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<li class="toctree-l2 current"><a class="current reference internal" href="#">For-Loops and While-Loops</a><ul>
<li class="toctree-l3"><a class="reference internal" href="#For-Loops">For-Loops</a><ul>
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<div class="section" id="For-Loops-and-While-Loops">
<h1>For-Loops and While-Loops<a class="headerlink" href="#For-Loops-and-While-Loops" title="Permalink to this headline"></a></h1>
<p>In this section, we will introduce the essential “for-loop” control flow paradigm along with the formal definition of an “iterable”. The utility of these items cannot be understated. Moving forward, you will likely find use for these concepts in nearly every piece of Python code that you write!</p>
<div class="admonition warning">
<p class="admonition-title fa fa-exclamation-circle"><strong>Note</strong>:</p>
<p>There are reading-comprehension exercises included throughout the text. These are meant to help you put your reading to practice. Solutions for the exercises are included at the bottom of this page.</p>
</div>
<div class="section" id="For-Loops">
<h2>For-Loops<a class="headerlink" href="#For-Loops" title="Permalink to this headline"></a></h2>
<p>A “for-loop” allows you to iterate over a collection of items, and execute a block of code once for each iteration. For example, the following code will sum up all the positive numbers in a tuple:</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="n">total</span> <span class="o">=</span> <span class="mi">0</span>
<span class="k">for</span> <span class="n">num</span> <span class="ow">in</span> <span class="p">(</span><span class="o">-</span><span class="mf">22.0</span><span class="p">,</span> <span class="mf">3.5</span><span class="p">,</span> <span class="mf">8.1</span><span class="p">,</span> <span class="o">-</span><span class="mi">10</span><span class="p">,</span> <span class="mf">0.5</span><span class="p">):</span>
<span class="k">if</span> <span class="n">num</span> <span class="o">></span> <span class="mi">0</span><span class="p">:</span>
<span class="n">total</span> <span class="o">=</span> <span class="n">total</span> <span class="o">+</span> <span class="n">num</span>
</pre></div>
</div>
<p>The general syntax for a “for-loop” is:</p>
<div class="highlight-none notranslate"><div class="highlight"><pre><span></span>for <var> in <iterable>:
block of code
</pre></div>
</div>
<p>Where <code class="docutils literal notranslate"><span class="pre"><var></span></code> is any valid variable-identifier and <code class="docutils literal notranslate"><span class="pre"><iterable></span></code> is any <strong>iterable</strong>. We will discuss iterables more formally in the next section; suffice it to know that every sequence-type object is an iterable. The <code class="docutils literal notranslate"><span class="pre">for</span></code>statement must end in a colon character, and the body the for-loop is <a class="reference external" href="https://www.pythonlikeyoumeanit.com/Module2_EssentialsOfPython/Introduction.html#Python-Uses-Whitespace-to-Delimit-Scope">whitespace-delimited</a>.</p>
<p>The for-loop behaves as follows:</p>
<ul class="simple">
<li><p>Request the next member of the iterable.</p></li>
<li><p>If the iterable is empty, exit the for-loop without running its body.</p></li>
<li><p>If the iterable did produce a member, assign that member to <code class="docutils literal notranslate"><span class="pre"><var></span></code> (if <code class="docutils literal notranslate"><span class="pre"><var></span></code> was not previously defined, it becomes defined).</p></li>
<li><p>Execute the enclosed body of code.</p></li>
<li><p>Go back to the first step.</p></li>
</ul>
<p>To be concrete, let’s consider the example:</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="c1"># demonstrating a basic for-loop</span>
<span class="n">total</span> <span class="o">=</span> <span class="mi">0</span>
<span class="k">for</span> <span class="n">item</span> <span class="ow">in</span> <span class="p">[</span><span class="mi">1</span><span class="p">,</span> <span class="mi">3</span><span class="p">,</span> <span class="mi">5</span><span class="p">]:</span>
