# Re: [R] how to convert multiple dummy variables to 1 factor variable?

From: Peter Dalgaard <p.dalgaard_at_biostat.ku.dk>
Date: Sun 22 Oct 2006 - 07:35:51 GMT

> On Sat, 2006-10-21 at 21:04 -0400, Wensui Liu wrote:
> > Dear Listers,
> >
> > I am wondering how to convert multiple dummy variables to 1 factor variable.
> >
> > Thanks.
> >
> > wensui
>
> I was thinking of a function that is essentially the reverse of
> model.matrix() which is used by R modeling functions. I did not see one,
> though it is possible that I missed it.
>
> However, I suppose that something along the lines of the following would
> work.
>
> Say we have a matrix as follows, where the columns represent the
> presence or absence of the factor levels, as one would see in a model
> matrix. There should be a single '1' in each row as each row corresponds
> to a single observation.
>
> > mat
> Level1 Level2 Level3 Level4 Level5
> [1,] 0 1 0 0 0
> [2,] 1 0 0 0 0
> [3,] 0 0 0 1 0
> [4,] 0 0 1 0 0
> [5,] 0 0 0 0 1
>
>
> # Create a new factor based upon the index of each 1 in each row
> # Use the matrix column names as the labels for each level
> NewFactor <- factor(apply(mat, 1, function(x) which(x == 1)),
> labels = colnames(mat))
>
> > NewFactor
> [1] Level2 Level1 Level4 Level3 Level5
> Levels: Level1 Level2 Level3 Level4 Level5

How about

factor(mat%*%(1:5), labels = colnames(mat))

?

--
O__  ---- Peter Dalgaard             Ă˜ster Farimagsgade 5, Entr.B
c/ /'_ --- Dept. of Biostatistics     PO Box 2099, 1014 Cph. K
(*) \(*) -- University of Copenhagen   Denmark          Ph:  (+45) 35327918
~~~~~~~~~~ - (p.dalgaard@biostat.ku.dk)                  FAX: (+45) 35327907

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Received on Sun Oct 22 17:42:02 2006

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