Wikipedia: Brainfuck: Examples / 2009-09-14 / SML Screenshots

en.wikipedia.org/wiki/Brainfuck

The brainfuck programming language is an esoteric programming language noted for its extreme minimalism. It is a Turing tarpit, designed to challenge and amuse programmers, and is not suitable for practical use. Its name has been variously bowdlerized. The name of the language is generally not capitalized, although it is a proper noun.

Language Design
Urban Müller created brainfuck in 1993 with the intention of designing a language which could be implemented with the smallest possible compiler, inspired by the 1024-byte compiler for the FALSE programming language. Several brainfuck compilers have been made smaller than 200 bytes. The classic distribution is Müller's version 2, containing a compiler for the Amiga, an interpreter, example programs, and a readme document.

The language consists of eight commands, listed below. A brainfuck program is a sequence of these commands, possibly interspersed with other characters (which are ignored). The commands are executed sequentially, except as noted below; an instruction pointer begins at the first command, and each command it points to is executed, after which it normally moves forward to the next command. The program terminates when the instruction pointer moves past the last command.

The brainfuck language uses a simple machine model consisting of the program and instruction pointer, as well as an array of at least 30,000 byte cells initialized to zero; a movable data pointer (initialized to point to the leftmost byte of the array); and two streams of bytes for input and output (most often connected to a keyboard and a monitor respectively, and using the ASCII character encoding).

Examples (screenshot above)
Hello World!

The following program prints "Hello World!" and a newline to the screen:

+++ +++ +++ +           initialize counter (cell #0) to 10
[                       use loop to set the next four cells to 70/100/30/10
    > +++ +++ +             add  7 to cell #1
    > +++ +++ +++ +         add 10 to cell #2 
    > +++                   add  3 to cell #3
    > +                     add  1 to cell #4
    <<< ++ .                   print 'H'
>+.                     print 'e'
+++ +++ +.              print 'l'
.                       print 'l'
+++ .                   print 'o'
>++ .                   print ' '
<.                      print 'o'
+++ .                   print 'r'
--- --- .               print 'l'
--- --- --.             print 'd'
>+.                     print '!'
>.                      print '\n'

For readability, this code has been spread across many lines and blanks and comments have been added. Brainfuck ignores all characters except the eight commands +-[],. so no special syntax for comments is needed. The code could just as well have been written as:

++++++++++[>+++++++>++++++++++>+++>+<<<++.>+.+++++++..+++.>++.<.+++.------.--------.>+.>.

The first line initialises a[0] = 10 by simply incrementing ten times from 0. The loop from line 2 effectively sets the initial values for the array: a[1] = 70 (close to 72, the ASCII code for the character 'H'), a[2] = 100 (close to 101 or 'e'), a[3] = 30 (close to 32, the code for space) and a[4] = 10 (newline). The loop works by multiplying the value of a[0], 10, by 7, 10, 3, and 1, saving the results in other cells. After the loop is finished, a[0] is zero. >++. then moves the pointer to a[1] which holds 70, adds two to it (producing 72 which is the ASCII character code of a capital H), and outputs it.

The next line moves the array pointer to a[2] and adds one to it, producing 101, a lower-case 'e', which is then output.

As 'l' happens to be the seventh letter after 'e', to output 'll' another seven are added (+++++++) to a[2] and the result is outputted twice.

'o' is the third letter after 'l', so a[2] is incremented three more times and output the result.

The rest of the program goes on in the same way. For the space and capital letters, different array cells are selected and incremented or decremented as needed.