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<div class="section" id="sparse-array-exercise">
<span id="exercise-sparse-array"></span><h1>Sparse Array Exercise<a class="headerlink" href="#sparse-array-exercise" title="Permalink to this headline"></a></h1>
<div class="section" id="sparse-array">
<h2>Sparse Array<a class="headerlink" href="#sparse-array" title="Permalink to this headline"></a></h2>
<blockquote>
<div><p>Goal:</p>
</div></blockquote>
<p>Learn how to emulate a built-in class.</p>
<div class="section" id="id1">
<h3>Sparse Array:<a class="headerlink" href="#id1" title="Permalink to this headline"></a></h3>
<p>Oftentimes, at least in computation programming, we have large arrays of data that hold mostly zeros.</p>
<p>These are referred to as “sparse” as the information in them is widely scattered, or sparse.</p>
<p>Since they are mostly zeros, it can be memory and computationally efficient to store only the value that are non-zero.</p>
<p>But you want it to look like a regular array in user code.</p>
<p>In the real world, these are usually 2 dimensional arrays. But to keep it a bit simpler, we’ll make a 1 dimensional sparse array in this class.</p>
<p>(feel free to make it 2d for an extra challenge!).</p>
</div>
<div class="section" id="a-sparse-array-class">
<h3>A Sparse array class<a class="headerlink" href="#a-sparse-array-class" title="Permalink to this headline"></a></h3>
<p>A sparse array class should present to the user the same interface as a regular list.</p>
<p>Some ideas of how to do that:</p>
<ul class="simple">
<li><p>Internally, it can store the values in a dict, with the index as the keys, so that only the indexes with non-zero values will be stored.</p></li>
<li><p>It should take a sequence of values as an initializer:</p></li>
</ul>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="n">sa</span> <span class="o">=</span> <span class="n">SparseArray</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">0</span><span class="p">,</span><span class="mi">0</span><span class="p">,</span><span class="mi">0</span><span class="p">,</span><span class="mi">0</span><span class="p">,</span><span class="mi">3</span><span class="p">,</span><span class="mi">0</span><span class="p">,</span><span class="mi">0</span><span class="p">,</span><span class="mi">4</span><span class="p">])</span>
</pre></div>
</div>
<ul class="simple">
<li><p>you should be able to tell how long it is:</p></li>
</ul>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="nb">len</span><span class="p">(</span><span class="n">my_array</span><span class="p">)</span>
</pre></div>
</div>
<p>This will give its “virtual” length – with the zeros</p>
<ul class="simple">
<li><p>It should support getting and setting particular elements via indexing:</p></li>
</ul>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="n">sa</span><span class="p">[</span><span class="mi">5</span><span class="p">]</span> <span class="o">=</span> <span class="mi">12</span>
<span class="n">sa</span><span class="p">[</span><span class="mi">3</span><span class="p">]</span> <span class="o">=</span> <span class="mi">0</span> <span class="c1"># the zero won't get stored!</span>
<span class="n">val</span> <span class="o">=</span> <span class="n">sa</span><span class="p">[</span><span class="mi">13</span><span class="p">]</span> <span class="c1"># it should get a zero if not set</span>
</pre></div>
</div>
<ul class="simple">
<li><p>It should support deleting an element by index:</p></li>
</ul>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="k">del</span> <span class="n">sa</span><span class="p">[</span><span class="mi">4</span><span class="p">]</span>
</pre></div>
</div>
<ul class="simple">
<li><p>It should raise an <code class="docutils literal notranslate"><span class="pre">IndexError</span></code> if you try to access an index beyond the end.</p></li>
<li><p>it should have an append() method.</p></li>
<li><p>Can you make it support slicing?</p></li>
<li><p>How else can you make it like a list?</p></li>
</ul>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [10]: </span><span class="n">my_array</span> <span class="o">=</span> <span class="n">SparseArray</span><span class="p">(</span> <span class="p">(</span><span class="mi">1</span><span class="p">,</span><span class="mi">0</span><span class="p">,</span><span class="mi">0</span><span class="p">,</span><span class="mi">0</span><span class="p">,</span><span class="mi">2</span><span class="p">,</span><span class="mi">0</span><span class="p">,</span><span class="mi">0</span><span class="p">,</span><span class="mi">0</span><span class="p">,</span><span class="mi">5</span><span class="p">)</span> <span class="p">)</span>
<span class="gp">In [11]: </span><span class="n">my_array</span><span class="p">[</span><span class="mi">4</span><span class="p">]</span>
<span class="gh">Out[11]: </span><span class="go">2</span>
<span class="gp">In [12]: </span><span class="n">my_array</span><span class="p">[</span><span class="mi">2</span><span class="p">]</span>
<span class="gh">Out[12]: </span><span class="go">0</span>
</pre></div>
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