Time for action – comparing with assert_array_almost_equal_nulp

Let's see the assert_array_almost_equal_nulp function in action:

  1. Determine the machine epsilon with the finfo function:
    eps = np.finfo(float).eps
print "EPS", eps

    The epsilon would be:

    EPS 2.22044604925e-16
  2. Compare two almost equal floats: Compare 1.0 with 1 + epsilon (eps) using the assert_almost_equal_nulp function. Do the same for 1 + 2 * epsilon (eps):
    print "1",
np.testing.assert_array_almost_equal_nulp(1.0, 1.0 + eps)
print "2",
np.testing.assert_array_almost_equal_nulp(1.0, 1.0 + 2 * eps)

    The result:

    1 None
2
Traceback (most recent call last):
  …
assert_array_almost_equal_nulp
raiseAssertionError(msg)
AssertionError: X and Y are not equal to 1 ULP (max is 2)

What just happened?

We determined the machine epsilon with the finfo function. We then compared 1.0 with 1 + epsilon (eps) with the assert_almost_equal_nulp function. This test passed, however, adding another epsilon resulted in an exception.

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