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02-Iteration.html
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<h1 class="title">Iteration over Data Structures</h1>
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<div class="quarto-title-meta">
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</header>
<p>Today, we will look at how we iterate through data structures. Consider a the following list:</p>
<div id="435adbdb" class="cell" data-execution_count="1">
<div class="sourceCode cell-code" id="cb1"><pre class="sourceCode python code-with-copy"><code class="sourceCode python"><span id="cb1-1"><a href="#cb1-1" aria-hidden="true" tabindex="-1"></a>heartrates <span class="op">=</span> [<span class="dv">68</span>, <span class="dv">54</span>, <span class="dv">72</span>, <span class="dv">66</span>, <span class="dv">90</span>, <span class="dv">102</span>, <span class="dv">49</span>]</span>
<span id="cb1-2"><a href="#cb1-2" aria-hidden="true" tabindex="-1"></a>heartrates</span></code><button title="Copy to Clipboard" class="code-copy-button"><i class="bi"></i></button></pre></div>
<div class="cell-output cell-output-display" data-execution_count="1">
<pre><code>[68, 54, 72, 66, 90, 102, 49]</code></pre>
</div>
</div>
<p>If we want to compute some simple calculations on this list, we have some built-in functions to do that, such as <code>sum()</code>, <code>max()</code>, and so forth.</p>
<p>But what is going on behind the scenes in these functions? They all have to iterate through each element of the List. For <code>sum()</code>, the function has to iterate through each element of the list to add up the total sum. For <code>max()</code>, the function has to iterate through each element of the list and see if it has encountered a value bigger than it has seen before.</p>
<p>When we use a function on a data structure, we are almost always iterating through the elements of the data structure, and doing something with the elements as we go. So far in our journey in learning Python, we have left the iteration for the built-in function to carry out (which are optimized for performance). But this course will be focused on building custom functions that require us to iterate through the data on our own. This process will create a huge amount of flexibility and power in what you can do with your data!</p>
<section id="iterable-data-structures" class="level2">
<h2 class="anchored" data-anchor-id="iterable-data-structures">Iterable Data Structures</h2>
<p>It turns out that we can iterate over <em>many</em> types of data structures in Python. These Data Structures are considered as <strong>“Iterable”</strong>:</p>
<ul>
<li><p>List</p></li>
<li><p>Tuple</p></li>
<li><p>String (yes, actually!)</p></li>
<li><p>Series (but not recommended)</p></li>
<li><p>DataFrame</p></li>
<li><p>Ranges (to be introduced in exercises)</p></li>
<li><p>Dictionary</p></li>
</ul>
<p>When a data structure is considered iterable, there are a few things you can do with it:</p>
<ul>
<li><p>Access elements or subset of the data structure via the bracket <code>[ ]</code> operator.</p></li>
<li><p>Use the <code>in</code>, <code>not in</code> statements to check for presence of an element in the data structure.</p></li>
<li><p>Examine the length via <code>len()</code>.</p></li>
<li><p>Iterate through the data structure via a For-Loop.</p></li>
</ul>
<p>You have seen examples of the first three actions already. Let’s see how we can iterate through all of these iterable data structures via the For-Loop.</p>
</section>
<section id="for-loops" class="level2">
<h2 class="anchored" data-anchor-id="for-loops">For Loops</h2>
<p>A “For-Loop” allows you to iterate over an iterable data structure, and execute a block of code once for each iteration. Here is what the syntax looks like:</p>
<pre><code>for <variable> in <iterable>:
block of code</code></pre>
<section id="example-iterating-through-a-list" class="level3">
<h3 class="anchored" data-anchor-id="example-iterating-through-a-list">Example: Iterating through a List</h3>
<p>The following code will iterate through each element of the list <code>heartrates</code> and print out each element:</p>
<div id="f7aa3542" class="cell" data-execution_count="2">
<div class="sourceCode cell-code" id="cb4"><pre class="sourceCode python code-with-copy"><code class="sourceCode python"><span id="cb4-1"><a href="#cb4-1" aria-hidden="true" tabindex="-1"></a>heartrates <span class="op">=</span> [<span class="dv">68</span>, <span class="dv">54</span>, <span class="dv">72</span>, <span class="dv">66</span>, <span class="dv">90</span>, <span class="dv">102</span>]</span>
