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Matrix Size

Here's another loose end. We notice that our story specifies a 5x5 matrix. We only really test location (1,1) and there is nothing in the tests to verify that the matrix can really be 5x5. Perhaps we should add a test to check this. 

# file: testnet.rb
...
  def test_max_matrix_size
    check_location(1,1)
    check_location(1,5)
    check_location(5,1)
    check_location(5,5)
  end

  def check_location(x,y)
    assert_equal true, @net.untied?(x,y)
    @net.tie(x,y)
    assert_equal false, @net.untied?(x,y)
    @net.untie(x,y)
    assert_equal true, @net.untied?(x,y)
  end
...

We expect this one to pass, but when we run it we get ...

  
$ ruby testnet.rb
Loaded suite testnet
Started...
..
Error occurred in test_max_matrix_size(TestNet): NameError: undefined method `[]' for nil
	./net.rb:14:in `untied?'
	testnet.rb:41:in `check_location'
	testnet.rb:36:in `test_max_matrix_size'
	testnet.rb:40
....
Finished in 0.067089 seconds.
5 runs, 13 assertions, 0 failures, 1 errors

What? It failed?! Why?

Oh yeah. Ruby arrays are indexed starting with 0. Our net matrix indices start with 1. We have a typical off-by-one boundary error.

That's easy to fix ... 

# file: net.rb
...
  def untied?(x,y)
    ! @tied[x-1][y-1]
  end

  def tie(x,y)
    @tied[x-1][y-1] = true
  end

  def untie(x,y)
    @tied[x-1][y-1] = false
  end
...

And now our test passes again!

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