[R] Creating dummy codes

[Peter Dalgaard]

"Doran, Harold" <HDoran at air.org> writes:

> Is there an efficient way to create a series of dummy codes from a single variable? For example, I have a variable, `grade' = {2, â¦, 12}. I want to create  k-1 dummy codes for grade such that grade 2 is the base (i.e, grade 2 =0).
> 
> I am hoping that the new variables can be labeled as grade.3, grade.4 etc. I'll then  use 
> 
> grade <- paste("grade.", 3:12, sep="") in as.formula to build the model.
> 
> Thanks,
> 
> Harold
> 
> 
> 	[[alternative HTML version deleted]]

[unfortunately, at least to some of us, the above is only slightly
less unreadable...]

No, you don't want to do that. Instead, do

fgrade <- factor(grade)
lm(y ~ fgrade) # or glm or coxph or whatever

If you really want the dummy variables, look at

model.matrix(~fgrade)

(minus the first (Intercept) column)

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