<span id="cb4-2"><a href="#cb4-2" aria-hidden="true" tabindex="-1"></a><span class="cf">for</span> rate <span class="kw">in</span> heartrates:</span>
<span id="cb4-3"><a href="#cb4-3" aria-hidden="true" tabindex="-1"></a> <span class="bu">print</span>(<span class="st">"Current heartrate:"</span>, rate)</span></code><button title="Copy to Clipboard" class="code-copy-button"><i class="bi"></i></button></pre></div>
<div class="cell-output cell-output-stdout">
<pre><code>Current heartrate: 68
Current heartrate: 54
Current heartrate: 72
Current heartrate: 66
Current heartrate: 90
Current heartrate: 102</code></pre>
</div>
</div>
<p>Here is what the Python interpretor is doing:</p>
<ol type="1">
<li>Assign <code>heartrates</code> as a list.</li>
<li>Enter For-Loop: <code>rate</code> is assigned to the next element of <code>heartrates</code>. If it is the first time, <code>rate</code> is assigned as the first element of <code>heartrates</code>.</li>
<li>The “block of code” in the indented section is run, and the <code>rate</code> is printed.</li>
<li>Steps 2 and 3 are repeated until the last element of <code>heartrates</code>.</li>
</ol>
<p>Now you see why it’s called a “For-Loop”: <em>for</em> an element of the iterable data structure, do the “block of code”, and <em>loop</em> back to the top for the next element. You can have multiple lines of code in the indented section for the block of code.</p>
</section>
<section id="example-iterating-through-a-list-and-summing-its-total" class="level3">
<h3 class="anchored" data-anchor-id="example-iterating-through-a-list-and-summing-its-total">Example: Iterating through a List and summing its total</h3>
<p>The following code will add up all the elements of a list:</p>
<div id="67f92028" class="cell" data-execution_count="3">
<div class="sourceCode cell-code" id="cb6"><pre class="sourceCode python code-with-copy"><code class="sourceCode python"><span id="cb6-1"><a href="#cb6-1" aria-hidden="true" tabindex="-1"></a>heartrates <span class="op">=</span> [<span class="dv">68</span>, <span class="dv">54</span>, <span class="dv">72</span>, <span class="dv">66</span>, <span class="dv">90</span>, <span class="dv">102</span>, <span class="dv">49</span>]</span>
<span id="cb6-2"><a href="#cb6-2" aria-hidden="true" tabindex="-1"></a>total <span class="op">=</span> <span class="dv">0</span></span>
<span id="cb6-3"><a href="#cb6-3" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb6-4"><a href="#cb6-4" aria-hidden="true" tabindex="-1"></a><span class="cf">for</span> rate <span class="kw">in</span> heartrates:</span>
<span id="cb6-5"><a href="#cb6-5" aria-hidden="true" tabindex="-1"></a> total <span class="op">=</span> total <span class="op">+</span> rate</span>
<span id="cb6-6"><a href="#cb6-6" aria-hidden="true" tabindex="-1"></a> <span class="bu">print</span>(<span class="st">"Current total:"</span>, total)</span>
<span id="cb6-7"><a href="#cb6-7" aria-hidden="true" tabindex="-1"></a> </span>
<span id="cb6-8"><a href="#cb6-8" aria-hidden="true" tabindex="-1"></a><span class="bu">print</span>(<span class="st">"Final total:"</span>, total)</span></code><button title="Copy to Clipboard" class="code-copy-button"><i class="bi"></i></button></pre></div>
<div class="cell-output cell-output-stdout">
<pre><code>Current total: 68
Current total: 122
Current total: 194
Current total: 260
Current total: 350
Current total: 452
Current total: 501
Final total: 501</code></pre>
</div>
</div>
<p>We just reconstructed the <code>sum()</code> function!</p>
<p>Another way of seeing what happened is to use the following tool that allows you to step through Python code execution line-by-line and see the variables change.</p>
<iframe width="800" height="500" frameborder="0" src="https://pythontutor.com/iframe-embed.html#code=heartrates%20%3D%20%5B68,%2054,%2072,%2066,%2090,%20102,%2049%5D%0Atotal%20%3D%200%0A%0Afor%20rate%20in%20heartrates%3A%0A%20%20total%20%3D%20total%20%2B%20rate%0A%20%20print%28%22Current%20total%3A%22,%20total%29%0A%20%20%0Aprint%28%22Final%20total%3A%22,%20total%29&codeDivHeight=400&codeDivWidth=350&cumulative=false&curInstr=0&heapPrimitives=nevernest&origin=opt-frontend.js&py=311&rawInputLstJSON=%5B%5D&textReferences=false">
</iframe>
<p>If it doesn’t load properly, here is the <a href="https://pythontutor.com/render.html#code=heartrates%20%3D%20%5B68,%2054,%2072,%2066,%2090,%20102,%2049%5D%0Atotal%20%3D%200%0A%0Afor%20rate%20in%20heartrates%3A%0A%20%20total%20%3D%20total%20%2B%20rate%0A%20%20print%28%22Current%20total%3A%22,%20total%29%0A%20%20%0Aprint%28%22Final%20total%3A%22,%20total%29&cumulative=false&curInstr=0&heapPrimitives=nevernest&mode=display&origin=opt-frontend.js&py=311&rawInputLstJSON=%5B%5D&textReferences=false">link</a>.</p>
</section>
