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3b_strings.html
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Financial Modeling and Analytics
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American Community Survey
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Regression
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Stock Prices
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CAPM
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Searching Strings
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<div class="section" id="more-strings">
<h1>More Strings<a class="headerlink" href="#more-strings" title="Permalink to this headline">¶</a></h1>
<div class="section" id="searching-strings">
<h2>Searching Strings<a class="headerlink" href="#searching-strings" title="Permalink to this headline">¶</a></h2>
<p>To find a particular character in a string, we can use the <code class="docutils literal notranslate"><span class="pre">.find()</span></code> function.</p>
<p>For example, <code class="docutils literal notranslate"><span class="pre">'cat'.find('a')</span></code> takes the string <code class="docutils literal notranslate"><span class="pre">'cat'</span></code> and applies the function <code class="docutils literal notranslate"><span class="pre">.find()</span></code> to it. We specify the character <code class="docutils literal notranslate"><span class="pre">'a'</span></code> inside the function (inside the parentheses) to indicate that we are searching for the character <code class="docutils literal notranslate"><span class="pre">'a'</span></code>. This command in Python will return the number <span class="math notranslate nohighlight">\(1\)</span> because the character <code class="docutils literal notranslate"><span class="pre">'a'</span></code> first appears in the string <code class="docutils literal notranslate"><span class="pre">'cat'</span></code> at position <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="s1">'cat'</span><span class="o">.</span><span class="n">find</span><span class="p">(</span><span class="s1">'a'</span><span class="p">)</span>
</pre></div>
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<div class="output text_plain highlight-myst-ansi notranslate"><div class="highlight"><pre><span></span>1
</pre></div>
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</div>
<p>Note that <code class="docutils literal notranslate"><span class="pre">.find()</span></code> will give us only the position of the first instance of a desired character. Suppose that we want to find the position of the character <code class="docutils literal notranslate"><span class="pre">'o'</span></code> in the string <code class="docutils literal notranslate"><span class="pre">'google'</span></code>. The command <code class="docutils literal notranslate"><span class="pre">'google.find('o')</span></code> will return the number <span class="math notranslate nohighlight">\(1\)</span>. It will never give us back the number <span class="math notranslate nohighlight">\(2\)</span>, even though the character <code class="docutils literal notranslate"><span class="pre">'o'</span></code> also appears in that position in the string <code class="docutils literal notranslate"><span class="pre">'google'</span></code>.</p>
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<div class="highlight-ipython3 notranslate"><div class="highlight"><pre><span></span><span class="s1">'google'</span><span class="o">.</span><span class="n">find</span><span class="p">(</span><span class="s1">'o'</span><span class="p">)</span>
</pre></div>
</div>
</div>
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<div class="output text_plain highlight-myst-ansi notranslate"><div class="highlight"><pre><span></span>1
</pre></div>
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<div class="highlight-ipython3 notranslate"><div class="highlight"><pre><span></span><span class="s1">'This is a sentence'</span><span class="o">.</span><span class="n">find</span><span class="p">(</span><span class="s1">'is'</span><span class="p">)</span>
</pre></div>
</div>
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<div class="cell_output docutils container">
<div class="output text_plain highlight-myst-ansi notranslate"><div class="highlight"><pre><span></span>2
</pre></div>
</div>
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</div>
<p>If a given character is <em>not</em> found in a string, the <code class="docutils literal notranslate"><span class="pre">.find()</span></code> function will return <code class="docutils literal notranslate"><span class="pre">-1</span></code>.</p>
