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<div class="section" id="strings">
<h1>Strings<a class="headerlink" href="#strings" title="Permalink to this headline">¶</a></h1>
<p>Strings are a simple but powerful tool for storing non-numerical information.</p>
<p>String variables store textual data. To tell Python that we want to treat some text as a string, we use either single quotations <code class="docutils literal notranslate"><span class="pre">'</span></code> or double quotations <code class="docutils literal notranslate"><span class="pre">"</span></code>. Either format is a valid approach.</p>
<div class="cell docutils container">
<div class="cell_input docutils container">
<div class="highlight-ipython3 notranslate"><div class="highlight"><pre><span></span><span class="n">fruit</span> <span class="o">=</span> <span class="s1">'banana'</span>
</pre></div>
</div>
</div>
</div>
<div class="cell docutils container">
<div class="cell_input docutils container">
<div class="highlight-ipython3 notranslate"><div class="highlight"><pre><span></span><span class="n">vegetable</span> <span class="o">=</span> <span class="s2">"carrot"</span>
</pre></div>
</div>
</div>
</div>
<p>Use of single or double quotes can make nested quotations easier. For instance, what if you want to use an apostrophe inside a string variable? Consider the apostrophe in:</p>
<blockquote>
<div><p>This isn’t an interesting sentence.</p>
</div></blockquote>
<p>In that case, be sure to use double quotes to mark your string.</p>
<div class="cell docutils container">
<div class="cell_input docutils container">
<div class="highlight-ipython3 notranslate"><div class="highlight"><pre><span></span><span class="n">sent</span> <span class="o">=</span> <span class="s2">"This isn't an interesting sentence."</span> <span class="c1"># this code will work</span>
</pre></div>
</div>
</div>
</div>
<div class="cell docutils container">
<div class="cell_input docutils container">
<div class="highlight-ipython3 notranslate"><div class="highlight"><pre><span></span><span class="n">sent</span> <span class="o">=</span> <span class="s1">'This isn'</span><span class="n">t</span> <span class="n">an</span> <span class="n">interesting</span> <span class="n">sentence</span><span class="o">.</span> <span class="c1"># this has problems</span>
</pre></div>
</div>
</div>
<div class="cell_output docutils container">
<div class="output traceback highlight-ipythontb notranslate"><div class="highlight"><pre><span></span><span class="gt"> File</span><span class="nn"> "<ipython-input-4-fcbf08c4a876>"</span><span class="gt">, line </span><span class="mi">1</span>
<span class="n">sent</span> <span class="o">=</span> <span class="s1">'This isn'</span><span class="n">t</span> <span class="n">an</span> <span class="n">interesting</span> <span class="n">sentence</span><span class="o">.</span> <span class="c1"># this has problems</span>
<span class="o">^</span>
<span class="ne">SyntaxError</span>: invalid syntax
</pre></div>
</div>
</div>
</div>
<p>Because Python won’t be able to differentiate between the <code class="docutils literal notranslate"><span class="pre">'</span></code> character being used as an apostrophe and the <code class="docutils literal notranslate"><span class="pre">'</span></code> character being used to signify a single quote, the easiest way to enter the above example into Python is to use double quotes for the string.</p>
<p>Similarly, use single quotes to mark the string if you want to include double quotes inside the string. Consider the example:</p>
<blockquote>
<div><p>Bob said, “Yes, that is a boring sentence.”</p>
</div></blockquote>
<p>In this case, the easiest way to do things is to enter this as a string in Python is to use single quotes.</p>
<div class="cell docutils container">
<div class="cell_input docutils container">
<div class="highlight-ipython3 notranslate"><div class="highlight"><pre><span></span><span class="n">quote</span> <span class="o">=</span> <span class="s1">'Bob said, "Yes, that is a boring sentence."'</span>
</pre></div>
</div>
</div>
</div>
<div class="section" id="escape-characters">
<h2>Escape Characters<a class="headerlink" href="#escape-characters" title="Permalink to this headline">¶</a></h2>
<p>The trickiest problem with quotations is something like the following example:</p>
<blockquote>
<div><p>Bob said, “Yes, that’s a boring sentence.”</p>
</div></blockquote>
<p>Note that the textual data we want to include here has both double quotes and a single quote inside the data. To enter this text into Python as a string variable, we need to use an <strong>escape character</strong>.</p>
<p>Escape characters are special commands inside of a string variable that tell Python that the data is something special. Without an escape character inside a string, Python interprets every character literally. For instance, the literal interpretation of <code class="docutils literal notranslate"><span class="pre">n</span></code> is that it represents the letter “n”. The literal interpretation of <code class="docutils literal notranslate"><span class="pre">'</span></code> is that it represents a single quote. We can “escape” from this literal interpretation of a character by prefacing that character with <code class="docutils literal notranslate"><span class="pre">\</span></code>. The <code class="docutils literal notranslate"><span class="pre">\</span></code> symbol tells Python to treat the information following the <code class="docutils literal notranslate"><span class="pre">\</span></code> specially. In the example here, <code class="docutils literal notranslate"><span class="pre">\'</span></code> will tell Python that the <code class="docutils literal notranslate"><span class="pre">'</span></code> character is to be treated as part of the data, rather than a potential end for a single quoted string.</p>
<div class="cell docutils container">
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<div class="highlight-ipython3 notranslate"><div class="highlight"><pre><span></span><span class="n">quote</span> <span class="o">=</span> <span class="s1">'Bob said, "Yes, that</span><span class="se">\'</span><span class="s1">s a boring sentence."'</span>
<span class="nb">print</span><span class="p">(</span><span class="n">quote</span><span class="p">)</span>
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<div class="output stream highlight-myst-ansi notranslate"><div class="highlight"><pre><span></span>Bob said, "Yes, that's a boring sentence."
