Subtract numbers

Context

Brainfuck only allows increase / decrease values by 1.
So, like sum, to compute a A-B, we need to remove 1 to A, and do it B times

Initial state

• Memory: A, B
• Cursor: first cell
• Input: any

Process

• Move to B
• While B cell is not null, decrease B and A by 1
• Loop invariant: difference of both cells is A-B
• When second cell equals 0, then first equals A-B
• Move back to A

Code

>[-<->]<

Final state

• Memory: A-B, 0
• Cursor: first cell
• Input: any
• Output: unchanged