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<div class="section" id="conditional-loops">
<h1>Bedingte Schleifen<a class="headerlink" href="#conditional-loops" title="Link zu dieser Überschrift">¶</a></h1>
<div class="section" id="introduction">
<h2>Einführung<a class="headerlink" href="#introduction" title="Link zu dieser Überschrift">¶</a></h2>
<p>Bedingte Schleifen sind ein Weg etwas zu wiederholen, solange eine bestimmte Bedingung erfüllt, oder <strong>True</strong>, ist. Falls die Bedingung immer erfüllt ist (also nie <strong>False</strong> wird), läuft die Schleife endlos weiter. Falls die Bedingung schon anfänglich falsch ist, wird der Code in der Schleife niemals ausgeführt! In Python werden bedingte Schleifen mit der <code class="docutils literal"><span class="pre">while</span></code>-Anweisung definiert:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="n">word</span> <span class="o">=</span> <span class="s">''</span>
<span class="n">sentence</span> <span class="o">=</span> <span class="s">''</span>
<span class="k">print</span><span class="p">(</span><span class="s">'Please enter some words.'</span><span class="p">)</span>
<span class="k">print</span><span class="p">(</span><span class="s">'Include a period (.) when you are finished.'</span><span class="p">)</span>
<span class="k">while</span> <span class="s">'.'</span> <span class="ow">not</span> <span class="ow">in</span> <span class="n">word</span><span class="p">:</span>
<span class="n">word</span> <span class="o">=</span> <span class="nb">input</span><span class="p">(</span><span class="s">'next word: '</span><span class="p">)</span>
<span class="n">sentence</span> <span class="o">=</span> <span class="n">word</span> <span class="o">+</span> <span class="s">' '</span> <span class="o">+</span> <span class="n">sentence</span>
<span class="k">print</span><span class="p">()</span>
<span class="k">print</span><span class="p">(</span><span class="s">'Aha! You said:'</span><span class="p">)</span>
<span class="k">print</span><span class="p">(</span><span class="n">sentence</span><span class="p">)</span>
</pre></div>
</div>
<p>Wir nennen diesen Teil des Quelltextes die ‘Bedingung’: <code class="docutils literal"><span class="pre">'.'</span> <span class="pre">not</span> <span class="pre">in</span> <span class="pre">word</span></code></p>
<p>Ob die Bedingung wahr (True) zurückgibt oder nicht entscheidet, ob der Quelltext innerhalb der <code class="docutils literal"><span class="pre">while</span></code>-Schleife ausgeführt wird.</p>
<p>Lies den obigen Quellcode und schau, ob du im Kopf herleiten kannst, was er tut (und was die endgültige Ausgabe sein wird).</p>
<p>Dann kopier ihn in eine Datei, sagen wir <code class="file docutils literal"><span class="pre">satz.py</span></code>, und führe ihn aus – schau genau, was er tut. Trifft das, was du dir gedacht hast, zu?</p>
<div class="admonition note">
<p class="first admonition-title">Bemerkung</p>
<p class="last">Falls du Python 2 benutzt, wirst du <code class="docutils literal"><span class="pre">input``mit</span> <span class="pre">``raw_input</span></code> ersetzen müssen, um das Programm korrekt auszuführen.</p>
</div>
</div>
<div class="section" id="turtle-prison">
<h2>Schildkrötengefängnis<a class="headerlink" href="#turtle-prison" title="Link zu dieser Überschrift">¶</a></h2>
<div class="section" id="exercise">
<h3>Übung<a class="headerlink" href="#exercise" title="Link zu dieser Überschrift">¶</a></h3>
<p>The turtle has been up to its usual tricks again, robbing liquor
stores and building up huge gambling debts. It’s time for turtle to be
put into a cell that it can’t get out of.</p>
<p>Lass uns eine neue Version von <code class="docutils literal"><span class="pre">forward()</span></code> bauen. Eine, die die Schildkröte herumdreht, falls sie versucht, sich weiter als 100 von ihrem Ursprung zu entfernen. Wir brauchen eine <code class="docutils literal"><span class="pre">while</span></code>-Schleife und einige neue turtle-Funktionen:</p>
<ul class="simple">
<li><p class="first"><code class="docutils literal"><span class="pre">turtle.distance(0,</span> <span class="pre">0)</span></code>- Entfernung der Schildkröte vom Ursprung</p>
</li>
<li><p class="first"><code class="docutils literal"><span class="pre">turtle.towards(0,</span> <span class="pre">0)</span></code> - Der Winkel, der zurück zum Ursprungspunkt führt</p>
</li>
<li><p class="first"><code class="docutils literal"><span class="pre">turtle.setheading(Winkel)</span></code> - Lege die Ausrichtung der Schildkröte direkt fest</p>
</li>
</ul>
<p>Du kannst, wenn du magst, versuchen, mit der turtle in einem Interpreter herumzuspielen und die Funktionen zu benutzen, um genau zu sehen, was sie tut.</p>
<p>Jetzt wirst du die Gefängnislogik implementieren müssen, indem du die turtle-Funktionen, vielleicht eine <code class="docutils literal"><span class="pre">while</span></code>-Schleife und ein bisschen bedingte Logik, verwendest. Es ist ein bisschen haarig, aber bleib dran! Scheue nicht davor zurück, deine Idee mit einem Coach oder einem anderen Lernenden durchzusprechen.</p>
</div>
<div class="solution section" id="solution">
<h3>Lösung<a class="headerlink" href="#solution" title="Link zu dieser Überschrift">¶</a></h3>