</pre></div>
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</div>
</div>
<p>There are only a handful of escape characters to remember. The rest can be Googled if you need them. The escape characters worth remembering are:</p>
<ul class="simple">
<li><p><code class="docutils literal notranslate"><span class="pre">\'</span></code>: tells Python to treat <code class="docutils literal notranslate"><span class="pre">'</span></code> as a single quote (apostrophe) inside the string data</p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">\"</span></code>: tells Python to treat <code class="docutils literal notranslate"><span class="pre">"</span></code> as a double quote inside the string data</p></li>
<li><p>‘\n’ : tells Python to enter a line break.</p></li>
<li><p>‘\t’ : tells Python to enter a tab.</p></li>
</ul>
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<div class="highlight-ipython3 notranslate"><div class="highlight"><pre><span></span><span class="nb">print</span><span class="p">(</span><span class="s1">'Line one of text.</span><span class="se">\n</span><span class="s1">Line two of text'</span><span class="p">)</span>
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<div class="output stream highlight-myst-ansi notranslate"><div class="highlight"><pre><span></span>Line one of text.
Line two of text
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<div class="highlight-ipython3 notranslate"><div class="highlight"><pre><span></span><span class="nb">print</span><span class="p">(</span><span class="s1">'</span><span class="se">\t</span><span class="s1">This is an indented sentenece.'</span><span class="p">)</span>
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<div class="output stream highlight-myst-ansi notranslate"><div class="highlight"><pre><span></span> This is an indented sentenece.
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<p><span style="color:red"><strong>Concept check</strong>:</span> Set the variable, <code class="docutils literal notranslate"><span class="pre">v</span></code>, below equal to a string such that the second line evaluates to (aka prints out) the following:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">That</span> <span class="n">isn</span><span class="s1">'t a smart investment.</span>
<span class="n">You</span> <span class="n">should</span> <span class="n">but</span> <span class="n">Dogecoin</span> <span class="n">instead</span><span class="o">.</span>
<span class="p">(</span><span class="n">just</span> <span class="n">kidding</span><span class="p">)</span>
</pre></div>
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<div class="highlight-ipython3 notranslate"><div class="highlight"><pre><span></span><span class="n">v</span> <span class="o">=</span>
<span class="nb">print</span><span class="p">(</span><span class="n">v</span><span class="p">)</span>
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</div>
<div class="section" id="characters">
<h2>Characters<a class="headerlink" href="#characters" title="Permalink to this headline">¶</a></h2>
<p>A string is a collection of characters. The string <code class="docutils literal notranslate"><span class="pre">'cat'</span></code> is comprised of the individual characters <code class="docutils literal notranslate"><span class="pre">'c'</span></code>, <code class="docutils literal notranslate"><span class="pre">'a'</span></code>, and <code class="docutils literal notranslate"><span class="pre">'t'</span></code>. The string <code class="docutils literal notranslate"><span class="pre">'my\ncat'</span></code> is made up of the characters <code class="docutils literal notranslate"><span class="pre">'m'</span></code>, <code class="docutils literal notranslate"><span class="pre">'y'</span></code>, <code class="docutils literal notranslate"><span class="pre">'\n'</span></code>, <code class="docutils literal notranslate"><span class="pre">'c'</span></code>, <code class="docutils literal notranslate"><span class="pre">'a'</span></code>, <code class="docutils literal notranslate"><span class="pre">'t'</span></code>. Note that the third character in this latter example is an escape character <code class="docutils literal notranslate"><span class="pre">'\n</span></code>’ rather than an alphanumeric character (the letters <code class="docutils literal notranslate"><span class="pre">'a'</span></code>-<code class="docutils literal notranslate"><span class="pre">'z'</span></code> or numbers <code class="docutils literal notranslate"><span class="pre">'0'</span></code>-<code class="docutils literal notranslate"><span class="pre">'9'</span></code>) or a symbol (e.g. <code class="docutils literal notranslate"><span class="pre">'!'</span></code>, <code class="docutils literal notranslate"><span class="pre">'@'</span></code>, or <code class="docutils literal notranslate"><span class="pre">'#'</span></code>).</p>
