Converting R Factors into Binary Matrix Values

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))

Answer 2

Let's call your data.frame df:

library(reshape2)
dcast(df,C2*C3~C1,fill=0,length)

  C2 C3 A B
1  1  1 1 0
2  3  4 0 1
3  3  5 1 0

Answer 3

dat <- read.table(text =' C1 C2 C3
A  3  5
B  3  4
A  1  1',header=T)

Using transform

transform(dat,C1_A =ifelse(C1=='A',1,0),C1_B =ifelse(C1=='B',1,0))[,-1]
  C2 C3 C1_A C1_B
1  3  5    1    0
2  3  4    0    1
3  1  1    1    0

Or to get more flexbility , with within

within(dat,{ 
             C1_A =ifelse(C1=='A',1,0)
             C1_B =ifelse(C1=='B',1,0)})

  C1 C2 C3  C1_B C1_A
1  A  3  5    0    1
2  B  3  4    1    0
3  A  1  1    0    1