<section id="example-modifying-each-element-while-iterating" class="level3">
<h3 class="anchored" data-anchor-id="example-modifying-each-element-while-iterating">Example: Modifying each element while iterating</h3>
<p>Sometimes you want to modify each element of an iterable data structure. However, if you modify the variable that is changing in the For-Loop, it won’t change the original value in the data structure.</p>
<div id="02aa3d83" class="cell" data-execution_count="4">
<div class="sourceCode cell-code" id="cb8"><pre class="sourceCode python code-with-copy"><code class="sourceCode python"><span id="cb8-1"><a href="#cb8-1" aria-hidden="true" tabindex="-1"></a><span class="im">import</span> math</span>
<span id="cb8-2"><a href="#cb8-2" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb8-3"><a href="#cb8-3" aria-hidden="true" tabindex="-1"></a><span class="bu">print</span>(<span class="st">"Before:"</span>, heartrates)</span>
<span id="cb8-4"><a href="#cb8-4" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb8-5"><a href="#cb8-5" aria-hidden="true" tabindex="-1"></a><span class="cf">for</span> rate <span class="kw">in</span> heartrates:</span>
<span id="cb8-6"><a href="#cb8-6" aria-hidden="true" tabindex="-1"></a> rate <span class="op">=</span> math.log(rate)</span>
<span id="cb8-7"><a href="#cb8-7" aria-hidden="true" tabindex="-1"></a> </span>
<span id="cb8-8"><a href="#cb8-8" aria-hidden="true" tabindex="-1"></a><span class="bu">print</span>(<span class="st">"After:"</span>, heartrates)</span></code><button title="Copy to Clipboard" class="code-copy-button"><i class="bi"></i></button></pre></div>
<div class="cell-output cell-output-stdout">
<pre><code>Before: [68, 54, 72, 66, 90, 102, 49]
After: [68, 54, 72, 66, 90, 102, 49]</code></pre>
</div>
</div>
<p>The code <code>rate = math.log(rate)</code> changes the value of rate, but it is not connected to <code>heartrates</code> anymore. Instead, we need to change <code>heartrates[index]</code>, where <code>index</code> is an integer that goes through all the indicies of <code>heartrates</code>.</p>
<p>We can do this with the <code>enumerate()</code> function:</p>
<div id="88ab4828" class="cell" data-execution_count="5">
<div class="sourceCode cell-code" id="cb10"><pre class="sourceCode python code-with-copy"><code class="sourceCode python"><span id="cb10-1"><a href="#cb10-1" aria-hidden="true" tabindex="-1"></a>heartrates <span class="op">=</span> [<span class="dv">68</span>, <span class="dv">54</span>, <span class="dv">72</span>, <span class="dv">66</span>, <span class="dv">90</span>, <span class="dv">102</span>, <span class="dv">49</span>]</span>
<span id="cb10-2"><a href="#cb10-2" aria-hidden="true" tabindex="-1"></a><span class="bu">print</span>(<span class="st">"Before:"</span>, heartrates)</span>
<span id="cb10-3"><a href="#cb10-3" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb10-4"><a href="#cb10-4" aria-hidden="true" tabindex="-1"></a><span class="cf">for</span> index, value <span class="kw">in</span> <span class="bu">enumerate</span>(heartrates):</span>
<span id="cb10-5"><a href="#cb10-5" aria-hidden="true" tabindex="-1"></a> <span class="bu">print</span>(<span class="st">"Index:"</span>, index, <span class="st">" value:"</span>, value)</span>
<span id="cb10-6"><a href="#cb10-6" aria-hidden="true" tabindex="-1"></a> heartrates[index] <span class="op">=</span> math.log(value)</span>
<span id="cb10-7"><a href="#cb10-7" aria-hidden="true" tabindex="-1"></a> <span class="co">#heartrates[index] = math.log(heartrates[index]) #this is okay also.</span></span>
<span id="cb10-8"><a href="#cb10-8" aria-hidden="true" tabindex="-1"></a> </span>
<span id="cb10-9"><a href="#cb10-9" aria-hidden="true" tabindex="-1"></a><span class="bu">print</span>(<span class="st">"After:"</span>, heartrates)</span></code><button title="Copy to Clipboard" class="code-copy-button"><i class="bi"></i></button></pre></div>
<div class="cell-output cell-output-stdout">
<pre><code>Before: [68, 54, 72, 66, 90, 102, 49]
Index: 0 value: 68
Index: 1 value: 54
Index: 2 value: 72
Index: 3 value: 66
Index: 4 value: 90
Index: 5 value: 102
Index: 6 value: 49
After: [4.219507705176107, 3.9889840465642745, 4.276666119016055, 4.189654742026425, 4.499809670330265, 4.624972813284271, 3.8918202981106265]</code></pre>
</div>
</div>
<p>What’s going on here? The <code>enumerate()</code> function returns something that resembles a list of tuples for us to iterate through, where the first element of the tuple is the iteration index, and the second element of the tuple is the iteration element. We access this tuple through the short-hand <code>index, m</code> at the start of the For-Loop. Let’s see what <code>enumerate()</code> looks like:</p>
