Getting started with BrainFuck

Sum numbers

As you probably already noticed, there is no "sum 2 variables" in BF. Variables actually don't exist either :)

The + command even does not "add" values : it simply increments current cell, so does the - command that decrements current cell.

In order to compute A+B, we have to add 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