Automatically expanding an R factor into a collection of 1/0 indicator variables for every factor level

Question

I have an R data frame containing a factor that I want to "expand" so that for each factor level, there is an associated column in a new data frame, which contains a 1/0 indicator. E.g., suppose I have:

df.original <-data.frame(eggs = c("foo", "foo", "bar", "bar"), ham = c(1,2,3,4)) 

I want:

df.desired  <- data.frame(foo = c(1,1,0,0), bar=c(0,0,1,1), ham=c(1,2,3,4)) 

Because for certain analyses for which you need to have a completely numeric data frame (e.g., principal component analysis), I thought this feature might be built in. Writing a function to do this shouldn't be too hard, but I can foresee some challenges relating to column names and if something exists already, I'd rather use that.

Answer 1

Use the model.matrix function:

model.matrix( ~ Species - 1, data=iris ) 

Answer 2

If your data frame is only made of factors (or you are working on a subset of variables which are all factors), you can also use the acm.disjonctif function from the ade4 package :

R> library(ade4) R> df <-data.frame(eggs = c("foo", "foo", "bar", "bar"), ham = c("red","blue","green","red")) R> acm.disjonctif(df)   eggs.bar eggs.foo ham.blue ham.green ham.red 1        0        1        0         0       1 2        0        1        1         0       0 3        1        0        0         1       0 4        1        0        0         0       1 

Not exactly the case you are describing, but it can be useful too...