<div id="3051041a" class="cell" data-execution_count="6">
<div class="sourceCode cell-code" id="cb12"><pre class="sourceCode python code-with-copy"><code class="sourceCode python"><span id="cb12-1"><a href="#cb12-1" aria-hidden="true" tabindex="-1"></a><span class="bu">print</span>(<span class="bu">list</span>(<span class="bu">enumerate</span>(heartrates)))</span></code><button title="Copy to Clipboard" class="code-copy-button"><i class="bi"></i></button></pre></div>
<div class="cell-output cell-output-stdout">
<pre><code>[(0, 4.219507705176107), (1, 3.9889840465642745), (2, 4.276666119016055), (3, 4.189654742026425), (4, 4.499809670330265), (5, 4.624972813284271), (6, 3.8918202981106265)]</code></pre>
</div>
</div>
<p>Let’s see this example step by step:</p>
<iframe width="800" height="500" frameborder="0" src="https://pythontutor.com/iframe-embed.html#code=import%20math%0Aheartrates%20%3D%20%5B68,%2054,%2072,%2066,%2090,%20102,%2049%5D%0Aprint%28%22Before%3A%22,%20heartrates%29%0A%0Afor%20index,%20m%20in%20enumerate%28heartrates%29%3A%0A%20%20print%28%22Index%3A%22,%20index,%20%22%20%20%20m%3A%22,%20m%29%0A%20%20heartrates%5Bindex%5D%20%3D%20math.log%28m%29%0A%20%20%23heartrates%5Bindex%5D%20%3D%20math.log%28heartrates%5Bindex%5D%29%20%23this%20is%20okay%20also.%0A%20%20%0Aprint%28%22After%3A%22,%20heartrates%29&codeDivHeight=400&codeDivWidth=350&cumulative=false&curInstr=0&heapPrimitives=nevernest&origin=opt-frontend.js&py=311&rawInputLstJSON=%5B%5D&textReferences=false">
</iframe>
<p>If it doesn’t load properly, here is the <a href="https://pythontutor.com/render.html#code=import%20math%0Aheartrates%20%3D%20%5B68,%2054,%2072,%2066,%2090,%20102,%2049%5D%0Aprint%28%22Before%3A%22,%20heartrates%29%0A%0Afor%20index,%20m%20in%20enumerate%28heartrates%29%3A%0A%20%20print%28%22Index%3A%22,%20index,%20%22%20%20%20m%3A%22,%20m%29%0A%20%20heartrates%5Bindex%5D%20%3D%20math.log%28m%29%0A%20%20%23heartrates%5Bindex%5D%20%3D%20math.log%28heartrates%5Bindex%5D%29%20%23this%20is%20okay%20also.%0A%20%20%0Aprint%28%22After%3A%22,%20heartrates%29&cumulative=false&curInstr=0&heapPrimitives=nevernest&mode=display&origin=opt-frontend.js&py=311&rawInputLstJSON=%5B%5D&textReferences=false">link</a>.</p>
</section>
</section>
<section id="conditional-statements" class="level2">
<h2 class="anchored" data-anchor-id="conditional-statements">Conditional Statements</h2>
<p>As we iterate through values in a data structure, it is quite common to want to have your code do different things depending on the value. For instance, suppose you are recoding <code>heartrates</code>, and the numerical values should be “low” if it is between 0 and 60, “medium” if it is between 60 and 100, “high” if it is above 100, and “unknown” otherwise (when it is below 0 or other data type).</p>
<p>Stepping back, we have been working with a <em>linear</em> way of executing code - we have unconditionally executing every line of code in our program. Here, we are create a “control flow” via <strong>conditional statements</strong> in which the your code will run a certain section <em>if</em> some conditions are met.</p>
<p>Here is how the syntax looks like for conditional statements:</p>
<pre><code>if <expression1>:
block of code 1
elif <expression2>:
block of code 2
else:
block of code 3
block of code 4 </code></pre>
<p>There are three possible ways the code can run:</p>
<ol type="1">
<li>If <code><expression1></code> is evaluated as <code>True</code>, then <code>block of code 1</code> will be run. When done, it will continue to <code>block of code 4</code>.</li>
<li>If <code><expression1></code> is evaluated as <code>False</code>, then it will ask if <code><expression2></code> is <code>True</code> or not. If <code>True</code>, then <code>block of code 2</code> will be run. When done, it will continue to <code>block of code 4</code>.</li>
<li>If <code><expression1></code> and <code><expression2></code> are both evaluated as <code>False</code>, then <code>block of code 3</code> is run. When done, it will continue to <code>block of code 4</code>.</li>
</ol>
<p>An important takeaway is that <em>only one block of code can be run</em>.</p>
<p>Let’s see how this apply to the data recoding example. Let’s just assume the data we want to recode is just a single value in a variable <code>rate</code>, not the entire list <code>heartrates</code>:</p>
<div id="4a58cb43" class="cell" data-execution_count="7">
<div class="sourceCode cell-code" id="cb15"><pre class="sourceCode python code-with-copy"><code class="sourceCode python"><span id="cb15-1"><a href="#cb15-1" aria-hidden="true" tabindex="-1"></a>rate <span class="op">=</span> heartrates[<span class="dv">0</span>]</span>