<p>Becuase strings are comprised of characters, it is helpful to know how to access individual characters of those strings. We will work with the examples <code class="docutils literal notranslate"><span class="pre">'AAPL</span> <span class="pre">Stock'</span></code> and <code class="docutils literal notranslate"><span class="pre">'F</span> <span class="pre">Stock'</span></code> for a moment. These two strings are items that we may come accross in a written document. Suppose that we need to extract the ticker names from these strings. For instance, if these strings are items that we’ve extracted from a news article or a tweet, then finding the ticker names helps us determine what the subject of the article/tweet is.</p>
<p>The first character of a string is the <span class="math notranslate nohighlight">\(0^{th}\)</span> character in the string. This is a quirk of Python, along with a number of many other programming languages. Python begins counting at zero, rather than at one. Hence, the first character is character <span class="math notranslate nohighlight">\(0\)</span>, the second character is character number <span class="math notranslate nohighlight">\(1\)</span>, etc. We can select character number <span class="math notranslate nohighlight">\(n\)</span> with square brackets that follow immediately after the string name.</p>
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<div class="highlight-ipython3 notranslate"><div class="highlight"><pre><span></span><span class="n">string</span> <span class="o">=</span> <span class="s1">'cat'</span>
<span class="nb">print</span><span class="p">(</span><span class="n">string</span><span class="p">[</span><span class="mi">0</span><span class="p">])</span>
</pre></div>
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<div class="output stream highlight-myst-ansi notranslate"><div class="highlight"><pre><span></span>c
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<div class="highlight-ipython3 notranslate"><div class="highlight"><pre><span></span><span class="nb">print</span><span class="p">(</span><span class="n">string</span><span class="p">[</span><span class="mi">1</span><span class="p">])</span>
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<div class="output stream highlight-myst-ansi notranslate"><div class="highlight"><pre><span></span>a
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<p>Along with selecting a single character with its position in the string, we can choose a subset of characters with a set of sequential numbers. Such numbers are listed with a starting point and ending point. For instance, if we want the numbers <span class="math notranslate nohighlight">\(1\)</span>, <span class="math notranslate nohighlight">\(2\)</span>, <span class="math notranslate nohighlight">\(3\)</span>, then we would indicate the starting point of <span class="math notranslate nohighlight">\(1\)</span> and the ending point of <span class="math notranslate nohighlight">\(4\)</span>. Yes, <span class="math notranslate nohighlight">\(4\)</span>, <em>not</em> <span class="math notranslate nohighlight">\(3\)</span>. The ending point is the number <em>after</em> the last number to be included.</p>
<p>Consequently, <code class="docutils literal notranslate"><span class="pre">0:2</span></code> would select characters number <span class="math notranslate nohighlight">\(0\)</span> and <span class="math notranslate nohighlight">\(1\)</span> because the starting point is <span class="math notranslate nohighlight">\(0\)</span> and the ending point is <span class="math notranslate nohighlight">\(1\)</span>.</p>
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<div class="highlight-ipython3 notranslate"><div class="highlight"><pre><span></span><span class="nb">print</span><span class="p">(</span><span class="n">string</span><span class="p">[</span><span class="mi">0</span><span class="p">:</span><span class="mi">2</span><span class="p">])</span>
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<div class="output stream highlight-myst-ansi notranslate"><div class="highlight"><pre><span></span>ca
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<p>Similarly, <code class="docutils literal notranslate"><span class="pre">1:3</span></code> selects characters number <span class="math notranslate nohighlight">\(1\)</span> and <span class="math notranslate nohighlight">\(2\)</span> because the starting point is <span class="math notranslate nohighlight">\(1\)</span> and the ending point is <span class="math notranslate nohighlight">\(3\)</span>.</p>
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<div class="highlight-ipython3 notranslate"><div class="highlight"><pre><span></span><span class="nb">print</span><span class="p">(</span><span class="n">string</span><span class="p">[</span><span class="mi">1</span><span class="p">:</span><span class="mi">3</span><span class="p">])</span>
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<div class="output stream highlight-myst-ansi notranslate"><div class="highlight"><pre><span></span>at
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