path: root/tde-i18n-en_GB/docs/tdeedu/kturtle/programming-reference.docbook

<chapter id="reference">
<title
>&kturtle;'s &logo; Programming Reference</title>

<sect1 id="commands">
<title
>Commands</title>
<para
>Using commands you tell the turtle or &kturtle; to do something. Some commands need input, some give output. In this section we explain all the commands that can be used in &kturtle;.</para>

<sect2 id="moving-the-turtle">
<title
>Moving the turtle</title>
<para
>There are several commands to move the turtle over the screen.</para>
  
  <variablelist>
    <anchor id="forward"/>
    <varlistentry>
      <term
>forward</term>
      <listitem
><para
><screen
>forward X</screen>
<userinput
>forward</userinput
> moves the turtle forward by the amount of X pixels. When the pen is down the turtle will leave a trail. <userinput
>forward</userinput
> can be abbreviated to <userinput
>fw</userinput
></para
></listitem>
    </varlistentry>
  </variablelist>
  <variablelist>
    <anchor id="backward"/>
    <varlistentry>
      <term
>backward</term>
      <listitem
><para
><screen
>backward X</screen>
<userinput
>backward</userinput
> moves the turtle backward by the amount of X pixels. When the pen is down the turtle will leave a trail. <userinput
>backward</userinput
> can be abbreviated to <userinput
>bw</userinput
>.</para
></listitem>
    </varlistentry>
  </variablelist>
  <variablelist>
    <anchor id="turnleft"/>
    <varlistentry>
      <term
>turnleft</term>
      <listitem
><para
><screen
>turnleft X</screen>
<userinput
>turnleft</userinput
> commands the turtle to turn an amount of X degrees to the left. <userinput
>turnleft</userinput
> can be abbreviated to <userinput
>tl</userinput
>.</para
></listitem>
    </varlistentry>
  </variablelist>
  <variablelist>
    <anchor id="turnright"/>
    <varlistentry>
      <term
>turnright</term>
      <listitem
><para
><screen
>turnright X</screen>
<userinput
>turnright</userinput
>the turtle to turn an amount of X degrees to the right. <userinput
>turnright</userinput
> can be abbreviated to <userinput
>tr</userinput
>.</para
></listitem>
    </varlistentry>
  </variablelist>
  <variablelist>
    <anchor id="direction"/>
    <varlistentry>
      <term
>direction</term>
      <listitem
><para
><screen
>direction X</screen>
<userinput
>direction</userinput
> set the turtle's direction to an amount of X degrees counting from zero, and thus is not relative to the turtle's previous direction. <userinput
>direction</userinput
> can be abbreviated to <userinput
>dir</userinput
>.</para
></listitem>
    </varlistentry>
  </variablelist>
  <variablelist>
    <anchor id="center"/>
    <varlistentry>
      <term
>centre</term>
      <listitem
><para
><screen
>centre</screen>
<userinput
>centre</userinput
> moves the turtle to the centre on the canvas.</para
></listitem>
    </varlistentry>
  </variablelist>
  <variablelist>
    <anchor id="go"/>
    <varlistentry>
      <term
>go</term>
      <listitem
><para
><screen
>go X,Y</screen>
<userinput
>go</userinput
> commands the turtle to go to a certain place on the canvas. This place is X <glossterm linkend="pixels"
>pixels</glossterm
> from the left of the canvas, and Y <glossterm linkend="pixels"
>pixels</glossterm
> form the top of the canvas. Note that using the <userinput
>go</userinput
> command the turtle will not draw a line.</para
></listitem>
    </varlistentry>
  </variablelist>
  <variablelist>
    <anchor id="gox"/>
    <varlistentry>
      <term
>gox</term>
      <listitem
><para
><screen
>gox X</screen>
<userinput
>gox</userinput
> using this command the turtle will move to X <glossterm linkend="pixels"
>pixels</glossterm
> from the left of the canvas whilst staying at the same height.</para
></listitem>
    </varlistentry>
  </variablelist>
  <variablelist>
    <anchor id="goy"/>
    <varlistentry>
      <term
>goy</term>
      <listitem
><para
><screen
>goy Y</screen>
<userinput
>gox</userinput
> using this command the turtle will move to Y <glossterm linkend="pixels"
>pixels</glossterm
> from the top of the canvas whilst staying at the same distance from the left border of the canvas.</para
></listitem>
    </varlistentry>
  </variablelist>
</sect2>

