Getting started with BrainFuck

Copy value

Copying a value is a destructive operation. The initial value is deleted while 2 copies are created.

Optionally, an extra "Move" operation can be used to move one of the 2 copies at source location, to create a "conservative copy illusion".

Let's start

• Memory: A, 0, 0
• Cursor: on A
• Input: any

Process

• while A is not null
  • invariant: A cell + 1s next cell = A, and 1st next cell = 2nd next cell
  • decrease A
  • increase 1st next cell
  • increase 2nd next cell
• loop
• A cell = 0, 1st next cell = A - 0 = A and 2nd next cell = 1st next cell = A

Code

[    while A is not null
  -  decrease A
  >+ increase 1st next cell
  >+ increase 2nd next cell
  <<  go back to A
]    loop

Minified version

[->+>+<<]

Final state

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

Variant

As explained, we can move the 2nd copy to source location to have A, A, 0 instead of 0, A, A

[->+>+<<]    copy value
>>[-<<+>>]<< go to 2nd copy; move it to source location; then go back to source location

Test program

,         read a char
[->+>+<<] copy value
>.        go to 1st copy and print
>.        go to 2nd copy and print