<span class="n">total</span> <span class="o">=</span> <span class="n">total</span> <span class="o">+</span> <span class="n">item</span>
<span class="nb">print</span><span class="p">(</span><span class="n">total</span><span class="p">)</span> <span class="c1"># `total` has the value 1 + 3 + 5 = 9</span>
<span class="c1"># `item` is still defined here, and holds the value 5</span>
</pre></div>
</div>
<p>This code will perform the following steps:</p>
<ol class="arabic simple">
<li><p>Define the variable <code class="docutils literal notranslate"><span class="pre">total</span></code>, and assign it the value <code class="docutils literal notranslate"><span class="pre">0</span></code></p></li>
<li><p>Iterate on the list, producing value <code class="docutils literal notranslate"><span class="pre">1</span></code>, define the variable <code class="docutils literal notranslate"><span class="pre">item</span></code> and assign it the value <code class="docutils literal notranslate"><span class="pre">1</span></code></p></li>
<li><p>Assign <code class="docutils literal notranslate"><span class="pre">total</span></code> the value <code class="docutils literal notranslate"><span class="pre">0</span> <span class="pre">+</span> <span class="pre">1</span></code></p></li>
<li><p>Iterate on the list, producing the value <code class="docutils literal notranslate"><span class="pre">3</span></code> and assigning it to <code class="docutils literal notranslate"><span class="pre">item</span></code></p></li>
<li><p>Assign <code class="docutils literal notranslate"><span class="pre">total</span></code> the value <code class="docutils literal notranslate"><span class="pre">1</span> <span class="pre">+</span> <span class="pre">3</span></code></p></li>
<li><p>Iterate on the list, producing the value <code class="docutils literal notranslate"><span class="pre">5</span></code> and assigning it to <code class="docutils literal notranslate"><span class="pre">item</span></code></p></li>
<li><p>Assign <code class="docutils literal notranslate"><span class="pre">total</span></code> the value <code class="docutils literal notranslate"><span class="pre">4</span> <span class="pre">+</span> <span class="pre">5</span></code></p></li>
<li><p>Iterate on the list. Having reached its end, a <code class="docutils literal notranslate"><span class="pre">StopIteration</span></code> signal it raised by the list, and the for-loop sequence is exited.</p></li>
<li><p>Print the value of <code class="docutils literal notranslate"><span class="pre">total</span></code> (9)</p></li>
</ol>
<div class="section" id="Potential-Pitfall">
<h3>Potential Pitfall<a class="headerlink" href="#Potential-Pitfall" title="Permalink to this headline"></a></h3>
<p>Note that the variable <code class="docutils literal notranslate"><span class="pre">item</span></code> will persist after the for-loop block is exited. It will reference the last value from the for-loop iteration (in this case <code class="docutils literal notranslate"><span class="pre">item</span></code> has the value 5). That being said, <em>you should not write code that depends on the iterate-variable, outside of the context of the for-loop</em>. In the case that you try to loop over an <em>empty</em> iterable, the iterate-variable is never defined:</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="k">for</span> <span class="n">x</span> <span class="ow">in</span> <span class="p">[]:</span> <span class="c1"># the iterable is empty - the iterate-variable `x` will not be defined</span>
<span class="nb">print</span><span class="p">(</span><span class="s2">"Hello?"</span><span class="p">)</span> <span class="c1"># this code is never executed</span>
<span class="nb">print</span><span class="p">(</span><span class="n">x</span><span class="p">)</span> <span class="c1"># raises an error because `x` was never defined</span>
</pre></div>
</div>
<p>Because we are attempting to iterate over an empty list, <code class="docutils literal notranslate"><span class="pre">StopIteration</span></code> is raised immediately - before the variable <code class="docutils literal notranslate"><span class="pre">x</span></code> is even defined. Thus the code enclosed within the for-loop is never reached, and the subsequent <code class="docutils literal notranslate"><span class="pre">print(x)</span></code> statement will raise a <code class="docutils literal notranslate"><span class="pre">NameError</span></code>, because <code class="docutils literal notranslate"><span class="pre">x</span></code> was never defined!</p>
