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


  • 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


[    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


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

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