How can you use an array of counters to identify whether a number of counters is prime or composite?

If you can arrange the counters in the shape of a rectangle with at least two in each row and each column then the number is composite. The numbers of row and the numbers of columns are factors of the given number. If the only rectangle you can make is the "degenerate" one, with only one row or only one column, then the number is prime.