Convert factors to numbers

November 29, 2007

(This article was first published on Quantitative Ecology, and kindly contributed to R-bloggers)

If you have a vector of factors it is easy to get the factor level; however, I always forget how to extract the factor value. I ran into the answer here.

> x<-factor(c(round(rnorm(10),2),"A","B",NA))
> x
[1] 1.61 1.12 1.26 0.09 -0.13 0.16 -0.03 -0.1 0.09 -0.47
[11] A B
Levels: -0.03 0.09 -0.1 -0.13 0.16 -0.47 1.12 1.26 1.61 A B
> as.numeric(x)
[1] 9 7 8 2 4 5 1 3 2 6 10 11 NA
> as.numeric(levels(x)[x])
[1] 1.61 1.12 1.26 0.09 -0.13 0.16 -0.03 -0.10 0.09 -0.47
[11] NA NA NA
Warning message:
NAs introduced by coercion

And another method mentioned in the comments (that I like better):

> as.numeric(as.character(x))

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