What does my teacher mean?

Question

I'm taking a CIS course in college and my Prof. Stated that you can't do subtraction in the formal sense, but I see everyone doing it on here, anybody know what she was talking about?

Answer 1

That makes sense now, thank you. I asked her and I got something I didn't exactly understand. Not saying she isn't a great teacher though.

Answer 2

If i remeber correctly, and if im understanding the question,

when you do math on paper/in your head/etc you are simply taking one value and subtracting it from anothet (5-4 for example).
But in programming you are only ever adding two values. To visualize this, maybe look at it like:
( add 5 to negative 4)
or 5 + (-4)
or saying 5 plus -4.