<span id="cb15-2"><a href="#cb15-2" aria-hidden="true" tabindex="-1"></a><span class="bu">print</span>(rate)</span>
<span id="cb15-3"><a href="#cb15-3" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb15-4"><a href="#cb15-4" aria-hidden="true" tabindex="-1"></a><span class="cf">if</span> rate <span class="op">></span> <span class="dv">0</span> <span class="kw">and</span> rate <span class="op"><=</span> <span class="dv">60</span>:</span>
<span id="cb15-5"><a href="#cb15-5" aria-hidden="true" tabindex="-1"></a> rate <span class="op">=</span> <span class="st">"low"</span></span>
<span id="cb15-6"><a href="#cb15-6" aria-hidden="true" tabindex="-1"></a><span class="cf">elif</span> rate <span class="op">></span> <span class="dv">60</span> <span class="kw">and</span> rate <span class="op"><=</span> <span class="dv">100</span>:</span>
<span id="cb15-7"><a href="#cb15-7" aria-hidden="true" tabindex="-1"></a> rate <span class="op">=</span> <span class="st">"medium"</span></span>
<span id="cb15-8"><a href="#cb15-8" aria-hidden="true" tabindex="-1"></a><span class="cf">elif</span> rate <span class="op">></span> <span class="dv">100</span>:</span>
<span id="cb15-9"><a href="#cb15-9" aria-hidden="true" tabindex="-1"></a> rate <span class="op">=</span> <span class="st">"high"</span></span>
<span id="cb15-10"><a href="#cb15-10" aria-hidden="true" tabindex="-1"></a><span class="cf">else</span>:</span>
<span id="cb15-11"><a href="#cb15-11" aria-hidden="true" tabindex="-1"></a> rate <span class="op">=</span> <span class="st">"unknown"</span></span>
<span id="cb15-12"><a href="#cb15-12" aria-hidden="true" tabindex="-1"></a> </span>
<span id="cb15-13"><a href="#cb15-13" aria-hidden="true" tabindex="-1"></a><span class="bu">print</span>(rate)</span></code><button title="Copy to Clipboard" class="code-copy-button"><i class="bi"></i></button></pre></div>
<div class="cell-output cell-output-stdout">
<pre><code>4.219507705176107
low</code></pre>
</div>
</div>
<p>You don’t always need multiple <code>if</code>, <code>elif</code>, <code>else</code> statements when writing conditional statements. In its simplest form, a conditional statement requires only an <code>if</code> clause:</p>
<div id="30f5907d" class="cell" data-execution_count="8">
<div class="sourceCode cell-code" id="cb17"><pre class="sourceCode python code-with-copy"><code class="sourceCode python"><span id="cb17-1"><a href="#cb17-1" aria-hidden="true" tabindex="-1"></a>x <span class="op">=</span> <span class="op">-</span><span class="dv">12</span></span>
<span id="cb17-2"><a href="#cb17-2" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb17-3"><a href="#cb17-3" aria-hidden="true" tabindex="-1"></a><span class="cf">if</span> x <span class="op"><</span> <span class="dv">0</span>:</span>
<span id="cb17-4"><a href="#cb17-4" aria-hidden="true" tabindex="-1"></a> x <span class="op">=</span> x <span class="op">*</span> <span class="op">-</span><span class="dv">1</span></span>
<span id="cb17-5"><a href="#cb17-5" aria-hidden="true" tabindex="-1"></a> </span>
<span id="cb17-6"><a href="#cb17-6" aria-hidden="true" tabindex="-1"></a><span class="bu">print</span>(x)</span></code><button title="Copy to Clipboard" class="code-copy-button"><i class="bi"></i></button></pre></div>
<div class="cell-output cell-output-stdout">
<pre><code>12</code></pre>
</div>
</div>
<p>Then, you can add an <code>elif</code> or <code>else</code> statement, if you like. Here’s <code>if</code>-<code>elif</code>:</p>
<div id="5aba473c" class="cell" data-execution_count="9">
<div class="sourceCode cell-code" id="cb19"><pre class="sourceCode python code-with-copy"><code class="sourceCode python"><span id="cb19-1"><a href="#cb19-1" aria-hidden="true" tabindex="-1"></a>x <span class="op">=</span> <span class="fl">.25</span></span>
<span id="cb19-2"><a href="#cb19-2" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb19-3"><a href="#cb19-3" aria-hidden="true" tabindex="-1"></a><span class="cf">if</span> x <span class="op"><</span> <span class="dv">0</span>:</span>
<span id="cb19-4"><a href="#cb19-4" aria-hidden="true" tabindex="-1"></a> x <span class="op">=</span> x <span class="op">*</span> <span class="op">-</span><span class="dv">1</span></span>
<span id="cb19-5"><a href="#cb19-5" aria-hidden="true" tabindex="-1"></a><span class="cf">elif</span> x <span class="op">>=</span> <span class="dv">0</span> <span class="kw">and</span> x <span class="op"><</span> <span class="dv">1</span>:</span>
<span id="cb19-6"><a href="#cb19-6" aria-hidden="true" tabindex="-1"></a> x <span class="op">=</span> <span class="dv">1</span> <span class="op">/</span> x</span>
<span id="cb19-7"><a href="#cb19-7" aria-hidden="true" tabindex="-1"></a> </span>
