[R] How to convert a factor to a numeric?
Chris Bergstresser
chris at subtlety.com
Tue Mar 1 23:45:36 CET 2005
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]
[1] NA NA NA NA NA NA
Warning message:
NAs introduced by coercion
> as.numeric(as.character(f))
[1] NA NA NA NA NA NA
Warning message:
NAs introduced by coercion
> as.numeric(f)
[1] 1 2 3 1 2 3
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