[R] counting row repetitions without loop

[Waterman, DG (David)]

Hi,
 
I have a data frame consisting of coordinates on a 10*10 grid, i.e.
 
> example
    x  y
1   4  5
2   6  7
3   6  6
4   7  5
5   5  7
6   6  7
7   4  5
8   6  7
9   7  6
10  5  6

What I would like to do is return an 10*10 matrix consisting of counts
at each position, so in the above example I would have a matrix where,
for example, cell [4,5] contains 2 and [6,7] contains 3. At the moment I
have implemented this using a for loop over the rows of the data frame,
however the data frames I want to process are very long so the loop
takes many minutes to complete. Can I do this in a more efficient way?
 
Cheers,
David
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