<span id="cb19-8"><a href="#cb19-8" aria-hidden="true" tabindex="-1"></a><span class="bu">print</span>(x)</span></code><button title="Copy to Clipboard" class="code-copy-button"><i class="bi"></i></button></pre></div>
<div class="cell-output cell-output-stdout">
<pre><code>4.0</code></pre>
</div>
</div>
<p>Here’s <code>if</code>-<code>else</code>. The <code>in</code> statement asks whether an element (102) is found in an iterable data structure <code>heartrates</code>, and returns <code>True</code> if so:</p>
<div id="96e27b73" class="cell" data-execution_count="10">
<div class="sourceCode cell-code" id="cb21"><pre class="sourceCode python code-with-copy"><code class="sourceCode python"><span id="cb21-1"><a href="#cb21-1" aria-hidden="true" tabindex="-1"></a><span class="cf">if</span> <span class="dv">102</span> <span class="kw">in</span> heartrates:</span>
<span id="cb21-2"><a href="#cb21-2" aria-hidden="true" tabindex="-1"></a> <span class="bu">print</span>(<span class="st">"Found 102."</span>)</span>
<span id="cb21-3"><a href="#cb21-3" aria-hidden="true" tabindex="-1"></a><span class="cf">else</span>:</span>
<span id="cb21-4"><a href="#cb21-4" aria-hidden="true" tabindex="-1"></a> <span class="bu">print</span>(<span class="st">"Did not find 102."</span>)</span></code><button title="Copy to Clipboard" class="code-copy-button"><i class="bi"></i></button></pre></div>
<div class="cell-output cell-output-stdout">
<pre><code>Did not find 102.</code></pre>
</div>
</div>
<p>Finally, let’s put the data recoding example within a For-Loop:</p>
<div id="4396a9dc" class="cell" data-execution_count="11">
<div class="sourceCode cell-code" id="cb23"><pre class="sourceCode python code-with-copy"><code class="sourceCode python"><span id="cb23-1"><a href="#cb23-1" aria-hidden="true" tabindex="-1"></a>heartrates <span class="op">=</span> [<span class="dv">68</span>, <span class="dv">54</span>, <span class="dv">72</span>, <span class="dv">66</span>, <span class="dv">90</span>, <span class="dv">102</span>]</span>
<span id="cb23-2"><a href="#cb23-2" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb23-3"><a href="#cb23-3" aria-hidden="true" tabindex="-1"></a><span class="cf">for</span> index, rate <span class="kw">in</span> <span class="bu">enumerate</span>(heartrates):</span>
<span id="cb23-4"><a href="#cb23-4" aria-hidden="true" tabindex="-1"></a> <span class="cf">if</span> rate <span class="op">></span> <span class="dv">0</span> <span class="kw">and</span> rate <span class="op"><=</span> <span class="dv">60</span>:</span>
<span id="cb23-5"><a href="#cb23-5" aria-hidden="true" tabindex="-1"></a> heartrates[index] <span class="op">=</span> <span class="st">"low"</span></span>
<span id="cb23-6"><a href="#cb23-6" aria-hidden="true" tabindex="-1"></a> <span class="cf">elif</span> rate <span class="op">></span> <span class="dv">60</span> <span class="kw">and</span> rate <span class="op"><=</span> <span class="dv">100</span>:</span>
<span id="cb23-7"><a href="#cb23-7" aria-hidden="true" tabindex="-1"></a> heartrates[index] <span class="op">=</span> <span class="st">"medium"</span></span>
<span id="cb23-8"><a href="#cb23-8" aria-hidden="true" tabindex="-1"></a> <span class="cf">elif</span> rate <span class="op">></span> <span class="dv">100</span>:</span>
<span id="cb23-9"><a href="#cb23-9" aria-hidden="true" tabindex="-1"></a> heartrates[index] <span class="op">=</span> <span class="st">"high"</span></span>
<span id="cb23-10"><a href="#cb23-10" aria-hidden="true" tabindex="-1"></a> <span class="cf">else</span>:</span>
<span id="cb23-11"><a href="#cb23-11" aria-hidden="true" tabindex="-1"></a> heartrates[index] <span class="op">=</span> <span class="st">"unknown"</span></span>
<span id="cb23-12"><a href="#cb23-12" aria-hidden="true" tabindex="-1"></a> </span>
<span id="cb23-13"><a href="#cb23-13" aria-hidden="true" tabindex="-1"></a><span class="bu">print</span>(heartrates)</span></code><button title="Copy to Clipboard" class="code-copy-button"><i class="bi"></i></button></pre></div>
<div class="cell-output cell-output-stdout">
<pre><code>['medium', 'low', 'medium', 'medium', 'medium', 'high']</code></pre>
</div>
</div>
<p>Let’s see this in action step by step:</p>
<iframe width="800" height="500" frameborder="0" src="https://pythontutor.com/iframe-embed.html#code=heartrates%20%3D%20%5B68,%2054,%2072,%2066,%2090,%20102%5D%0A%0Afor%20index,%20rate%20in%20enumerate%28heartrates%29%3A%0A%20%20if%20rate%20%3E%200%20and%20rate%20%3C%3D%2060%3A%0A%20%20%20%20heartrates%5Bindex%5D%20%3D%20%22low%22%0A%20%20elif%20rate%20%3E%2060%20and%20rate%20%3C%3D%20100%3A%0A%20%20%20%20heartrates%5Bindex%5D%20%3D%20%22medium%22%0A%20%20elif%20rate%20%3E%20100%3A%0A%20%20%20%20heartrates%5Bindex%5D%20%3D%20%22high%22%0A%20%20else%3A%0A%20%20%20%20heartrates%5Bindex%5D%20%3D%20%22unknown%22%0A%20%20%20%20%0Aprint%28heartrates%29&codeDivHeight=400&codeDivWidth=350&cumulative=false&curInstr=0&heapPrimitives=nevernest&origin=opt-frontend.js&py=311&rawInputLstJSON=%5B%5D&textReferences=false">
