gvrng 4.4-1 / usr / share / doc / gvrng / tutorial.html

<h2>Getting started</h2>
<p>
Guido van Robot is a robot who can do a lot of things, but 
only if you program him.  We're gonna gradually show how
it's possible to teach Guido to navigate his world.
</p>
<p>
Without our help, Guido already understands five primitive instructions:
</p>
<ul>
    <li>turnleft</li>
    <li>move</li>
    <li>putbeeper</li>
    <li>pickbeeper</li>
    <li>turnoff</li>
</ul>
<p>
The primitive instructions are called "builtin" instructions,
because he already knows them before we ever give him a program
to run.  Guido also understands the special words "define," "do," 
"if," and "while," but let's start out simple. 
</p>
<h2>Using primitive instructions (<i>turnleft</i>, <i>move</i>, etc.)</h2>
<p>
You can write simple programs by combining the five builtin
instructions turnleft, move, putbeeper, pickbeeper, and turnoff.
</p>
<p>
For example, let's have guido move a square (move), turn 
left (turnleft), move another square (move), put a beeper down (putbeeper), 
then turn off (turnoff). 
</p>
<pre>
move
turnleft
move
putbeeper
turnoff
</pre>
<p>
That's a pretty simple program, but we have to format if very carefully
for Guido.  Notice how every instruction is on its own line.  
</p>
<h2>Teaching Guido new things (</i>define</i>)</h2>
<p>
Let's modify the program so that instead of turning left, Guido turns right. 
The problem is, Guido does not know how to turn right, so we teach him. 
Look at the program below, and see how we teach Guido:
</p>
<p>
<pre>
define turnright:
    turnleft
    turnleft
    turnleft

move
turnright
move
putbeeper
turnoff
</pre>
<p>
We take the first four lines of the program to define to Guido what 
it means to "turnright."  Turning right is 3 left turns.  Try acting
this yourself to see how it works.  Again, Guido is very particular
about how you format your program.  Let's write a program with 
two definitions now:
</p>
<pre>
define turnright:
    turnleft
    turnleft
    turnleft

define move_ten_squares:
    move
    move
    move
    move
    move
    move
    move
    move
    move
    move

move_ten_squares
turnright
move_ten_squares
putbeeper
turnoff
</pre>
<p>
Notice how all the instructions in each definition get indented at the 
same level.  Guido needs you to format the instructions like that for
him, or else he will get confused.  Remember, his brain is made out 
of sand, and he does not possess human intelligence.  On the other
hand, he is very loyal when you give him the right format.  So please
bear with him.
</p>
<h2>Repeating instructions (<i>do</i>)</h2>
<p>
Since Guido has a computer brain, there is one thing he's very good
at--counting.  If you want to tell him to do something ten times, just
tell him to do it 10 times.  If you want to tell him to do something
three times, again, just tell him:
</p>
<pre>
define turnright:
    do 3:
        turnleft

define move_ten_squares:
    do 10:
        move

move_ten_squares
turnright
move_ten_squares
putbeeper
turnoff
</pre>
<p>
The above program does exactly what the one before it did, but 
we didn't have to type as much.  Also, Guido can repeat more
than one instruction:
</p>
<pre>
do 5:
    move
    pickbeeper
turnoff
</pre>
The following program does the same thing:
<pre>
move
pickbeeper    
move
pickbeeper    
move
pickbeeper    
move
pickbeeper    
move
pickbeeper    
turnoff
</pre>
<p>
But really, who wants to type all that?  It's better to use the "do"
command.
</p>
<p>
<h2>Making decisions (<i>if</i>, <i>else</i>)</h2>
<p>
So far Guido does exactly what we tell him, like a good loyal robot,
but every now and then, we'd like for him to make decisions on his
own.  Let's write a really small program for Guido to show his 
decisionmaking ability.  We want him to move one square, unless
there's a wall in front of him.  If there's a wall in front of him,
we'd prefer for him not to run in the wall, so he can just turn off.
</p>
<pre>
if front_is_clear:
    move
turnoff
</pre>
<p>
Guido understand what "if" means.  He also knows what "front_is_clear"
means.  He has already been wired to know those words.  Still, it's up
to you tell him what to do "if" his "front_is_clear."  We are saying
"if" his "front_is_clear," he can "move."  Then, regardless of the 
situation, the next command is to "turnoff."
</p>
Let's give him a more complex instruction:
<pre>
if front_is_clear:
   move
   pickbeeper
   pickbeeper
   pickbeeper
else:
   turnleft
   turnleft
turnoff
</pre>
</p>
Here we are telling him that if his front is clear, it's okay for him
to move and then pick up three beepers.  Or "else," if his front is
not clear, then we want him to turnleft twice.
Then, regardless, he has to turnoff.
</p>
<h2>Other conditionals</h2>
<p>
We said earlier that Guido understand five commands without us 
having to teach him anything--move, turnleft, putbeeper, pickbeeper,
and turnoff.  Those were called "builtin" commands.
</p>
<p>
We also introduced the idea that Guido can check for a condition--
for example "front_is_clear."  The word "front_is_clear" is an 
example of a builtin conditional.  It turns out there are 
eighteen builtin conditionals:
</p>
<ul>
    <li>front_is_clear
    <li>front_is_blocked
    <li>left_is_clear
    <li>left_is_blocked
    <li>right_is_clear
    <li>right_is_blocked

    <li>next_to_a_beeper
    <li>not_next_to_a_beeper
    <li>any_beepers_in_beeper_bag
    <li>no_beepers_in_beeper_bag

    <li>facing_north
    <li>not_facing_north
    <li>facing_south
    <li>not_facing_south
    <li>facing_east
    <li>not_facing_east
    <li>facing_west
    <li>not_facing_west
</ul>
<p>
All the conditionals can be used with the "if" keyword.  Here is an
example program:
</p>
<pre>
define pick_up_beeper_only_if_its_there:
    if next_to_a_beeper:
        pickbeeper

define move_only_if_you_can:
    if front_is_clear:
        move

pick_up_beeper_only_if_its_there
move_only_if_you_can
pick_up_beeper_only_if_its_there
turnoff
</pre>
<h2>Repeating Conditionally (<i>while</i>)</h2>
<p>
One common task for Guido is that he wants to approach a wall,
but of course he does not want to slam into it.
<p>
Here is a program that does this:
<pre>
while front_is_clear:
    move
</pre>
<p>
The "while" command is like the "if" and "do" commands; it 
works with a block of commands.  Like the "if" command, the 
"while" command only does its block if the condition is true.
<p>
In our example, the "while" command only does the "move" if
"front_is_clear."
<p>
But the powerful thing about the "while" command is that it 
keeps doing the "move" as long as "front_is_clear."  It
repeats the command.  This is called "looping."  Guido
keeps looping through the "while" statement and executing
"move" as long as his front is clear.