Brainfuck (not capitalized except at the start of a sentence) is an extremely minimal Turing-complete programming language with just 8 commands.
Any character not "><+-.," (excluding quotation marks) is ignored. Brainfuck is represented by an array with 30,000 cells initialized to zero and a data pointer pointing at the current cell. There are eight commands: + : Increments the value at the current cell by one. - : Decrements the value at the current cell by one. > : Moves the data pointer to the next cell (cell on the right). < : Moves the data pointer to the previous cell (cell on the left). . : Prints the ASCII value at the current cell (i.e. 65 = 'A'). , : Reads a single input character into the current cell. [ : If the value at the current cell is zero, skips to the corresponding ] . Otherwise, move to the next instruction. ] : If the value at the current cell is zero, move to the next instruction. Otherwise, move backwards in the instructions to the corresponding [ . [ and ] form a while loop. Obviously, they must be balanced. Let's look at some basic brainfuck programs. ++++++ [ > ++++++++++ < - ] > +++++ . This program prints out the letter 'A'. First, it increments cell #1 to 6. Cell #1 will be used for looping. Then, it enters the loop ([) and moves to cell #2. It increments cell #2 10 times, moves back to cell #1, and decrements cell #1. This loop happens 6 times (it takes 6 decrements for cell #1 to reach 0, at which point it skips to the corresponding ] and continues on). At this point, we're on cell #1, which has a value of 0, while cell #2 has a value of 60. We move on cell #2, increment 5 times, for a value of 65, and then print cell #2's value. 65 is 'A' in ASCII, so 'A' is printed to the terminal. , [ > + < - ] > . This program reads a character from the user input and copies the character into cell #1. Then we start a loop. Move to cell #2, increment the value at cell #2, move back to cell #1, and decrement the value at cell #1. This continues on until cell #1 is 0, and cell #2 holds cell #1's old value. Because we're on cell #1 at the end of the loop, move to cell #2, and then print out the value in ASCII. Also keep in mind that the spaces are purely for readability purposes. You could just as easily write it as: ,[>+<-]>. Try and figure out what this program does: ,>,< [ > [ >+ >+ << -] >> [- << + >>] <<< -] >> This program takes two numbers for input, and multiplies them. The gist is it first reads in two inputs. Then it starts the outer loop, conditioned on cell #1. Then it moves to cell #2, and starts the inner loop conditioned on cell #2, incrementing cell #3. However, there comes a problem: At the end of the inner loop, cell #2 is zero. In that case, inner loop won't work anymore since next time. To solve this problem, we also increment cell #4, and then recopy cell #4 into cell #2. Then cell #3 is the result.
And that’s brainfuck. Not that hard, eh? For fun, you can write your own brainfuck programs, or you can write a brainfuck interpreter in another language. The interpreter is fairly simple to implement, but if you’re a masochist, try writing a brainfuck interpreter… in brainfuck.
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Originally contributed by Prajit Ramachandran, and updated by 3 contributors