<div class="highlight-python"><div class="highlight"><pre><span class="k">def</span> <span class="nf">forward</span><span class="p">(</span><span class="n">distance</span><span class="p">):</span>
<span class="k">while</span> <span class="n">distance</span> <span class="o">></span> <span class="mi">0</span><span class="p">:</span>
<span class="k">if</span> <span class="n">turtle</span><span class="o">.</span><span class="n">distance</span><span class="p">(</span><span class="mi">0</span><span class="p">,</span><span class="mi">0</span><span class="p">)</span> <span class="o">></span> <span class="mi">100</span><span class="p">:</span>
<span class="n">angle</span> <span class="o">=</span> <span class="n">turtle</span><span class="o">.</span><span class="n">towards</span><span class="p">(</span><span class="mi">0</span><span class="p">,</span><span class="mi">0</span><span class="p">)</span>
<span class="n">turtle</span><span class="o">.</span><span class="n">setheading</span><span class="p">(</span><span class="n">angle</span><span class="p">)</span>
<span class="n">turtle</span><span class="o">.</span><span class="n">forward</span><span class="p">(</span><span class="mi">1</span><span class="p">)</span>
<span class="n">distance</span> <span class="o">=</span> <span class="n">distance</span> <span class="o">-</span> <span class="mi">1</span>
</pre></div>
</div>
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</div>
<div class="section" id="draw-a-spiral">
<h2>Zeichne eine Spirale<a class="headerlink" href="#draw-a-spiral" title="Link zu dieser Überschrift">¶</a></h2>
<p>Schleifen können mit der <code class="docutils literal"><span class="pre">break</span></code>-Anweisung unterbrochen werden. Das ist besonders nützlich, wenn du eine <em>Endlosschleife</em> schreibst, welches eine Schleife ist, deren Bedingung immer wahr (<strong>True</strong>) ist.</p>
<div class="section" id="id1">
<h3>Übung<a class="headerlink" href="#id1" title="Link zu dieser Überschrift">¶</a></h3>
<p>Schreibe eine <code class="docutils literal"><span class="pre">while</span></code>-Schleife mit einer Bedingung, die immer wahr (<strong>True</strong>) ist, um eine Spirale zu malen. Unterbrich die Schleife, wenn die turtle eine gewisse Distanz zum Mittelpunkt erreicht hat. Benutze die Funktion <code class="docutils literal"><span class="pre">turtle.distance(x,</span> <span class="pre">y)</span></code>, um die Entfernung der Schildkröte zu dem durch die <code class="docutils literal"><span class="pre">x</span></code>- und <code class="docutils literal"><span class="pre">y</span></code>-Koordinaten bestimmten Punkt zu errechnen.</p>
<p>Um das zu tun, brauchst Du die Funktionen <code class="docutils literal"><span class="pre">turtle.xcor()</span></code> und <code class="docutils literal"><span class="pre">turtle.ycor()</span></code>, welche die Position der Schildkröte auf der X- und Y-Achse zurückgeben.</p>
<div class="admonition note">
<p class="first admonition-title">Bemerkung</p>
<p class="last">Um eine Spirale zu zeichnen, muss die Schildkröte um einen konstanten Winkel gedreht werden und dabei um eine anwachsende Strecke vorwärts bewegt werden.</p>
</div>
</div>
<div class="solution section" id="id2">
<h3>Lösung<a class="headerlink" href="#id2" title="Link zu dieser Überschrift">¶</a></h3>
<div class="highlight-python"><div class="highlight"><pre><span class="k">def</span> <span class="nf">draw_spiral</span><span class="p">(</span><span class="n">radius</span><span class="p">):</span>
<span class="n">original_xcor</span> <span class="o">=</span> <span class="n">turtle</span><span class="o">.</span><span class="n">xcor</span><span class="p">()</span>
<span class="n">original_ycor</span> <span class="o">=</span> <span class="n">turtle</span><span class="o">.</span><span class="n">ycor</span><span class="p">()</span>
<span class="n">speed</span> <span class="o">=</span> <span class="mi">1</span>
<span class="k">while</span> <span class="bp">True</span><span class="p">:</span>
<span class="n">turtle</span><span class="o">.</span><span class="n">forward</span><span class="p">(</span><span class="n">speed</span><span class="p">)</span>
<span class="n">turtle</span><span class="o">.</span><span class="n">left</span><span class="p">(</span><span class="mi">10</span><span class="p">)</span>
<span class="n">speed</span> <span class="o">+=</span> <span class="mf">0.1</span>
<span class="k">if</span> <span class="n">turtle</span><span class="o">.</span><span class="n">distance</span><span class="p">(</span><span class="n">original_xcor</span><span class="p">,</span> <span class="n">original_ycor</span><span class="p">)</span> <span class="o">></span> <span class="n">radius</span><span class="p">:</span>
<span class="k">break</span>
</pre></div>
</div>
</div>
<div class="section" id="bonus">
<h3>Bonus<a class="headerlink" href="#bonus" title="Link zu dieser Überschrift">¶</a></h3>
<p>Kannst du eine Bedingung für die Schleife formulieren, so dass du du keine Endlosschleife <code class="docutils literal"><span class="pre">while</span> <span class="pre">True</span></code> oder die <code class="docutils literal"><span class="pre">break</span></code>-Anweisung benötigst? Welche Version findest du einfacher zu verstehen?</p>
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