Herbert challenges your ability to see patterns and create algorithms to produce these patterns.

This Challenge Round consists of 25 levels. On each level there are some white buttons; to "solve" a level you must press all the white buttons. To press the buttons, you have at your disposal a programmable robot named Herbert. You must program Herbert, in a language called “h”, to press all the white buttons while avoiding obstacles such as walls and gray buttons (walls block Herbert's path and gray buttons will reset any previously pressed white buttons to their unpressed state). You are only allotted a certain number of "bytes" (a unit of program size) per level; your program must use no more than this number of bytes. This last restriction is not too stringent for the earlier levels where the emphasis is just on writing a program that works, but on the later levels you'll find that your program must not only work, it must be small and cleanly designed as well. On each level, points are awarded for each white button pressed, a bonus is awarded for solving the level, and extra points are awarded for those solutions that use less than the allotted maximum number of bytes.

Herbert is programmed in a simple but powerful language called “h”. “h” contains elements of traditional high-level languages: statements, procedures, parameters, arguments, and recursion. However, “h” is syntactically simpler, and contains some concepts (procedural arguments) that are not found in traditional languages.

Here are the basic elements of “h”:

Element		Syntax		Examples
Statement		s(go straight), r (turn right), or l (turn left), a procedure call, or a procedural parameter.		s
Procedure definition		x[(P1,P2,...,Pk)]:y1y2y3...yn where x is any lowercase letter (except s, r, or l), Pi is a parameter, yi is any statement, 0 <= k <=15, and n >= 0. If k = 0 (no parameters) then the parentheses are not used.		a:ssssr b(C,D):sra(B)C
Parameter		X where X is any uppercase letter.		A
Expression		[-] X [(+ or -) X [(+ or -) X [(+ or -) X ... ]]] where each X is either a numeric parameter or a number.		5 A-5 -A-B-C+1
Argument		Any number, expression, or sequence of zero or more statements.		4 rsr A-1
Procedure call		x[(a1,a2,...,ak)] where x is the name of a procedure and ai is an argument (one for each parameter). If k = 0 (no parameters) then the parentheses are not used. No call is made if any numeric argument is 0 or less.		a(1,B-1,srs) b
Recursion		x[(P1,P2,...,Pk)]:y1y2y3...ymx[(a1,a2,a3,..,ak)]y1y2y3...yn where x is any lowercase letter (except s, r, or l), Pi is a parameter, ai is an argument, yi is any statement, 0<= k <= 15, m >= 0, and n >= 0.		a(A):sa(A-1) b:sssrb
Main Procedure		y1y2y3...yn where yi is any statement and n >= 1.		sa(1,rsr)r
Program		<procedure definition>       :      : <main procedure>		a:ssssra sssa
Byte		Any lowercase or uppercase letter		s A a(1)

Here is a short tutorial to get you familiar with “h” and with Herbert.

First, make sure that your system is able to run Herbert with all of its functionality. See help for more information on minimum systems requirements and required security settings.

Next, click here to run a practice version of Herbert.

You can now follow along in the tutorial by both writing programs yourself, and checking them against the examples given, or by copying and pasting the examples from this page into Herbert to see how they work.

In this tutorial we'll learn how to program in "h" by writing a program that makes Herbert go around in a square. We'll start out with a simple solution and then show how procedures and parameters can be used to simplify and/or generalize the solution.

Our first task is to program Herbert to go around in a square 5 units per edge, and come back to where he started, facing in the direction he started.

The tutorial consists of a single level (level 0) which is free of buttons and obstructions. Go to the Options menu and turn on "Trace" and make sure "Path" is enabled, so you can better see what your program is doing. At any time during this tutorial, feel free to adjust Herbert's speed by using the scroll bar on the bottom of the Herbert window. To watch a program more carefully, you may halt it at any time by choosing "Halt" from the "Run" menu, pressing the Halt (Pause) toolbar button, or simply clicking anywhere in the text area. You can also place a vertical bar "|" (a breakpoint) at the point in the program where you want it to stop. Once the program is halted, you can start stepping through the program one command at a time ("Step" in the "Run" menu or on the toolbar), resume execution ("Resume"), or reset Herbert to his starting position ("Reset").

