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

[Marc Schwartz]

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


HTH,

Marc Schwartz