If I do -10 mod(%) 3 in Python, excel, google calc etc I get result 2.

If i do -10 mod(%) 3 in RobotC I get -1.

I have obviously missed something somewhere as I would expect 1! I am clearly wrong, but why different results in RobotC?

Why?

This is a fun one. Lets start with what the modulo operator does: Lets say you do 10 % 3. It finds the largest multiple of 3 that is less than 10, which is 9. Think of it like this: 10 / 3 = 3.333, so if you round down, the answer is 3, and 3 * 3 = 9. Then it gives you the difference between 10 and 9, which is (10 - 9) = 1. Now when you do (-10) % 3, (-10) / 3 = -3.333. When you round down, that equals -4. 3 * -4 = -12. The difference is (-10 - (-12)) = 2. Now some compilers will round toward zero, not down, which is what you might expect. So, in that case, when you do (-10) % 3, (-10) / 3 = -3.333, rounded toward zero = -3, 3 * -3 = -9, and (-10 - (-9)) = -1. Does that answer your question?

Yes thanks, it's not a bug, it's a feature!

I assumed this was a standard mathematical operation that would be the same in every compiler. You know what they say about assumption? Useful to know, thanks for the detailed explanation.

Yep, some compilers may not play nice with your code.
P.S: It took me quite a while to see that was a reference to my signature.