Type the following into the program text area:

sssssrsssssrsssssrsssssr

Now choose "Go" from the "Run" menu. Note that "s" makes Herbert go forward one unit, while "r" makes him turn one-quarter turn to the right. As you might guess, "l" (the letter "ell") would make him turn one-quarter turn to the left. These three commands are the only ones Herbert understands.

Now reset Herbert by choosing "Reset" from the "Run" menu (or simply go ahead and edit the program; Herbert will be automatically reset on your first edit) and try:

a:ssssr
aaaa

Here you've defined a procedure called "a", which does "ssssr" whenever called. By calling it 4 times in a row this is effectively the same as the first example - but it's a smaller program, 11 bytes versus 24. Any single lowercase letter is legal as a procedure name (except for "s", "r", and "l" which already mean something to Herbert).

Even simpler is

a:ssssra
a

Here "a" does "ssssr" as before, and then calls itself (which then does "ssssr" and calls itself, ...). When a procedure calls itself this is called "recursion". This example is an infinite recursion; Herbert doesn't stop when he's done with the square. There is no problem with this in itself, since Herbert has an infinite stack, but you can stop him if you want by choosing "Halt" on the "Run" menu. You may wish to limit the depth of the recursion in the program itself, for instance if you wanted him to do something else after he's gone around in a square. To do this you must use a parameter:

a(A):sssssra(A-1)
a(4)

Parameters are always single uppercase letters. When a procedure with parameters is called, you must supply "arguments" for each parameter (here the argument for "A" is initially 4, then 3, then 2, etc.). The rule with arguments in "h" is that if any numeric argument in a procedure call evaluates to 0 or less, the call is not made.

You can also use a second parameter to vary the size of the square.

a(A,B):f(B)ra(A-1,B)
f(A):sf(A-1)
a(4,5)

Now change the "5" to "8" and see what happens. Study the procedure "f" here (we've used the letter "f" to stand for "forward"). This procedure is a very important one and will appear time and time again in your solutions.

Parameters can be supplied with "procedural" arguments as well as numeric ones. Try

a(A,B,C):f(B)Ca(A-1,B,C)
f(A):sf(A-1)
a(4,5,r)

and then try changing the "r" to an "l" (ell).

That's all there is to it! For more practice with "h", try the following variations of our square program. What does each one do?

a(A,B,C):f(B)Ca(A-1,B,C)
b(A):a(4,5,r)lb(A-1)
f(A):sf(A-1)
b(4)

a(A,B,C):f(B)Ca(A-1,B,C)
b(A):a(4,A,r)b(A-1)
f(A):sf(A-1)
b(10)

a(A,B,C):f(B)Ca(A-1,B,C)
b(A):a(2,11-A,r)b(A-1)
f(A):sf(A-1)
b(10)

a(A,B,C):f(B)Ca(A-1,B,C)
f(A):sf(A-1)
a(4,5,rslsr)

• "h" programs can be a maximum of 999 characters (including special characters and digits). Your program can have a maximum of 15 lines, and each line can have a maximum of 127 characters (excluding the function prototype; that is, including everything after the ":").
   
• Numbers are limited to the range between -2^15 and +2^15 - 1 (-32768 to 32767). You will receive a runtime error if a numeric argument evaluates to a number outside this range.
   
• Herbert's stack is circular and of size 64K. For a procedure call, the size of each stack frame is 4 * (1 + number of arguments). For evaluating procedural arguments, a stack frame of size 8 is used. What this means in practice is that programs that recurse infinitely, without returning, and without nested procedural arguments, can run forever. Other programs are limited to a maximum call depth of approximately 2,000 calls (depending on the number of arguments being passed). You will receive a runtime error if you attempt to return from a call stack which exceeds this size.
   
• There is no inherent time limit to running Herbert programs. Keep in mind, however, that your score will be based on the number of buttons pressed (and score uploaded) by the end of the round.
   
• Level 0, in both the tutorial and full version of Herbert, is a practice level. Programs left on this level are not saved and restored across program sessions.