</iframe>
<p>If it doesn’t load properly, you can find it <a href="https://pythontutor.com/render.html#code=heartrates%20%3D%20%5B68,%2054,%2072,%2066,%2090,%20102%5D%0A%0Afor%20index,%20rate%20in%20enumerate%28heartrates%29%3A%0A%20%20if%20rate%20%3E%200%20and%20rate%20%3C%3D%2060%3A%0A%20%20%20%20heartrates%5Bindex%5D%20%3D%20%22low%22%0A%20%20elif%20rate%20%3E%2060%20and%20rate%20%3C%3D%20100%3A%0A%20%20%20%20heartrates%5Bindex%5D%20%3D%20%22medium%22%0A%20%20elif%20rate%20%3E%20100%3A%0A%20%20%20%20heartrates%5Bindex%5D%20%3D%20%22high%22%0A%20%20else%3A%0A%20%20%20%20heartrates%5Bindex%5D%20%3D%20%22unknown%22%0A%20%20%20%20%0Aprint%28heartrates%29&cumulative=false&curInstr=0&heapPrimitives=nevernest&mode=display&origin=opt-frontend.js&py=311&rawInputLstJSON=%5B%5D&textReferences=false">here</a></p>
</section>
<section id="exercises" class="level2">
<h2 class="anchored" data-anchor-id="exercises">Exercises</h2>
<ul>
<li><p>Create iteration to search for an element</p></li>
<li><p>Create iteration for populating a dictionary via <code>zip()</code></p></li>
<li><p>Create iteration for loading in multiple Dataframes</p></li>
<li><p>Create iteration to find whether a word is in the sentiment dictionary and store the score in a growing list</p></li>
</ul>
</section>
</main>
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const note = window.document.getElementById(id);
if (note) {
return note.innerHTML;
} else {
return "";
}
});
}
const xrefs = window.document.querySelectorAll('a.quarto-xref');
const processXRef = (id, note) => {
// Strip column container classes
const stripColumnClz = (el) => {
el.classList.remove("page-full", "page-columns");
if (el.children) {
for (const child of el.children) {
stripColumnClz(child);
}
}
}
stripColumnClz(note)
if (id === null || id.startsWith('sec-')) {
// Special case sections, only their first couple elements
const container = document.createElement("div");
if (note.children && note.children.length > 2) {
container.appendChild(note.children[0].cloneNode(true));
for (let i = 1; i < note.children.length; i++) {
const child = note.children[i];
if (child.tagName === "P" && child.innerText === "") {
continue;
} else {
container.appendChild(child.cloneNode(true));
break;
}
}
if (window.Quarto?.typesetMath) {
window.Quarto.typesetMath(container);
}
return container.innerHTML
} else {
if (window.Quarto?.typesetMath) {
window.Quarto.typesetMath(note);
}
return note.innerHTML;
}
} else {
// Remove any anchor links if they are present
const anchorLink = note.querySelector('a.anchorjs-link');
if (anchorLink) {
anchorLink.remove();
}
if (window.Quarto?.typesetMath) {
window.Quarto.typesetMath(note);
}
// TODO in 1.5, we should make sure this works without a callout special case
if (note.classList.contains("callout")) {
return note.outerHTML;
} else {
return note.innerHTML;
}
}
}
for (var i=0; i<xrefs.length; i++) {
const xref = xrefs[i];
tippyHover(xref, undefined, function(instance) {
instance.disable();
let url = xref.getAttribute('href');
let hash = undefined;
if (url.startsWith('#')) {
hash = url;
} else {
try { hash = new URL(url).hash; } catch {}
}
if (hash) {
const id = hash.replace(/^#\/?/, "");
const note = window.document.getElementById(id);
if (note !== null) {
try {
const html = processXRef(id, note.cloneNode(true));
instance.setContent(html);
} finally {
instance.enable();
instance.show();
}
} else {
// See if we can fetch this
fetch(url.split('#')[0])
.then(res => res.text())
.then(html => {
const parser = new DOMParser();
const htmlDoc = parser.parseFromString(html, "text/html");
const note = htmlDoc.getElementById(id);
if (note !== null) {
const html = processXRef(id, note);
instance.setContent(html);
}
}).finally(() => {
instance.enable();
instance.show();
});
}
} else {
// See if we can fetch a full url (with no hash to target)
// This is a special case and we should probably do some content thinning / targeting
fetch(url)
.then(res => res.text())
.then(html => {
const parser = new DOMParser();
const htmlDoc = parser.parseFromString(html, "text/html");
const note = htmlDoc.querySelector('main.content');
if (note !== null) {
// This should only happen for chapter cross references
// (since there is no id in the URL)
// remove the first header
if (note.children.length > 0 && note.children[0].tagName === "HEADER") {
note.children[0].remove();
}
const html = processXRef(null, note);