<sect2 id="pen">
<title
>The turtle has a pen</title>
<para
>The turtle has a pen that draws a line when the turtle moves. There are a few commands to control the pen. In this section we explain these commands.</para>
  <variablelist>
    <anchor id="penup"/>
    <varlistentry>
      <term
>penup</term>
      <listitem
><para
><screen
>penup</screen>
<userinput
>penup</userinput
> lifts the pen from the canvas. When the pen is <quote
>up</quote
> no line will be drawn when the turtle moves. See also <userinput
>pendown</userinput
>. <userinput
>penup</userinput
> can be abbreviated to <userinput
>pu</userinput
>.</para
></listitem>
    </varlistentry>
  </variablelist>
  <variablelist>
    <anchor id="pendown"/>
    <varlistentry>
      <term
>pendown</term>
      <listitem
><para
><screen
>pendown</screen>
<userinput
>pendown</userinput
> presses the pen down on the canvas. When the pen is press <quote
>down</quote
> on the canvas a line will be drawn when the turtle moves. See also <userinput
>penup</userinput
>.<userinput
>pendown</userinput
> can be abbreviated to <userinput
>pd</userinput
>.</para
></listitem>
    </varlistentry>
  </variablelist>
  <variablelist>
    <anchor id="penwidth"/>
    <varlistentry>
      <term
>penwidth</term>
      <listitem
><para
><screen
>penwidth X</screen>
<userinput
>penwidth</userinput
> sets the width of the pen (the line width) to an amount of X <glossterm linkend="pixels"
>pixels</glossterm
>. <userinput
>penwidth</userinput
> can be abbreviated to <userinput
>pw</userinput
>.</para
></listitem>
    </varlistentry>
  </variablelist>
  <variablelist>
    <anchor id="pencolor"/>
    <varlistentry>
      <term
>pencolour</term>
      <listitem
><para
><screen
>pencolour R,G,B</screen>
<userinput
>pencolor</userinput
> sets the color of the pen. <userinput
>pencolor</userinput
> takes an <glossterm linkend="rgb"
>RGB combination</glossterm
> as input. <userinput
>pencolor</userinput
> can be abbreviated to <userinput
>pc</userinput
>.</para
></listitem>
    </varlistentry>
  </variablelist>
</sect2>

<sect2 id="canvas">
<title
>Commands to control the canvas</title>
<para
>There are several commands to control the canvas.</para>
  <variablelist>
    <anchor id="canvassize"/>
    <varlistentry>
      <term
>canvassize</term>
      <listitem
><para
><screen
>canvassize X,Y</screen>
With the <userinput
>canvassize</userinput
> command you can set the size of the canvas. It takes X and Y as input, where X is the new canvas width in <glossterm linkend="pixels"
>pixels</glossterm
>, and Y is the new height of the canvas in <glossterm linkend="pixels"
>pixels</glossterm
>. <userinput
>canvassize</userinput
> can be abbreviated to <userinput
>cs</userinput
>.</para
></listitem>
    </varlistentry>
  </variablelist>
  <variablelist>
    <anchor id="canvascolor"/>
    <varlistentry>
      <term
>canvascolour</term>
      <listitem
><para
><screen
>canvascolour R,G,B</screen>
<userinput
>canvascolor</userinput
> set the color of the canvas. <userinput
>canvascolor</userinput
> takes an <glossterm linkend="rgb"
>RGB combination</glossterm
> as input. <userinput
>canvascolor</userinput
> can be abbreviated to <userinput
>cc</userinput
>.</para
></listitem>
    </varlistentry>
  </variablelist>
  <variablelist>
    <anchor id="wrapon"/>
    <varlistentry>
      <term
>wrapon</term>
      <listitem
><para
><screen
>wrapon</screen>
With the <userinput
>wrapon</userinput
> command you can set <glossterm linkend="wrapping"
>wrapping</glossterm
> <quote
>on</quote
> for the canvas. Please see the glossary if you want to know what <glossterm linkend="wrapping"
>wrapping</glossterm
> is.</para
></listitem>
    </varlistentry>
  </variablelist>
  <variablelist>
    <anchor id="wrapoff"/>
    <varlistentry>
      <term
>wrapoff</term>
      <listitem
><para
><screen
>wrapoff</screen>
With the <userinput
>wrapoff</userinput
> command you can set <glossterm linkend="wrapping"
>wrapping</glossterm
> <quote
>off</quote
> for the canvas: this means the turtle can move off the canvas and can get <quote
>lost</quote
>. Please see the glossary if you want to know what <glossterm linkend="wrapping"
>wrapping</glossterm
> is.</para
></listitem>
    </varlistentry>
  </variablelist>
</sect2>