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<div class="highlight-ipython3 notranslate"><div class="highlight"><pre><span></span><span class="s1">'google'</span><span class="o">.</span><span class="n">find</span><span class="p">(</span><span class="s1">'a'</span><span class="p">)</span>
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<div class="output text_plain highlight-myst-ansi notranslate"><div class="highlight"><pre><span></span>-1
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<p>Suppose that we are given a tweet and want to check whether the tweet concerns a company of interest. The tweet data will be a string, because tweets are textual data.</p>
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<div class="highlight-ipython3 notranslate"><div class="highlight"><pre><span></span><span class="n">tweet</span> <span class="o">=</span> <span class="s1">'Very excited about AAPL earnings next week.'</span>
<span class="n">company</span> <span class="o">=</span> <span class="s1">'AAPL'</span>
<span class="k">if</span> <span class="n">tweet</span><span class="o">.</span><span class="n">find</span><span class="p">(</span><span class="n">company</span><span class="p">):</span>
<span class="nb">print</span><span class="p">(</span><span class="s1">'This tweet discusses'</span><span class="p">,</span> <span class="n">company</span><span class="p">)</span>
<span class="k">else</span><span class="p">:</span>
<span class="nb">print</span><span class="p">(</span><span class="s1">'Nothing to report.'</span><span class="p">)</span>
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<div class="output stream highlight-myst-ansi notranslate"><div class="highlight"><pre><span></span>This tweet discusses AAPL
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<p><span style="color:red"><strong>Concept check</strong>:</span> Find the position of the ticker <code class="docutils literal notranslate"><span class="pre">NVDA</span></code> in the following string.</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">'Analysts expect earnings for NVDA to rise over the next four quarters.'</span>
</pre></div>
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<div class="section" id="string-concatenation">
<h2>String Concatenation<a class="headerlink" href="#string-concatenation" title="Permalink to this headline">¶</a></h2>
<p>The way to combine (also known as <em>concatenating</em>) two or more strings is to use the <code class="docutils literal notranslate"><span class="pre">+</span></code> character. Just like <code class="docutils literal notranslate"><span class="pre">+</span></code> combines integers and/or floating point numbers, <code class="docutils literal notranslate"><span class="pre">+</span></code> combines strings.</p>
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<div class="highlight-ipython3 notranslate"><div class="highlight"><pre><span></span><span class="n">word1</span> <span class="o">=</span> <span class="s1">'Apple'</span>
<span class="n">word2</span> <span class="o">=</span> <span class="s1">'Inc.'</span>
<span class="n">word1</span> <span class="o">+</span> <span class="n">word2</span>
</pre></div>
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<div class="output text_plain highlight-myst-ansi notranslate"><div class="highlight"><pre><span></span>'AppleInc.'
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<p>Notice that <code class="docutils literal notranslate"><span class="pre">+</span></code> does not join together strings with spaces between them. There are very reasonable applications in which no spacing should occur between strings, and so Python will not assume that you’d like to have spaces between strings.</p>
<p>In the above example, we can safely say that the combined string would look more reasonable with a space between the two words. To achive this, simply include a <code class="docutils literal notranslate"><span class="pre">'</span> <span class="pre">'</span></code> string as a middle string in what becomes a concatenation of three strings.</p>
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<div class="highlight-ipython3 notranslate"><div class="highlight"><pre><span></span><span class="n">word1</span> <span class="o">+</span> <span class="s1">' '</span> <span class="o">+</span> <span class="n">word2</span>
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<div class="output text_plain highlight-myst-ansi notranslate"><div class="highlight"><pre><span></span>'Apple Inc.'