<div class="admonition note">
<p class="admonition-title fa fa-exclamation-circle"><strong>Reading Comprehension: A basic for-loop</strong></p>
<p>Using a for-loop and an if-statement, print each letter in the string <code class="docutils literal notranslate"><span class="pre">"abcdefghij"</span></code>, if that letter is a vowel.</p>
</div>
</div>
</div>
<div class="section" id="While-Loops">
<h2>While-Loops<a class="headerlink" href="#While-Loops" title="Permalink to this headline"></a></h2>
<p>A “while-loop” allows you to repeat a block of code until a condition is no longer true:</p>
<div class="highlight-none notranslate"><div class="highlight"><pre><span></span>while <condition>:
block of code
</pre></div>
</div>
<p>Where <code class="docutils literal notranslate"><span class="pre"><condition></span></code> is an expression that returns <code class="docutils literal notranslate"><span class="pre">True</span></code> or <code class="docutils literal notranslate"><span class="pre">False</span></code>, or is any object on which <code class="docutils literal notranslate"><span class="pre">bool</span></code> can be called. The “body” of the while-loop is the code indented beneath the while-loop statement.</p>
<p>The while-loop behaves as follows:</p>
<ul class="simple">
<li><p>Call <code class="docutils literal notranslate"><span class="pre">bool(<condition>)</span></code> and execute the indented block of code if <code class="docutils literal notranslate"><span class="pre">True</span></code> is returned. Otherwise, “exit” the while-loop, skipping past the indented code.</p></li>
<li><p>If the indented block code is executed, go back to the first step.</p></li>
</ul>
<p>To be concrete, let’s consider the example:</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="c1"># demonstrating a basic while-loop</span>
<span class="n">total</span> <span class="o">=</span> <span class="mi">0</span>
<span class="k">while</span> <span class="n">total</span> <span class="o"><</span> <span class="mi">2</span><span class="p">:</span>
<span class="n">total</span> <span class="o">+=</span> <span class="mi">1</span> <span class="c1"># equivalent to: `total = total + 1`</span>
<span class="nb">print</span><span class="p">(</span><span class="n">total</span><span class="p">)</span> <span class="c1"># `total` has the value 2</span>
</pre></div>
</div>
<p>This code will perform the following steps:</p>
<ol class="arabic simple">
<li><p>Define the variable <code class="docutils literal notranslate"><span class="pre">total</span></code>, and assign it the value <code class="docutils literal notranslate"><span class="pre">0</span></code></p></li>
<li><p>Evaluate <code class="docutils literal notranslate"><span class="pre">0</span> <span class="pre"><</span> <span class="pre">2</span></code>, which returns <code class="docutils literal notranslate"><span class="pre">True</span></code>: enter the enclosed code-block</p></li>
<li><p>Execute the code block: assign <code class="docutils literal notranslate"><span class="pre">total</span></code> the value <code class="docutils literal notranslate"><span class="pre">0</span> <span class="pre">+</span> <span class="pre">1</span></code></p></li>
<li><p>Evaluate <code class="docutils literal notranslate"><span class="pre">1</span> <span class="pre"><</span> <span class="pre">2</span></code>, which returns <code class="docutils literal notranslate"><span class="pre">True</span></code>: enter the enclosed code-block</p></li>
<li><p>Execute the code block: assign <code class="docutils literal notranslate"><span class="pre">total</span></code> the value <code class="docutils literal notranslate"><span class="pre">1</span> <span class="pre">+</span> <span class="pre">1</span></code></p></li>
<li><p>Evaluate <code class="docutils literal notranslate"><span class="pre">2</span> <span class="pre"><</span> <span class="pre">2</span></code>, which returns <code class="docutils literal notranslate"><span class="pre">False</span></code>: <em>skip</em> the enclosed code-block</p></li>