<sect2 id="clean">
<title
>Commands to clean up</title>
<para
>There are two commands to clean up the canvas after you have made a mess.</para>
  <variablelist>
    <anchor id="clear"/>
    <varlistentry>
      <term
>clear</term>
      <listitem
><para
><screen
>clear</screen>
With <userinput
>clear</userinput
> you can clean all drawings from the canvas. All other things remain: the position and angle of the turtle, the canvascolor, the visibility of the turtle, and the canvas size. <userinput
>clear</userinput
> can be abbreviated to <userinput
>ccl</userinput
>.</para
></listitem>
    </varlistentry>
  </variablelist>
  <variablelist>
    <anchor id="reset"/>
    <varlistentry>
      <term
>reset</term>
      <listitem
><para
><screen
>reset</screen>
<userinput
>reset</userinput
> cleans much more thoroughly than the <userinput
>clear</userinput
> command. After a <userinput
>reset</userinput
> command everything is like is was when you had just started &kturtle;. The turtle is positioned at the middle of the screen, the canvas color is white, and the turtle draws a black line on the canvas.</para
></listitem>
    </varlistentry>
  </variablelist>
</sect2>

<sect2 id="sprites">
<title
>The turtle is a sprite</title>
<para
>Many people do not know what sprites are, so here a short explanation: sprites are small pictures that can be moved around the screen. (for more info see the glossary on <glossterm linkend="sprites"
>sprites</glossterm
>). So the turtle is a sprite.</para>
<para
>Next you will find a full overview on all commands to work with sprites.</para>
<para
>[The current version of &kturtle; does not yet support the use of sprites other than the turtle. With future versions you will be able to change the turtle into something of your own design]</para>
  <variablelist>
    <anchor id="show"/>
    <varlistentry>
      <term
>show</term>
      <listitem
><para
><screen
>show</screen>
<userinput
>show</userinput
> makes the turtle visible again after it has been hidden.<userinput
>show</userinput
> can be abbreviated to <userinput
>ss</userinput
>.</para
></listitem>
    </varlistentry>
  </variablelist>
  <variablelist>
    <anchor id="hide"/>
    <varlistentry>
      <term
>hide</term>
      <listitem
><para
><screen
>hide</screen>
<userinput
>hide</userinput
> hides the turtle. This can be used if the turtle does not fit in your drawing.<userinput
>hide</userinput
> can be abbreviated to <userinput
>sh</userinput
>.</para
></listitem>
    </varlistentry>
  </variablelist>
</sect2>

