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<section id="iteration">
<span id="id1"></span><h1>Iteration<a class="headerlink" href="#iteration" title="Link to this heading"></a></h1>
<p>Repetition, Repetition, Repetition, Repe…</p>
<section id="for-loops">
<h2>For Loops<a class="headerlink" href="#for-loops" title="Link to this heading"></a></h2>
<p>We’ve seen simple iteration over a sequence with <code class="docutils literal notranslate"><span class="pre">for</span> <span class="pre">...</span> <span class="pre">in</span></code>:</p>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [170]: </span><span class="k">for</span> <span class="n">x</span> <span class="ow">in</span> <span class="s2">"a string"</span><span class="p">:</span>
<span class="gp"> .....: </span> <span class="nb">print</span><span class="p">(</span><span class="n">x</span><span class="p">)</span>
<span class="gp"> .....:</span>
<span class="go">a</span>
<span class="go">s</span>
<span class="go">t</span>
<span class="go">r</span>
<span class="go">i</span>
<span class="go">n</span>
<span class="go">g</span>
</pre></div>
</div>
<section id="no-indexing-required">
<h3>No Indexing Required<a class="headerlink" href="#no-indexing-required" title="Link to this heading"></a></h3>
<p>Contrast this with other languages, where you must build and use an <code class="docutils literal notranslate"><span class="pre">index</span></code> to iterate through an array.</p>
<div class="highlight-javascript notranslate"><div class="highlight"><pre><span></span><span class="k">for</span><span class="p">(</span><span class="kd">var</span><span class="w"> </span><span class="nx">i</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="mf">0</span><span class="p">;</span><span class="w"> </span><span class="nx">i</span><span class="w"> </span><span class="o"><</span><span class="w"> </span><span class="nx">arr</span><span class="p">.</span><span class="nx">length</span><span class="p">;</span><span class="w"> </span><span class="nx">i</span><span class="o">++</span><span class="p">)</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="kd">var</span><span class="w"> </span><span class="nx">value</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="nx">arr</span><span class="p">[</span><span class="nx">i</span><span class="p">];</span>
<span class="w"> </span><span class="nx">alert</span><span class="p">(</span><span class="nx">i</span><span class="w"> </span><span class="o">+</span><span class="w"> </span><span class="s2">") "</span><span class="w"> </span><span class="o">+</span><span class="w"> </span><span class="nx">value</span><span class="p">);</span>
</pre></div>
</div>
<p>If you <em>do</em> need an index, you can use <code class="docutils literal notranslate"><span class="pre">enumerate</span></code>:</p>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [140]: </span><span class="k">for</span> <span class="n">idx</span><span class="p">,</span> <span class="n">letter</span> <span class="ow">in</span> <span class="nb">enumerate</span><span class="p">(</span><span class="s1">'Python'</span><span class="p">):</span>
<span class="gp"> .....: </span> <span class="nb">print</span><span class="p">(</span><span class="n">idx</span><span class="p">,</span> <span class="n">letter</span><span class="p">,</span> <span class="n">end</span><span class="o">=</span><span class="s1">' '</span><span class="p">)</span>
<span class="gp"> .....:</span>
<span class="go">0 P 1 y 2 t 3 h 4 o 5 n</span>
</pre></div>
</div>
</section>
<section id="range-and-for-loops">
<h3><code class="docutils literal notranslate"><span class="pre">range</span></code> and <code class="docutils literal notranslate"><span class="pre">for</span></code> Loops<a class="headerlink" href="#range-and-for-loops" title="Link to this heading"></a></h3>
<p>The <code class="docutils literal notranslate"><span class="pre">range</span></code> builtin is useful for looping a known number of times:</p>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [171]: </span><span class="k">for</span> <span class="n">i</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">5</span><span class="p">):</span>
<span class="gp"> .....: </span> <span class="nb">print</span><span class="p">(</span><span class="n">i</span><span class="p">)</span>
<span class="gp"> .....:</span>
<span class="go">0</span>
<span class="go">1</span>
<span class="go">2</span>
<span class="go">3</span>
<span class="go">4</span>
</pre></div>
</div>
<p>But you don’t really need to do anything at all with <code class="docutils literal notranslate"><span class="pre">i</span></code></p>
