Getting started with BrainFuck

Subtract numbers

Like we did for addition : in order to compute A-B, we have to subtract 1 to A, B times

Let's start

• Memory: A, B
• Cursor: on B
• Input: any

Process

• while B is not null
  • invariant: A cell - B cell = A-B
  • decrease B
  • increase A
• loop
• B cell = 0, A cell = A-B - 0 = A-B

Code

[    while B is not null
  -  decrease B
  <- go to A and increase
  >  go back to B
]    loop

Minified version

[-<->]

Final state

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