway it is maybe more about personal habits than about bad factor
"features"

Hi

r-help-bounces at r-project.org napsal dne 12.12.2010 21:00:37:

At 12.12.2010 00:48 +0200, Tal Galili wrote:

Hello dear R-help mailing list,

My question is *not* about how factors are implemented in R (which is,

if I

understand correctly, that factors keeps numbers and assign levels to

them).

My question *is* about why so many functions that work on factors don't
treat them as characters by default?

Here are two simple examples:
Example one turning the characters inside a factor into numeric:

x <- factor(4:6)
as.numeric(x) # output: 1 2 3
as.numeric(as.character(x)) # output: 4 5 6  # isn't this what we

wanted?

Example two, using strsplit on a factor:

x <- factor(paste(letters[4:6], 4:6, sep="A"))
strsplit(x, "A") # will result in an error:  # Error in strsplit(x,

"A") :

non-character argument
strsplit(as.character(x), "A") # will work and split


So what is the reason this is the case?
Is it that implementing a switch of factors to characters as the

default in

some of the basic function will cause old code to break?
Is it a better design in some other way?

I am curious to know the reason for this.

In my view the answer can be found implicitly in the language

definition.

"Factors are currently implemented using an integer array to specify
the actual levels and a second array of names that are mapped to the
integers. Rather unfortunately users often make use of the
implementation in order to make some calculations easier."

It is the "unfortunate" use of factors that seems generally accepted,
even if the language definition continues:

"This, however, is an implementation issue and is not guaranteed to
hold in all implementations of R."

Personally, like some others, I avoid factors, except in cases, where
they represent a statistical concept.

On contrary I find factors quite useful. Consider possibility to change
its levels

set.seed(111)
x <- factor(sample(1:4, 20, replace=T), labels=c("one", "two", "three",

"four"))

x

 [1] three three two   three two   two   one   three two   one   three
three
[13] one   one   one   two   one   four  two   three
Levels: one two three four

levels(x)[3:4] <- "more"
x

 [1] more more two  more two  two  one  more two  one  more more one  one
one
[16] two  one  more two  more
Levels: one two more

I believe that if x is character, it can be also done but factor way seems
to me more convenient. I also use point distinction in plots by
pch=as.numeric(some.factor) quite often.

Anyway it is maybe more about personal habits than about bad factor
"features"

Regards
Petr

Certainly I would agree with you that, if only reading the "R
Language Definition" and not the documentation of the function
factor, one would rather expect functions like as.numeric or strsplit
to operate on the levels of a factor and not on the underlying,
implementation specific, integer array.

Heinz

Thank you for your reading,
Tal

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