<sect2 id="writing">
<title
>Can the turtles write text?</title>
<para
>The answer is: <quote
>yes</quote
>. The turtle can write: it writes just about everything you command it to.</para>
  <variablelist>
    <anchor id="print"/>
    <varlistentry>
      <term
>print</term>
      <listitem
><para
><screen
>print X</screen>
The <userinput
>print</userinput
> command is used to command the turtle to write something on the canvas. <userinput
>print</userinput
> takes numbers and strings as input. You can <userinput
>print</userinput
> various numbers and strings using the <quote
>+</quote
> symbol. See here a small example: <screen
>year = 2004
author = "Cies"
print "KTurtle was made in " + year + " by " + author
</screen>
      </para
></listitem>
    </varlistentry>
  </variablelist>
  <variablelist>
    <anchor id="fontsize"/>
    <varlistentry>
      <term
>fontsize</term>
      <listitem
><para
><screen
>fontsize X</screen>
<userinput
>fontsize</userinput
> sets the size of the font that is used by <userinput
>print</userinput
>. <userinput
>fontsize</userinput
> takes one input which should be a number. The size is set in <glossterm linkend="pixels"
>pixels</glossterm
>.</para
></listitem>
    </varlistentry>
  </variablelist>
</sect2>

<sect2 id="dice">
<title
>A command that rolls a dice for you</title>
<para
>There is one command that rolls a dice for you</para>
  <variablelist>
    <anchor id="random"/>
    <varlistentry>
      <term
>random</term>
      <listitem
><para
><screen
>random X,Y</screen>
<userinput
>random</userinput
> is a command that takes input and gives output. As input are required two numbers, the first (X) sets the minimum output, the second (Y) sets the maximum. The output is a randomly chosen number that is equal or greater then the minimum and equal or smaller than the maximum. Here a small example: <screen>
repeat 500 [
  x = random 1,20
  forward x
  turnleft 10 - x
]
</screen
> Using the <userinput
>random</userinput
> command you can add a bit of chaos to your program.</para
></listitem>
    </varlistentry>
  </variablelist>
</sect2>
</sect1>

<sect1 id="containers">
<title
>Containers</title>
<para
>Containers are letters or words that can be used by the programmer to store a number or a text. Containers that contain a number are called <link linkend="variables"
>variables</link
>, containers that can contain text are called <link linkend="strings"
>strings</link
>.</para>

<para
>Containers that are not used yet are 0 by default. An example: <screen>
print N
</screen
> This will print a 0. </para>

<sect2 id="variables">
<title
>Variables: number containers</title>
<para
>Let us start with an example: <screen>
x = 3
print x
</screen
> In the first line the letter <userinput
>x</userinput
> made into a variable (number container). As you see the value of the variable <userinput
>x</userinput
> is set to 3. On the second line the value is printed.</para>
<para
>Note that if we wanted to print an <quote
>x</quote
> that we should have written <screen
>print "x"
</screen>
</para>
<para
>That was easy, now a bit harder example: <screen>
A = 2004
B = 25
AB = A + B

# the next command prints "2029"
print AB
backward 30
# the next command prints "2004 plus 25"
print "" +  A + " plus " + B
backward 30
# the next command prints "1979"
print A - B
</screen
> In the first two lines the variables <userinput
>A</userinput
> and <userinput
>B</userinput
> are set to 2004 and 25. On the third line the variable <userinput
>AB</userinput
> is set to <userinput
>A + B</userinput
>, which is 2029. The rest of the example consists of 3 <userinput
>print</userinput
> commands with <userinput
>backward 30</userinput
> in between. The <userinput
>backward 30</userinput
> is there to make sure every output is on a new line. In this example you also see that variables can be used in <link linkend="math"
>mathematical calculations</link
>.</para>
</sect2>

<sect2 id="strings">
<title
>Strings: text containers</title>
<para
>Strings are a lot like variables. The biggest difference is that strings cannot be used in <link linkend="math"
>mathematical calculations</link
> and <link linkend="questions"
>questions</link
>. An example of the use of strings: <screen>
x = "Hello "
name = inputwindow "Please enter your name..."
print x + name + ", how are you?"
</screen
> On the first line the string <userinput
>x</userinput
> is set to <quote
>Hello </quote
>. On the second line the string <userinput
>name</userinput
> is set to the output of the <userinput
>inputwindow</userinput
> command. On the third line the program prints a composition of three strings on the canvas.</para>
<para
>This program ask you to enter your name. When you, for instance, enter the name <quote
>Paul</quote
>, the program prints <quote
>Hello Paul, how are you?</quote
>. Please note that the plus (+) is the only math symbol that you can use with strings.</para>
</sect2>
</sect1>