<li><p>Print the value of <code class="docutils literal notranslate"><span class="pre">total</span></code> (2)</p></li>
</ol>
<p>Note that if we started off with <code class="docutils literal notranslate"><span class="pre">total</span> <span class="pre">=</span> <span class="pre">3</span></code>, the condition-expression <code class="docutils literal notranslate"><span class="pre">3</span> <span class="pre"><</span> <span class="pre">2</span></code> would evaluate to <code class="docutils literal notranslate"><span class="pre">False</span></code> outright, and the indented body of code would never be reached.</p>
<div class="admonition warning">
<p class="admonition-title fa fa-exclamation-circle"><strong>Warning!</strong></p>
<p>It is possible to write a while-loop such that its conditional statement is always True, in which case your code will run ceaselessly! If this ever happens to you in a Jupyter notebook, either interrupt or restart your kernel.</p>
</div>
<div class="admonition note">
<p class="admonition-title fa fa-exclamation-circle"><strong>Reading Comprehension: A basic while-loop</strong></p>
<p>Given a list of nonzero, positive numbers, <code class="docutils literal notranslate"><span class="pre">x</span></code>, append the sum of that list to the end of it. Do this until the last value in <code class="docutils literal notranslate"><span class="pre">x</span></code> is at least 100. Use a while-loop.</p>
<p>If you start with <code class="docutils literal notranslate"><span class="pre">x</span> <span class="pre">=</span> <span class="pre">[1]</span></code>, then by the end of your while-loop <code class="docutils literal notranslate"><span class="pre">x</span></code> should be <code class="docutils literal notranslate"><span class="pre">[1,</span> <span class="pre">1,</span> <span class="pre">2,</span> <span class="pre">4,</span> <span class="pre">8,</span> <span class="pre">16,</span> <span class="pre">32,</span> <span class="pre">64,</span> <span class="pre">128]</span></code>.</p>
</div>
</div>
<div class="section" id="break,-continue,-&-else-clauses-on-loops">
<h2><code class="docutils literal notranslate"><span class="pre">break</span></code>, <code class="docutils literal notranslate"><span class="pre">continue</span></code>, & <code class="docutils literal notranslate"><span class="pre">else</span></code> clauses on loops<a class="headerlink" href="#break,-continue,-&-else-clauses-on-loops" title="Permalink to this headline"></a></h2>
<p>The <code class="docutils literal notranslate"><span class="pre">continue</span></code> and <code class="docutils literal notranslate"><span class="pre">break</span></code> statements can be used within the bodies of both for-loops and while-loops. They provide added means for “short-circuiting” or prematurely exiting a given loop, respectively.</p>
<p>Encountering <code class="docutils literal notranslate"><span class="pre">break</span></code> within a given loop causes that loop to be exited immediately:</p>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="c1"># breaking out of a loop early</span>
<span class="o">>>></span> <span class="k">for</span> <span class="n">item</span> <span class="ow">in</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="mi">4</span><span class="p">,</span> <span class="mi">5</span><span class="p">]:</span>
<span class="o">...</span> <span class="k">if</span> <span class="n">item</span> <span class="o">==</span> <span class="mi">3</span><span class="p">:</span>
<span class="o">...</span> <span class="nb">print</span><span class="p">(</span><span class="n">item</span><span class="p">,</span> <span class="s2">" ...break!"</span><span class="p">)</span>
<span class="o">...</span> <span class="k">break</span>
<span class="o">...</span> <span class="nb">print</span><span class="p">(</span><span class="n">item</span><span class="p">,</span> <span class="s2">" ...next iteration"</span><span class="p">)</span>
</pre></div>
</div>
<div class="highlight-none notranslate"><div class="highlight"><pre><span></span>1 ...next iteration
2 ...next iteration
3 ...break!
