Playground

Choosing an example replaces the program, the input and the cell names. Undo brings your program back.

Worked out from the text of the program, without running it.

Text to print Hello World!

Link

The link holds the program, the input, the settings, the cell names and the breakpoints, compressed in the part after the #. Nothing is sent to a server.

The link

Saved in this browser

Name Save

Files

You can also drop a file anywhere on this page. Files up to 64 MB are accepted. Up to 300,000 characters a program can be edited (with colours up to 150,000); a larger one is kept as it is and can be run, stepped and analysed, but not edited.

What the page shows

The interpreter

Interpreters differ in these points, and a program can depend on them. A change resets the program.

Cell size

8 bits (0 to 255)16 bits (0 to 65,535)32 bits (0 to 4,294,967,295)

How large the number in one cell can get. The classic size is 8 bits, one byte.

When a cell passes its limit

it wraps roundthe program stops with an error

Wrapping means 255 + 1 = 0 and 0 − 1 = 255; most interpreters wrap, and many programs count on it.

Tape length

cells

How many cells there are. The first interpreter had 30,000; the most here is 16,777,216.

Moving off an end of the tape

is an errorwraps round to the other end

What < does on cell 0 and > on the last cell. An error finds mistakes; wrapping is what some programs expect.

End of input

0−1cell unchanged

What , stores when nothing is left to read. Interpreters disagree; a careful program works with all three.

When the input box is used up

ask me for morethat is the end of the input

Asking works like a terminal: the program waits until you type something or say that the input has ended.

Line breaks in the input

one byte: 10two bytes: 13, 10

How a line break typed in the input box is sent. Almost every program expects 10. In the output, 10 starts a new line and 13 is not shown.

Breakpoint character

none#!

When set, a running program pauses at this character. It is off at first, so that a # or ! in a comment stays a comment.

Engine of fast mode

checked interpreterWebAssembly compiler

The checked interpreter obeys every setting, counts steps and can be paused. WebAssembly is much faster for heavy programs, but needs wrapping cells and end of input 0, does not notice a pointer that leaves the tape on the left, and cannot pause or ask for input.

Back to the classic settings

Write a brainfuck program, run it, and step through it forward and back. The tape shows every cell, and one sentence says what each command does.

The eight commands

Everything else is a comment. The tape starts with every cell at zero and the pointer on cell 0.

Keyboard

Ctrl + Enter or F8

Play, or pause when playing

F10 or Ctrl + .

Step

F7 or Ctrl + ,

Step back

F2

Step over a loop

Shift + F2

Finish the loop you are in

F4

Play to the text cursor

Shift + F8

Start again

Ctrl + Shift + Enter

Play fast

Esc

Pause, or stop a fast run

F9

Breakpoint on the line of the text cursor

Ctrl + Z, Ctrl + Y

Undo and redo in the editor

Tab, Shift + Tab

Leave the editor: Tab is never caught, so the keyboard cannot get stuck in it

← → on the tabs

Next and previous tab of this panel

On a Mac, use ⌘ where this list says Ctrl; the function keys may need fn.

Playing and stepping

• Play works in the page, at the speed of the slider, and stops at breakpoints. Step back takes back up to 200,000 steps; older steps are forgotten.
• Play fast works in the background with an optimising interpreter. It ignores breakpoints. Stop ends it at once, whatever the program is doing; Pause and inspect hands the program to the debugger where it stands.
• Breakpoints. Click left of a line (or F9). A run pauses when it arrives at that line from another line. The breakpoint character (Settings) pauses wherever it stands; common interpreters use # or !, and here you choose which, or none.
• Input. The comma reads bytes from the input box. When the box is used up, the program either asks you or gets the end of input: 0, −1 or no change, as set.
• Cells. Press a cell to give it a name or change its value. Names are saved with the program and in the link.
• The program map. The strip beside Steps so far is the whole program from its first command to its last. Its height is the loop depth, and the block marks the command that runs next.

Use it in another page

A small version of the playground can be put in any page of this site: see the instructions in embed.js and an example.

The rest of the site

This page is one screen, so it has no footer. Every page of the site is in the sitemap.