[R] A co-occurrence matrix

[Alexey Shipunov]

Dear R experts,

I have a matrix (from some sort of 
classification) like this:

      object  group
 [1,] 1       1
 [2,] 2       2
 [3,] 3       1
 [4,] 4       1
 [5,] 5       3

And I need something like this:

      [,1] [,2] [,3] [,4] [,5]
 [1,] 1    0    1    1    0
 [2,] 0    1    0    0    0
 [3,] 1    0    1    1    0
 [4,] 1    0    1    1    0
 [5,] 0    0    0    0    1

where all zeros mean that these objects are not 
in same group, and vice versa.

Is there a relatively simple way to construct co-
uccurence matrices of type shown above?

Any help would be appreciated,


=================================
Dr. Alexey B. Shipunov
Section of Molecular Systematics
Jodrell Laboratory
Royal Botanic Gardens, Kew,
Richmond, Surrey, TW9 3DS, U.K.
e-mail: a.shipunov at rbgkew.org.uk