<p>In fact, it’s a common convention to make this clear with a “nothing” name:</p>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [21]: </span><span class="k">for</span> <span class="n">__</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">5</span><span class="p">):</span>
<span class="gp"> ....: </span> <span class="nb">print</span><span class="p">(</span><span class="s2">"*"</span><span class="p">)</span>
<span class="gp"> ....:</span>
<span class="go">*</span>
<span class="go">*</span>
<span class="go">*</span>
<span class="go">*</span>
<span class="go">*</span>
</pre></div>
</div>
</section>
<section id="no-namespace">
<h3>No Namespace<a class="headerlink" href="#no-namespace" title="Link to this heading"></a></h3>
<p>Be alert that a loop does not create a local namespace:</p>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [172]: </span><span class="n">x</span> <span class="o">=</span> <span class="mi">10</span>
<span class="gp">In [173]: </span><span class="k">for</span> <span class="n">x</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">3</span><span class="p">):</span>
<span class="gp"> .....: </span> <span class="k">pass</span>
<span class="gp"> .....:</span>
<span class="gp">In [174]: </span><span class="n">x</span>
<span class="gh">Out[174]: </span><span class="go">2</span>
</pre></div>
</div>
</section>
<section id="loop-control">
<h3>Loop Control<a class="headerlink" href="#loop-control" title="Link to this heading"></a></h3>
<p>Sometimes you want to interrupt or alter the flow of control through a loop.</p>
<p>Loops can be controlled in two ways, with <code class="docutils literal notranslate"><span class="pre">break</span></code> and <code class="docutils literal notranslate"><span class="pre">continue</span></code>.</p>
<p>The <code class="docutils literal notranslate"><span class="pre">break</span></code> keyword will cause a loop to immediately terminate:</p>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [141]: </span><span class="k">for</span> <span class="n">i</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">101</span><span class="p">):</span>
<span class="gp"> .....: </span> <span class="nb">print</span><span class="p">(</span><span class="n">i</span><span class="p">)</span>
<span class="gp"> .....: </span> <span class="k">if</span> <span class="n">i</span> <span class="o">></span> <span class="mi">50</span><span class="p">:</span>
<span class="gp"> .....: </span> <span class="k">break</span>
<span class="gp"> .....:</span>
<span class="go">0 1 2 3 4 5... 46 47 48 49 50 51</span>
</pre></div>
</div>
<p>The <code class="docutils literal notranslate"><span class="pre">continue</span></code> keyword will skip later statements in the loop block, but
allow iteration to continue:</p>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [143]: </span><span class="k">for</span> <span class="ow">in</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">101</span><span class="p">):</span>
<span class="gp"> .....: </span> <span class="k">if</span> <span class="n">i</span> <span class="o">></span> <span class="mi">50</span><span class="p">:</span>
<span class="gp"> .....: </span> <span class="k">break</span>
<span class="gp"> .....: </span> <span class="k">if</span> <span class="n">i</span> <span class="o"><</span> <span class="mi">25</span><span class="p">:</span>
<span class="gp"> .....: </span> <span class="k">continue</span>
<span class="gp"> .....: </span> <span class="nb">print</span><span class="p">(</span><span class="n">i</span><span class="p">,</span> <span class="n">end</span><span class="o">=</span><span class="s1">' '</span><span class="p">)</span>
<span class="gp"> .....:</span>
<span class="go"> 25 26 27 28 29 ... 41 42 43 44 45 46 47 48 49 50</span>
</pre></div>
</div>
<p>Take some time to look at these examples carefully, and make sure you understand them. It’s probably a good idea to write a bit of code to experiment as well.</p>
</section>
<section id="else">
<h3>else<a class="headerlink" href="#else" title="Link to this heading"></a></h3>
<p>For loops can also take an optional <code class="docutils literal notranslate"><span class="pre">else</span></code> block.</p>
<p>This is <strong>not</strong> a feature of most languages, but it can be handy.</p>
<p>Executed only when the loop exits normally (not via break):</p>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [147]: </span><span class="k">for</span> <span class="n">x</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">10</span><span class="p">):</span>
