[R] counting row repetitions without loop

From: Waterman, DG \(David\) <david.waterman_at_diamond.ac.uk>
Date: Wed, 6 Feb 2008 14:08:16 -0000


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?  

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