</pre></div>
</div>
<p>An <code class="docutils literal notranslate"><span class="pre">else</span></code> clause can be added to the end of any loop. The body of this else-statement will be executed <em>only if the loop was not exited via a ``break`` statement</em>.</p>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="c1"># including an else-clause at the end of the loop</span>
<span class="o">>>></span> <span class="k">for</span> <span class="n">item</span> <span class="ow">in</span> <span class="p">[</span><span class="mi">2</span><span class="p">,</span> <span class="mi">4</span><span class="p">,</span> <span class="mi">6</span><span class="p">]:</span>
<span class="o">...</span> <span class="k">if</span> <span class="n">item</span> <span class="o">==</span> <span class="mi">3</span><span class="p">:</span>
<span class="o">...</span> <span class="nb">print</span><span class="p">(</span><span class="n">item</span><span class="p">,</span> <span class="s2">" ...break!"</span><span class="p">)</span>
<span class="o">...</span> <span class="k">break</span>
<span class="o">...</span> <span class="nb">print</span><span class="p">(</span><span class="n">item</span><span class="p">,</span> <span class="s2">" ...next iteration"</span><span class="p">)</span>
<span class="o">...</span> <span class="k">else</span><span class="p">:</span>
<span class="o">...</span> <span class="nb">print</span><span class="p">(</span><span class="s2">"if you are reading this, then the loop completed without a 'break'"</span><span class="p">)</span>
</pre></div>
</div>
<div class="highlight-none notranslate"><div class="highlight"><pre><span></span>2 ...next iteration
4 ...next iteration
6 ...next iteration
if you are reading this, then the loop completed without a 'break'
</pre></div>
</div>
<p>The <code class="docutils literal notranslate"><span class="pre">continue</span></code> statement, when encountered within a loop, causes the loop-statement to be revisited immediately.</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="c1"># demonstrating a `continue` statement in a loop</span>
<span class="o">>>></span> <span class="n">x</span> <span class="o">=</span> <span class="mi">1</span>
<span class="o">>>></span> <span class="k">while</span> <span class="n">x</span> <span class="o"><</span> <span class="mi">4</span><span class="p">:</span>
<span class="o">...</span> <span class="nb">print</span><span class="p">(</span><span class="s2">"x = "</span><span class="p">,</span> <span class="n">x</span><span class="p">,</span> <span class="s2">">> enter loop-body <<"</span><span class="p">)</span>
<span class="o">...</span> <span class="k">if</span> <span class="n">x</span> <span class="o">==</span> <span class="mi">2</span><span class="p">:</span>
<span class="o">...</span> <span class="nb">print</span><span class="p">(</span><span class="s2">"x = "</span><span class="p">,</span> <span class="n">x</span><span class="p">,</span> <span class="s2">" continue...back to the top of the loop!"</span><span class="p">)</span>
<span class="o">...</span> <span class="n">x</span> <span class="o">+=</span> <span class="mi">1</span>
<span class="o">...</span> <span class="k">continue</span>
<span class="o">...</span> <span class="n">x</span> <span class="o">+=</span> <span class="mi">1</span>
<span class="o">...</span> <span class="nb">print</span><span class="p">(</span><span class="s2">"--reached end of loop-body--"</span><span class="p">)</span>
</pre></div>
</div>
<div class="highlight-none notranslate"><div class="highlight"><pre><span></span>x = 1 >> enter loop-body <<
--reached end of loop-body--
x = 2 >> enter loop-body <<
x = 2 continue...back to the top of the loop!