<span class="gp"> .....: </span> <span class="k">if</span> <span class="n">x</span> <span class="o">==</span> <span class="mi">11</span><span class="p">:</span>
<span class="gp"> .....: </span> <span class="k">break</span>
<span class="gp"> .....: </span><span class="k">else</span><span class="p">:</span>
<span class="gp"> .....: </span> <span class="nb">print</span><span class="p">(</span><span class="s1">'finished'</span><span class="p">)</span>
<span class="go">finished</span>
<span class="gp">In [148]: </span><span class="k">for</span> <span class="n">x</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">10</span><span class="p">):</span>
<span class="gp"> .....: </span> <span class="k">if</span> <span class="n">x</span> <span class="o">==</span> <span class="mi">5</span><span class="p">:</span>
<span class="gp"> .....: </span> <span class="nb">print</span><span class="p">(</span><span class="n">x</span><span class="p">)</span>
<span class="gp"> .....: </span> <span class="k">break</span>
<span class="gp"> .....: </span><span class="k">else</span><span class="p">:</span>
<span class="gp"> .....: </span> <span class="nb">print</span><span class="p">(</span><span class="s1">'finished'</span><span class="p">)</span>
<span class="go">5</span>
</pre></div>
</div>
<p>This is a really nice, unique Python feature!</p>
<p>If Python didn’t have <code class="docutils literal notranslate"><span class="pre">else</span></code> on loops, you’d need to set a flag, something like:</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="n">it_did_break</span> <span class="o">=</span> <span class="kc">False</span>
<span class="k">for</span> <span class="n">x</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">10</span><span class="p">):</span>
<span class="k">if</span> <span class="n">x</span> <span class="o">==</span> <span class="mi">11</span><span class="p">:</span>
<span class="n">it_did_break</span> <span class="o">=</span> <span class="kc">True</span>
<span class="k">break</span>
<span class="k">if</span> <span class="ow">not</span> <span class="n">it_did_break</span><span class="p">:</span>
<span class="nb">print</span><span class="p">(</span><span class="s1">'finished'</span><span class="p">)</span>
</pre></div>
</div>
<p>That’s klunkier, no?</p>
<p>Make sure to try this a bit yourself too, to make sure you get it.</p>
</section>
</section>
<section id="while-loops">
<h2>While Loops<a class="headerlink" href="#while-loops" title="Link to this heading"></a></h2>
<p>While loops are different – they are not for iterating over a collection, but rather for repeating something an unknown number of times – and maybe even forever – or until the program terminates.</p>
<p>The <code class="docutils literal notranslate"><span class="pre">while</span></code> keyword is for when you don’t know how many loops you need.</p>
<p>It continues to execute the body until the associated condition does not evaluate to True:</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="k">while</span> <span class="n">a_condition</span><span class="p">:</span>
<span class="n">some_code</span>
<span class="n">in_the_body</span>
</pre></div>
</div>
<section id="while-vs-for">
<h3><code class="docutils literal notranslate"><span class="pre">while</span></code> vs. <code class="docutils literal notranslate"><span class="pre">for</span></code><a class="headerlink" href="#while-vs-for" title="Link to this heading"></a></h3>
<p><code class="docutils literal notranslate"><span class="pre">while</span></code> is more general than <code class="docutils literal notranslate"><span class="pre">for</span></code></p>
<p>– you can always express <code class="docutils literal notranslate"><span class="pre">for</span></code> as <code class="docutils literal notranslate"><span class="pre">while</span></code>, but not always vice-versa.</p>
<p><code class="docutils literal notranslate"><span class="pre">while</span></code> is more error-prone – requires some care to terminate.</p>
<p>The loop body must make progress, so the associated condition can become <code class="docutils literal notranslate"><span class="pre">False</span></code>.</p>
<p>Care must be taken to avoid an unintended error – infinite loops:</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="n">i</span> <span class="o">=</span> <span class="mi">0</span><span class="p">;</span>
<span class="k">while</span> <span class="n">i</span> <span class="o"><</span> <span class="mi">5</span><span class="p">:</span>
<span class="nb">print</span><span class="p">(</span><span class="n">i</span><span class="p">)</span>
</pre></div>
</div>
</section>