<sect1 id="math">
<title
>Can the Turtle do maths?</title>
<para
>Yes, &kturtle; will do your math. You can add (+), substract (-), multiply (*), and divide (/). Here is an example in which we use all of them: <screen>
a = 20 - 5
b = 15 * 2
c = 30 / 30
d = 1 + 1
print "a: "+a+", b: "+b+", c: "+c+", d: "+d 
</screen
> Do you know what value a, b, c and d have?</para>
<para
>If you just want a simple calculation to be done you can do something like this: <screen
>print 2004-12
</screen
></para>
<para
>Now an example with brackets: <screen>
print ( ( 20 - 5 ) * 2 / 30 ) + 1
</screen
> The expressions inside brackets will be calculated first. In this example, 20-5 will be calculated, then multiplied by 2, divided by 30, and then 1 is added (giving 2).</para>
</sect1>

<sect1 id="questions">
<title
>Asking questions, getting answers...</title>
<para
><link linkend="if"
><userinput
>if</userinput
></link
> and <link linkend="while"
><userinput
>while</userinput
></link
> are <link linkend="controlling-execution"
>execution controllers</link
> that we will discuss in the next section. In this section we use the <userinput
>if</userinput
> command to explain questions.</para>
<para
>A simple example of questions: <screen>
x = 6
if x &gt; 5 [
  print "hello"
]
</screen
> In this example the question is the <userinput
>x &gt; 5</userinput
> part. If the answer to this question is <quote
>true</quote
> the code between the brackets will be executed. Questions are an important part of programming and often used together with <link linkend="controlling-execution"
>execution controllers</link
>, like <link linkend="if"
><userinput
>if</userinput
></link
>. All numbers and <link linkend="variables"
>variables</link
> (number containers) can be compared to each other with questions.</para>
<para
>Here are all possible questions: <table>
<title
>Types of questions</title>
<tgroup cols="3">
<tbody>
<row>
<entry
><userinput
>a == b</userinput
></entry>
<entry
>equals</entry>
<entry
>answer is <quote
>true</quote
> if <userinput
>a</userinput
> equals <userinput
>b</userinput
></entry>
</row>
<row>
<entry
><userinput
>a != b</userinput
></entry>
<entry
>not-equal</entry>
<entry
>answer is <quote
>true</quote
> if <userinput
>a</userinput
> does not equal <userinput
>b</userinput
></entry>
</row>
<row>
<entry
><userinput
>a &gt; b</userinput
></entry>
<entry
>greater than</entry>
<entry
>answer is <quote
>true</quote
> if <userinput
>a</userinput
> is greater than <userinput
>b</userinput
></entry>
</row>
<row>
<entry
><userinput
>a &lt; b</userinput
></entry>
<entry
>smaller than</entry>
<entry
>answer is <quote
>true</quote
> if <userinput
>a</userinput
> is smaller than <userinput
>b</userinput
></entry>
</row>
<row>
<entry
><userinput
>a &gt;= b</userinput
></entry>
<entry
>greater than or equals</entry>
<entry
>answer is <quote
>true</quote
> if <userinput
>a</userinput
> is greater than or equals <userinput
>b</userinput
></entry>
</row>
<row>
<entry
><userinput
>a &lt;= b</userinput
></entry>
<entry
>smaller than or equals</entry>
<entry
>answer is <quote
>true</quote
> if <userinput
>a</userinput
> is smaller than or equals <userinput
>b</userinput
></entry>
</row>
</tbody>
</tgroup>
</table>
</para>
<para
>Questions can also be glued to each other with <quote
>question glue</quote
>, this way a few questions can become one big question. <screen>
a = 1
b = 5
if a &lt; 5 and b == 5 [
  print "hello"
]
</screen
> <anchor id="question-glue"/>In this example the glue-word <userinput
>and</userinput
> is used to glue 2 questions (<userinput
>a &lt; 5</userinput
>, <userinput
>b == 5</userinput
>) together. If one side of the <userinput
>and</userinput
> would answer <quote
>false</quote
> the whole question would answer <quote
>false</quote
>, because with the glue-word <userinput
>and</userinput
> both sides need to be <quote
>true</quote
> in order to answer <quote
>true</quote
>.</para>
<para
><userinput
>and</userinput
> is not the only glue-word there are two others. They are all in the next table: <table>
<title
>Question glue-words</title>
<tgroup cols="2">
<tbody>
<row>
<entry
><userinput
>and</userinput
></entry>
<entry
>both sides need to be <quote
>true</quote
> in order to answer <quote
>true</quote
></entry>
</row>
<row>
<entry
><userinput
>or</userinput
></entry>
<entry
>if one of the sides is <quote
>true</quote
> the answer is <quote
>true</quote
></entry>
</row>
<row>
<entry
><userinput
>not</userinput
></entry>
<entry
>only if both of the sides are <quote
>false</quote
> the answer is <quote
>false</quote
></entry>
</row>
</tbody>
</tgroup>
</table>
</para>
</sect1>