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<p>Note that tricky issues arrive if we use <code class="docutils literal notranslate"><span class="pre">+</span></code> in circumstances that include both strings and numbers. Obviously, one can’t add strings and numbers together in any sort of algebraic interpretation of the word “add”. Importantly, however, is that Python won’t assume that, in the presence of both strings and numbers, a <code class="docutils literal notranslate"><span class="pre">+</span></code> character should be interpreted as concatenation. To clarify these last two sentences, let’s attempt two different lines of code.</p>
<p>First, enter a number plus a string.</p>
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<div class="cell_input docutils container">
<div class="highlight-ipython3 notranslate"><div class="highlight"><pre><span></span><span class="mi">8</span> <span class="o">+</span> <span class="s1">'1'</span>
</pre></div>
</div>
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<div class="cell_output docutils container">
<div class="output traceback highlight-ipythontb notranslate"><div class="highlight"><pre><span></span><span class="gt">---------------------------------------------------------------------------</span>
<span class="ne">TypeError</span><span class="g g-Whitespace"> </span>Traceback (most recent call last)
<span class="o"><</span><span class="n">ipython</span><span class="o">-</span><span class="nb">input</span><span class="o">-</span><span class="mi">9</span><span class="o">-</span><span class="mi">5043558</span><span class="n">bd679</span><span class="o">></span> <span class="ow">in</span> <span class="o"><</span><span class="n">module</span><span class="o">></span>
<span class="ne">----> </span><span class="mi">1</span> <span class="mi">8</span> <span class="o">+</span> <span class="s1">'1'</span>
<span class="ne">TypeError</span>: unsupported operand type(s) for +: 'int' and 'str'
</pre></div>
</div>
</div>
</div>
<p>Python prints out an error. The message, in a nutshell, informs us that we can’t use <code class="docutils literal notranslate"><span class="pre">+</span></code> in the presence of an <code class="docutils literal notranslate"><span class="pre">int</span></code> and a <code class="docutils literal notranslate"><span class="pre">str</span></code> variable type.</p>
<p>Second, try things the other way around. Enter a string plus a number.</p>
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<div class="highlight-ipython3 notranslate"><div class="highlight"><pre><span></span><span class="s1">'1'</span> <span class="o">+</span> <span class="mi">8</span>
</pre></div>
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<div class="output traceback highlight-ipythontb notranslate"><div class="highlight"><pre><span></span><span class="gt">---------------------------------------------------------------------------</span>
<span class="ne">TypeError</span><span class="g g-Whitespace"> </span>Traceback (most recent call last)
<span class="o"><</span><span class="n">ipython</span><span class="o">-</span><span class="nb">input</span><span class="o">-</span><span class="mi">25</span><span class="o">-</span><span class="mi">73173934</span><span class="n">f639</span><span class="o">></span> <span class="ow">in</span> <span class="o"><</span><span class="n">module</span><span class="o">></span>
<span class="ne">----> </span><span class="mi">1</span> <span class="s1">'1'</span> <span class="o">+</span> <span class="mi">8</span>
<span class="ne">TypeError</span>: can only concatenate str (not "int") to str
</pre></div>
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<p>Again, Python prints out an error. This time the message is different. It warns us that Python can only concatenate <code class="docutils literal notranslate"><span class="pre">str</span></code> variables with other <code class="docutils literal notranslate"><span class="pre">str</span></code> variables (and not <code class="docutils literal notranslate"><span class="pre">int</span></code> variables.</p>
<p>Why do the messages differ? In the first case, Python sees a number (integer) first. It then assumes that the <code class="docutils literal notranslate"><span class="pre">+</span></code> character must be addition, and gets confused and raises an error when, after the <code class="docutils literal notranslate"><span class="pre">+</span></code> character, it encounters a string. In the second case, Python sees a string first. It then assumes that the <code class="docutils literal notranslate"><span class="pre">+</span></code> character is meant to signify concatenation, and thus becomes confused and raises and error when it sees an integer.</p>
<p>The solution to the problem of combining strings and numbers together is called <em>type conversion</em>, and it will be discussed later in this chapter.</p>
<p><span style="color:red"><strong>Concept check</strong>:</span> Add a new line of code to the end of the block below to combine the variables <code class="docutils literal notranslate"><span class="pre">first</span></code> and <code class="docutils literal notranslate"><span class="pre">second</span></code> so that the result is a complete sentence that reads:</p>
<blockquote>
<div><p>Strings are really important pieces of data.</p>
</div></blockquote>
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<div class="highlight-ipython3 notranslate"><div class="highlight"><pre><span></span><span class="n">first</span> <span class="o">=</span> <span class="s1">'Strings are really important'</span>
<span class="n">second</span> <span class="o">=</span> <span class="s1">'pieces of data.'</span>
</pre></div>
</div>
</div>
</div>
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