providing a set of values to test (x-y)

I've been tasked with providing a comprehensive test for some code that calculates x-y.

You can assume that the code is not simply (x-y)

My idea is that a good set of values would be: 3 and 7

I think this because they are prime. Can anyone give me their opinion on this.

Many thanks.

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My opinion is that this is a coding problem, and would fit better on the coding website than here on the math website. – Gerry Myerson Nov 4 '12 at 11:54
@Fendorio, it is impossible to completely test it because there are infinitely many numbers and you can only test finitely many! To check that - is correct you need to proof by induction. – sperners lemma Nov 4 '12 at 11:56

• $x=7$ and $y=7$ (test it gives the correct answer for equal inputs)
• $x=3$ and $y=7$ (test it gives the correct answer for $x<y$)
• $x=7$ and $y=3$ (test it gives the correct answer for $x>y$)
It is probably also worth testing that you get the correct answer if $x$ or $y$ or both take the maximum value (or minimum value, i.e. most negative) allowed in your implementation language.