<sect1 id="controlling-execution">
<title
>Controlling execution</title>
<para
>The execution controllers enable you &mdash; as their name implies &mdash; to control execution.</para>

<sect2 id="easy">
<title
>Have the turtle wait</title>
<para
>If you have done some programming in &kturtle; you have might noticed that the turtle can be very quick at drawing. This command makes the turtle wait for a given amount of time.</para>
  <variablelist>
    <anchor id="wait"/>
    <varlistentry>
      <term
>wait</term>
      <listitem
><para
><screen
>wait X</screen>
<userinput
>wait</userinput
> makes the turtle wait for X seconds. <screen>
repeat 36 [
  forward 5
  turnright 10
  wait 0.5
]
</screen
> This code draws a circle, but the turtle will wait half a second after each step. This gives the impression of a slow-moving turtle.</para
></listitem>
    </varlistentry>
  </variablelist>
</sect2>

<sect2 id="execute-if">
<title
>Execute "if"</title>
<para
></para>
  <variablelist>
    <anchor id="if"/>
    <varlistentry>
      <term
>if</term>
      <listitem
><para
><screen
>if <link linkend="questions"
>question</link
> [ ... ]</screen>
The code that is placed between the brackets will only be executed <userinput
>if</userinput
> the answer to the <link linkend="questions"
>question</link
> is <quote
>true</quote
>. Please read for more information on <link linkend="questions"
>questions</link
> in the <link linkend="questions"
>question section</link
>. <screen>
x = 6
if x &gt; 5 [
  print "x is greater than five!"
]
</screen
> On the first line <userinput
>x</userinput
> is set to 6. On the second line the <link linkend="questions"
>question</link
> <userinput
>x &gt; 5</userinput
> is asked. Since the answer to this question is <quote
>true</quote
> the execution controller <userinput
>if</userinput
> will allow the code between the brackets to be executed</para
></listitem>
    </varlistentry>
  </variablelist>
</sect2>

<sect2 id="execute-while">
<title
>Execute "while"</title>
<para
></para>
  <variablelist>
    <anchor id="while"/>
    <varlistentry>
      <term
>while</term>
      <listitem
><para
><screen
>while <link linkend="questions"
>question</link
> [ ... ]</screen>
The execution controller <userinput
>while</userinput
> is a lot like <link linkend="if"
><userinput
>if</userinput
></link
>. The difference is that <userinput
>while</userinput
> keeps repeating the code between the brackets till the answer to the <link linkend="questions"
>question</link
> is <quote
>false</quote
>. <screen>
x = 1
while x &lt; 5 [
  forward 10
  wait 1
  x = x + 1
]
</screen
> On the first line <userinput
>x</userinput
> is set to 1. On the second line the <link linkend="questions"
>question</link
> <userinput
>x &lt; 5</userinput
> is asked. Since the answer to this question is <quote
>true</quote
> the execution controller <userinput
>while</userinput
> starts executing the code between the brackets till the answer to the <link linkend="questions"
>question</link
> is <quote
>false</quote
>. In this case the code between the brackets will be executed 4 times, because every time the fifth line is executed <userinput
>x</userinput
> increases by 1..</para
></listitem>
    </varlistentry>
  </variablelist>
</sect2>