<section id="terminating-a-while-loop">
<h3>Terminating a while Loop<a class="headerlink" href="#terminating-a-while-loop" title="Link to this heading"></a></h3>
<p>Use <code class="docutils literal notranslate"><span class="pre">break</span></code>:</p>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [150]: </span><span class="k">while</span> <span class="kc">True</span><span class="p">:</span>
<span class="gp"> .....: </span> <span class="n">i</span> <span class="o">+=</span> <span class="mi">1</span>
<span class="gp"> .....: </span> <span class="k">if</span> <span class="n">i</span> <span class="o">></span> <span class="mi">10</span><span class="p">:</span>
<span class="gp"> .....: </span> <span class="k">break</span>
<span class="gp"> .....: </span> <span class="nb">print</span><span class="p">(</span><span class="n">i</span><span class="p">)</span>
<span class="gp"> .....:</span>
<span class="go">1 2 3 4 5 6 7 8 9 10</span>
</pre></div>
</div>
<p>Set a flag:</p>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [156]: </span><span class="kn">import</span><span class="w"> </span><span class="nn">random</span>
<span class="gp">In [157]: </span><span class="n">keep_going</span> <span class="o">=</span> <span class="kc">True</span>
<span class="gp">In [158]: </span><span class="k">while</span> <span class="n">keep_going</span><span class="p">:</span>
<span class="gp"> .....: </span> <span class="n">num</span> <span class="o">=</span> <span class="n">random</span><span class="o">.</span><span class="n">choice</span><span class="p">(</span><span class="nb">range</span><span class="p">(</span><span class="mi">5</span><span class="p">))</span>
<span class="gp"> .....: </span> <span class="nb">print</span><span class="p">(</span><span class="n">num</span><span class="p">)</span>
<span class="gp"> .....: </span> <span class="k">if</span> <span class="n">num</span> <span class="o">==</span> <span class="mi">3</span><span class="p">:</span>
<span class="gp"> .....: </span> <span class="n">keep_going</span> <span class="o">=</span> <span class="kc">False</span>
<span class="gp"> .....:</span>
<span class="go">3</span>
</pre></div>
</div>
<p>Use a condition:</p>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [161]: </span><span class="k">while</span> <span class="n">i</span> <span class="o"><</span> <span class="mi">10</span><span class="p">:</span>
<span class="gp"> .....: </span> <span class="n">i</span> <span class="o">+=</span> <span class="n">random</span><span class="o">.</span><span class="n">choice</span><span class="p">(</span><span class="nb">range</span><span class="p">(</span><span class="mi">4</span><span class="p">))</span>
<span class="gp"> .....: </span> <span class="nb">print</span><span class="p">(</span><span class="n">i</span><span class="p">)</span>
<span class="gp"> .....:</span>
<span class="go">0 0 2 3 4 6 8 8 8 9 12</span>
</pre></div>
</div>
</section>
<section id="similarities">
<h3>Similarities<a class="headerlink" href="#similarities" title="Link to this heading"></a></h3>
<p>Both <code class="docutils literal notranslate"><span class="pre">for</span></code> and <code class="docutils literal notranslate"><span class="pre">while</span></code> loops can use <code class="docutils literal notranslate"><span class="pre">break</span></code> and <code class="docutils literal notranslate"><span class="pre">continue</span></code> for
internal flow control.</p>
<p>Both <code class="docutils literal notranslate"><span class="pre">for</span></code> and <code class="docutils literal notranslate"><span class="pre">while</span></code> loops can have an optional <code class="docutils literal notranslate"><span class="pre">else</span></code> block.</p>
<p>In both loops, the statements in the <code class="docutils literal notranslate"><span class="pre">else</span></code> block are only executed if the
loop terminates normally (no <code class="docutils literal notranslate"><span class="pre">break</span></code>).</p>
</section>
</section>
<section id="pythonic-iteration">
<h2>Pythonic Iteration<a class="headerlink" href="#pythonic-iteration" title="Link to this heading"></a></h2>
<p>I’ve already said it, but it bears repeating:</p>
<p>for loops are for iterating over something (an “iterable”) – you almost never want to iterate over the indexes, and then access items with the index.</p>
<section id="nifty-for-loop-tricks">
<h3>Nifty for loop tricks<a class="headerlink" href="#nifty-for-loop-tricks" title="Link to this heading"></a></h3>
<p><strong>tuple unpacking:</strong></p>
