[R] How to convert a factor to a numeric?

From: Chris Bergstresser <chris_at_subtlety.com>
Date: Wed 02 Mar 2005 - 09:45:36 EST

Hi all --

    I've got two columns, both of which correspond to three factor levels (e.g., column1 is "a", "b", or "c"; column2 is "x", "y", or "z").   I'd like to generate a third column, consisting on whether the two factors are correctly aligned for a given case (in this example, "a" corresponds to "x", "b" to "y", and "c" to "z"). For example:

    a x TRUE
    a y FALSE
    b y TRUE
    c z TRUE
    b x FALSE

    Several questions:

    The easiest way seemed to me to be comparing the numeric values across columns, but the encodings are (a=1, b=2, c=3) and (x=1, y=3, z=2). Is there a way to change the underlying value representing each factor, so I could just run an equality on them?

    Is there a simple way to check for correspondence without recoding the factors?

    In the help for factor(), it says "In particular, 'as.numeric' applied to a factor is meaningless, and may happen by implicit coercion.   To "revert" a factor 'f' to its original numeric values, 'as.numeric(levels(f))[f]' is recommended and slightly more efficient than 'as.numeric(as.character(f))'." However, I get the following results. What's going on?

> f = gl(3, 1, 6, labels=c("a", "b", "c"))
> f

[1] a b c a b c
Levels: a b c
> as.numeric(levels(f))[f]

Warning message:
NAs introduced by coercion
> as.numeric(as.character(f))

Warning message:
NAs introduced by coercion
> as.numeric(f)

[1] 1 2 3 1 2 3

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