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

Use the model.matrix function:

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

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...

A quick way using the reshape2 package:

require(reshape2)

> dcast(df.original, ham ~ eggs, length)

Using ham as value column: use value_var to override.
  ham bar foo
1   1   0   1
2   2   0   1
3   3   1   0
4   4   1   0

Note that this produces precisely the column names you want.