x = 3 >> enter loop-body <<
--reached end of loop-body--
</pre></div>
</div>
<div class="admonition note">
<p class="admonition-title fa fa-exclamation-circle"><strong>Reading Comprehension: conducting flow in a loop</strong></p>
<p>Loop over a list of integers repeatedly, summing up all of its even values, and adding the content to a total. Repeat this process until the the total exceeds 100, or if you have looped over the list more than 50 times. Print the total only if it exceeds 100.</p>
</div>
</div>
<div class="section" id="Links-to-Official-Documentation">
<h2>Links to Official Documentation<a class="headerlink" href="#Links-to-Official-Documentation" title="Permalink to this headline"></a></h2>
<ul class="simple">
<li><p><a class="reference external" href="https://docs.python.org/3/reference/compound_stmts.html#the-for-statement">‘for’ statement</a></p></li>
<li><p><a class="reference external" href="https://docs.python.org/3/reference/compound_stmts.html#the-while-statement">‘while’ statement</a></p></li>
<li><p><a class="reference external" href="https://docs.python.org/3/tutorial/controlflow.html#break-and-continue-statements-and-else-clauses-on-loops">‘break’, ‘continue’, and ‘else’ clauses</a></p></li>
<li><p><a class="reference external" href="https://docs.python.org/3/tutorial/controlflow.html#pass-statements">‘pass’ statment</a></p></li>
</ul>
</div>
<div class="section" id="Reading-Comprehension-Exercise-Solutions:">
<h2>Reading Comprehension Exercise Solutions:<a class="headerlink" href="#Reading-Comprehension-Exercise-Solutions:" title="Permalink to this headline"></a></h2>
<p><strong>A basic for-loop: Solution</strong></p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="k">for</span> <span class="n">letter</span> <span class="ow">in</span> <span class="s2">"abcdefghij"</span><span class="p">:</span>
<span class="k">if</span> <span class="n">letter</span> <span class="ow">in</span> <span class="s2">"aeiou"</span><span class="p">:</span>
<span class="nb">print</span><span class="p">(</span><span class="n">letter</span><span class="p">)</span>
</pre></div>
</div>
<p><strong>A basic while-loop: Solution</strong></p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="k">while</span> <span class="n">x</span><span class="p">[</span><span class="o">-</span><span class="mi">1</span><span class="p">]</span> <span class="o"><</span> <span class="mi">100</span><span class="p">:</span>
<span class="n">x</span><span class="o">.</span><span class="n">append</span><span class="p">(</span><span class="nb">sum</span><span class="p">(</span><span class="n">x</span><span class="p">))</span>
</pre></div>
</div>
<p><strong>Conducting flow in a loop: Solution</strong></p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="n">x</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">1</span><span class="p">,</span> <span class="mi">2</span><span class="p">,</span> <span class="mi">8</span><span class="p">,</span> <span class="mi">10</span><span class="p">,</span> <span class="o">-</span><span class="mi">3</span><span class="p">,</span> <span class="mi">0</span><span class="p">]</span>
<span class="n">num_loop</span> <span class="o">=</span> <span class="mi">0</span>
<span class="n">total</span> <span class="o">=</span> <span class="mi">0</span>
<span class="k">while</span> <span class="n">total</span> <span class="o"><</span> <span class="mi">100</span><span class="p">:</span>
<span class="k">for</span> <span class="n">item</span> <span class="ow">in</span> <span class="n">x</span><span class="p">:</span>
<span class="c1"># return to for-loop if</span>
<span class="c1"># `item` is odd-valued</span>
<span class="k">if</span> <span class="n">item</span> <span class="o">%</span> <span class="mi">2</span> <span class="o">==</span> <span class="mi">1</span><span class="p">:</span>
<span class="k">continue</span>
<span class="k">else</span><span class="p">:</span>
<span class="n">total</span> <span class="o">+=</span> <span class="n">item</span>
<span class="n">num_loop</span> <span class="o">+=</span> <span class="mi">1</span>
<span class="c1"># break from while-loop if</span>
<span class="c1"># more than 50 items tallied</span>
<span class="k">if</span> <span class="mi">50</span> <span class="o"><</span> <span class="n">num_loop</span><span class="p">:</span>
<span class="k">break</span>
<span class="k">else</span><span class="p">:</span>
<span class="nb">print</span><span class="p">(</span><span class="n">total</span><span class="p">)</span>
</pre></div>
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