<p>remember this?</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="n">x</span><span class="p">,</span> <span class="n">y</span> <span class="o">=</span> <span class="mi">3</span><span class="p">,</span> <span class="mi">4</span>
</pre></div>
</div>
<p>You can do that in a for loop, also:</p>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [4]: </span><span class="n">l</span> <span class="o">=</span> <span class="p">[(</span><span class="mi">1</span><span class="p">,</span> <span class="mi">2</span><span class="p">),</span> <span class="p">(</span><span class="mi">3</span><span class="p">,</span> <span class="mi">4</span><span class="p">),</span> <span class="p">(</span><span class="mi">5</span><span class="p">,</span> <span class="mi">6</span><span class="p">)]</span>
<span class="gp">In [5]: </span><span class="k">for</span> <span class="n">i</span><span class="p">,</span> <span class="n">j</span> <span class="ow">in</span> <span class="n">l</span><span class="p">:</span>
<span class="go"> print("i:{}, j:{}".format(i, j))</span>
<span class="go">i:1, j:2</span>
<span class="go">i:3, j:4</span>
<span class="go">i:5, j:6</span>
</pre></div>
</div>
</section>
<section id="looping-through-two-iterables-at-once">
<h3>Looping through two iterables at once:<a class="headerlink" href="#looping-through-two-iterables-at-once" title="Link to this heading"></a></h3>
<blockquote>
<div><p><code class="docutils literal notranslate"><span class="pre">zip</span></code></p>
</div></blockquote>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [10]: </span><span class="n">l1</span> <span class="o">=</span> <span class="p">[</span><span class="mi">1</span><span class="p">,</span> <span class="mi">2</span><span class="p">,</span> <span class="mi">3</span><span class="p">]</span>
<span class="gp">In [11]: </span><span class="n">l2</span> <span class="o">=</span> <span class="p">[</span><span class="mi">3</span><span class="p">,</span> <span class="mi">4</span><span class="p">,</span> <span class="mi">5</span><span class="p">]</span>
<span class="gp">In [12]: </span><span class="k">for</span> <span class="n">i</span><span class="p">,</span> <span class="n">j</span> <span class="ow">in</span> <span class="nb">zip</span><span class="p">(</span><span class="n">l1</span><span class="p">,</span> <span class="n">l2</span><span class="p">):</span>
<span class="go"> print("i:{}, j:{}".format(i, j))</span>
<span class="go">i:1, j:3</span>
<span class="go">i:2, j:4</span>
<span class="go">i:3, j:5</span>
</pre></div>
</div>
<p>There can be more than two:</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="k">for</span> <span class="n">i</span><span class="p">,</span> <span class="n">j</span><span class="p">,</span> <span class="n">k</span><span class="p">,</span> <span class="n">l</span> <span class="ow">in</span> <span class="nb">zip</span><span class="p">(</span><span class="n">l1</span><span class="p">,</span> <span class="n">l2</span><span class="p">,</span> <span class="n">l3</span><span class="p">,</span> <span class="n">l4</span><span class="p">):</span>
</pre></div>
</div>
</section>
<section id="need-the-index-and-the-item">
<h3>Need the index and the item?<a class="headerlink" href="#need-the-index-and-the-item" title="Link to this heading"></a></h3>
<blockquote>
<div><p><code class="docutils literal notranslate"><span class="pre">enumerate</span></code></p>
</div></blockquote>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [2]: </span><span class="n">l</span> <span class="o">=</span> <span class="p">[</span><span class="s1">'this'</span><span class="p">,</span> <span class="s1">'that'</span><span class="p">,</span> <span class="s1">'the other'</span><span class="p">]</span>
<span class="gp">In [3]: </span><span class="k">for</span> <span class="n">i</span><span class="p">,</span> <span class="n">item</span> <span class="ow">in</span> <span class="nb">enumerate</span><span class="p">(</span><span class="n">l</span><span class="p">):</span>
<span class="gp"> ...: </span> <span class="nb">print</span><span class="p">(</span><span class="s2">"the </span><span class="si">{:d}</span><span class="s2">th item is: </span><span class="si">{:s}</span><span class="s2">"</span><span class="o">.</span><span class="n">format</span><span class="p">(</span><span class="n">i</span><span class="p">,</span> <span class="n">item</span><span class="p">))</span>
<span class="gp"> ...:</span>
<span class="go">the 0th item is: this</span>
<span class="go">the 1th item is: that</span>
<span class="go">the 2th item is: the other</span>
</pre></div>
</div>
</section>
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