Question

I would like to convert my dataframe into a matrix that expands a single factor column into multiple ones and assigns a 1/0 depending on the factor. For example

C1 C2 C3
A  3  5
B  3  4
A  1  1

Should turn into something like

C1_A C1_B C2 C3
1      0  3  5
0      1  3  4
1      0  1  1

How can I do this in R? I tried data.matrix, as.matrix which did not return what I wanted. They assign an “integer” value to a single factor column, there is no expansion.

Answer 1

Assuming dat is your data frame:

cbind(dat, model.matrix( ~ 0 + C1, dat))

  C1 C2 C3 C1A C1B
1  A  3  5   1   0
2  B  3  4   0   1
3  A  1  1   1   0

This solution works with any number of factor levels and without manually specifying column names.

If you want to exclude the column C1, you could use this command:

cbind(dat[-1], model.matrix( ~ 0 + C1, dat))