Unit testing failing due to 0.000000001f inaccuarcy

Question

A vector is out by 0.000000001f so the test is failing.

Doing the logic in my head yeilds -1.0f, -1.0f but when I wrote the code to pass the test (Using the built in Vector methods) the answer comes back as -0.999999999f, -0.999999999f which is still 'right'.

This is C# (Vector2 class provided by the XNA framework) by the way, so I tried adding an epsilon value to each the x and y parameter of the Vector but this didn't work.

The way I see it is that by coding the answer, I'm writing the production code. E.g. it would be the same. Surely this is not the right.

Any advice?

score 6 · Answer 1 · answered May 27 '09 at 21:34

6

Unit testing frameworks usually provide asserts for floats that take a precision. For example in Visual Studio Test you can write:

Assert.AreEqual(1.0f, 0.9999999999f, 1e-3); // passes
Assert.AreEqual(1.0f, 0.9f, 1e-3); // fails

answered May 27 '09 at 21:34

Darin Dimitrov

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Its NUnit. The code is not accessible at the moment. Does anyone know if NUnit has something similar? – Finglas May 27 '09 at 21:36
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Yes, NUnit has the same concept: http://www.nunit.org/index.php?p=equalityAsserts&r=2.2.7 – Darin Dimitrov May 27 '09 at 21:38
After all Microsoft haven't reinvented the wheel :-) – Darin Dimitrov May 27 '09 at 21:39

score 2 · Accepted Answer · edited May 23 '17 at 09:57

2

Please see my answer here.

It is not about C#, but the same logic applies.

edited May 23 '17 at 09:57

Community

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answered May 27 '09 at 21:31

Brian R. Bondy

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score 1 · Answer 3 · answered May 27 '09 at 21:32

1

Using an epsilon value should work, you could post the code that doesn't work to make your problem clearer. You should do something like this:

if ((TestSomething()->Vector.x - 1.0f) <= EPSILON) return true;

instead of

if (TestSomething()->Vector.x = 1.0f) return true;

answered May 27 '09 at 21:32

schnaader

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You forgot the Math.Abs() around the difference. – Cheeso May 27 '09 at 22:15

score 0 · Answer 4 · answered May 27 '09 at 21:32

Floating point numbers can have precision issues, besides comparison to 0 you rarely would want to compare two floating point numbers using ==.

Trying to add an epsilon value to the vectors won't work - you'll still fall for the same precision issues.

Instead, you need to see if the difference between the two is below some threshold:

if (Math.Abs(x - y) < epsilon)
{
     // Equal!
}

Unit testing failing due to 0.000000001f inaccuarcy

4 Answers4