instance.setContent(html);
}
}).finally(() => {
instance.enable();
instance.show();
});
}
}, function(instance) {
});
}
let selectedAnnoteEl;
const selectorForAnnotation = ( cell, annotation) => {
let cellAttr = 'data-code-cell="' + cell + '"';
let lineAttr = 'data-code-annotation="' + annotation + '"';
const selector = 'span[' + cellAttr + '][' + lineAttr + ']';
return selector;
}
const selectCodeLines = (annoteEl) => {
const doc = window.document;
const targetCell = annoteEl.getAttribute("data-target-cell");
const targetAnnotation = annoteEl.getAttribute("data-target-annotation");
const annoteSpan = window.document.querySelector(selectorForAnnotation(targetCell, targetAnnotation));
const lines = annoteSpan.getAttribute("data-code-lines").split(",");
const lineIds = lines.map((line) => {
return targetCell + "-" + line;
})
let top = null;
let height = null;
let parent = null;
if (lineIds.length > 0) {
//compute the position of the single el (top and bottom and make a div)
const el = window.document.getElementById(lineIds[0]);
top = el.offsetTop;
height = el.offsetHeight;
parent = el.parentElement.parentElement;
if (lineIds.length > 1) {
const lastEl = window.document.getElementById(lineIds[lineIds.length - 1]);
const bottom = lastEl.offsetTop + lastEl.offsetHeight;
height = bottom - top;
}
if (top !== null && height !== null && parent !== null) {
// cook up a div (if necessary) and position it
let div = window.document.getElementById("code-annotation-line-highlight");
if (div === null) {
div = window.document.createElement("div");
div.setAttribute("id", "code-annotation-line-highlight");
div.style.position = 'absolute';
parent.appendChild(div);
}
div.style.top = top - 2 + "px";
div.style.height = height + 4 + "px";
div.style.left = 0;
let gutterDiv = window.document.getElementById("code-annotation-line-highlight-gutter");
if (gutterDiv === null) {
gutterDiv = window.document.createElement("div");
gutterDiv.setAttribute("id", "code-annotation-line-highlight-gutter");
gutterDiv.style.position = 'absolute';
const codeCell = window.document.getElementById(targetCell);
const gutter = codeCell.querySelector('.code-annotation-gutter');
gutter.appendChild(gutterDiv);
}
gutterDiv.style.top = top - 2 + "px";
gutterDiv.style.height = height + 4 + "px";
}
selectedAnnoteEl = annoteEl;
}
};
const unselectCodeLines = () => {
const elementsIds = ["code-annotation-line-highlight", "code-annotation-line-highlight-gutter"];
elementsIds.forEach((elId) => {
const div = window.document.getElementById(elId);
if (div) {
div.remove();
}
});
selectedAnnoteEl = undefined;
};
// Handle positioning of the toggle
window.addEventListener(
"resize",
throttle(() => {
elRect = undefined;
if (selectedAnnoteEl) {
selectCodeLines(selectedAnnoteEl);
}
}, 10)
);
function throttle(fn, ms) {
let throttle = false;
let timer;
return (...args) => {
if(!throttle) { // first call gets through
fn.apply(this, args);
throttle = true;
} else { // all the others get throttled
if(timer) clearTimeout(timer); // cancel #2
timer = setTimeout(() => {
fn.apply(this, args);
timer = throttle = false;
}, ms);
}
};
}
// Attach click handler to the DT
const annoteDls = window.document.querySelectorAll('dt[data-target-cell]');
for (const annoteDlNode of annoteDls) {
annoteDlNode.addEventListener('click', (event) => {
const clickedEl = event.target;
if (clickedEl !== selectedAnnoteEl) {
unselectCodeLines();
const activeEl = window.document.querySelector('dt[data-target-cell].code-annotation-active');
if (activeEl) {
activeEl.classList.remove('code-annotation-active');
}
selectCodeLines(clickedEl);
clickedEl.classList.add('code-annotation-active');
} else {
// Unselect the line
unselectCodeLines();
clickedEl.classList.remove('code-annotation-active');
}
});
}
const findCites = (el) => {
const parentEl = el.parentElement;
if (parentEl) {
const cites = parentEl.dataset.cites;
if (cites) {
return {
el,
cites: cites.split(' ')
};
} else {
return findCites(el.parentElement)
}
} else {
return undefined;
}
};
var bibliorefs = window.document.querySelectorAll('a[role="doc-biblioref"]');
for (var i=0; i<bibliorefs.length; i++) {
const ref = bibliorefs[i];
const citeInfo = findCites(ref);
if (citeInfo) {
tippyHover(citeInfo.el, function() {
var popup = window.document.createElement('div');
citeInfo.cites.forEach(function(cite) {
var citeDiv = window.document.createElement('div');
citeDiv.classList.add('hanging-indent');
citeDiv.classList.add('csl-entry');
var biblioDiv = window.document.getElementById('ref-' + cite);
if (biblioDiv) {
citeDiv.innerHTML = biblioDiv.innerHTML;
}
popup.appendChild(citeDiv);
});
return popup.innerHTML;
});
}
}
});
</script>
</div> <!-- /content -->
</body></html>