<sect2 id="if-else">
<title
>If not, in other words: "else"</title>
<para
></para>
  <variablelist>
    <anchor id="else"/>
    <varlistentry>
      <term
>else</term>
      <listitem
><para
><screen
>if question [ ... ] else [ ... ]</screen>
<userinput
>else</userinput
> can be used in addition to the execution controller <link linkend="if"
><userinput
>if</userinput
></link
>. The code between the brackets after <userinput
>else</userinput
> is only executed if the answer to the <link linkend="questions"
>question</link
> that is asked is <quote
>false</quote
>. <screen>
x = 4
if x &gt; 5 [
  print "x is greater than five!"
] else [
  print "x is smaller than six!"
]
</screen
> The <link linkend="questions"
>question</link
> asks if <userinput
>x</userinput
> is greater than 5. Since <userinput
>x</userinput
> is set to 4 on the first line the answer to the question is <quote
>false</quote
>. This means the code between the brackets after <userinput
>else</userinput
> gets executed.</para
></listitem>
    </varlistentry>
  </variablelist>
</sect2>

<sect2 id="for-loop">
<title
>The "for" loop</title>
<para
></para>
  <variablelist>
    <anchor id="for"/>
    <varlistentry>
      <term
>for</term>
      <listitem
><para
><screen
>for <userinput
>start point</userinput
> to <userinput
>end point</userinput
> [ ... ]</screen>
The <userinput
>for</userinput
> loop is a <quote
>counting loop</quote
>, &ie; it keeps count for you. <screen>
for x = 1 to 10 [
  print x * 7
  forward 15
]
</screen
> Every time the code between the brackets is executed the <userinput
>x</userinput
> is increased by 1, till <userinput
>x</userinput
> reaches the value of 10. The code between the brackets prints the <userinput
>x</userinput
> multiplied by 7. After this program finishes its execution you will see the times table of 7 on the canvas.</para
></listitem>
    </varlistentry>
  </variablelist>
</sect2>

</sect1>


<sect1 id="learn">
<title
>Create your own commands</title>
<para
><userinput
>learn</userinput
> is a very special command, because it is used to create your own commands. The command you create can take <glossterm linkend="input-output"
>input</glossterm
> and return <glossterm linkend="input-output"
>output</glossterm
>. Let us take a look at how a new command is created: <screen>
learn circle (x) [
  repeat 36 [
    forward x
    turnleft 10
  ]
]
</screen
> The new command is called <userinput
>circle</userinput
>. <userinput
>circle</userinput
> takes one <glossterm linkend="input-output"
>input</glossterm
>, a number, to set the size of the circle. <userinput
>circle</userinput
> returns no <glossterm linkend="input-output"
>output</glossterm
>. The <userinput
>circle</userinput
> command can now be used like a normal command in the rest of the code. See this example: <screen
>learn circle (x) [ 
   repeat 36 [ 
     forward x 
     turnleft 10 
   ] 
 ] 
 
 go 30,30 
 circle(20) 
 
 go 40,40 
 circle(50) 
</screen>
</para>
<para
>In the next example a command with a return value is created. <screen>
reset

learn multiplyBySelf (n) [
  r = n * 1
  r = n * n
  return r
]
i = inputwindow "Please enter a number and press OK"
print i + " multiplied by itself is: " + multiplyBySelf (i)
</screen
> In this example a new command called <userinput
>multiplyBySelf</userinput
> is created. The input of this command is multiplied by it self and then returned, using the <anchor id="return"/><userinput
>return</userinput
> command. The <userinput
>return</userinput
> command is the way to output a value from a function you have created. </para>
</sect1>

</chapter>