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

[Wensui Liu]

Thank you so much, Marc and Peter,

Your method works great if I want to convert N dummies into N-level
factor. But what if I want to convert N dummies into (N+1)-level
factor? I tried both ways but none  works.

Again, thank you so much!

On 10/22/06, Marc Schwartz <MSchwartz at mn.rr.com> wrote:
> On Sun, 2006-10-22 at 09:35 +0200, Peter Dalgaard wrote:
> > Marc Schwartz <MSchwartz at mn.rr.com> writes:
> >
> > > 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))
> >
> > ?
>
> That'll do it too...and more efficiently of course.
>
> Thanks Peter.
>
> Regards,
>
> Marc
>
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-- 
WenSui Liu
(http://spaces.msn.com/statcompute/blog)
Senior Decision Support Analyst
Cincinnati Children Hospital Medical Center