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<div class="section" id="map-filter-and-reduce">
<span id="map-filter-reduce"></span><h1>Map Filter and Reduce<a class="headerlink" href="#map-filter-and-reduce" title="Permalink to this headline"></a></h1>
<p><code class="docutils literal notranslate"><span class="pre">map</span></code>, <code class="docutils literal notranslate"><span class="pre">filter</span></code>, and <code class="docutils literal notranslate"><span class="pre">reduce</span></code> are considered key parts of “functional programming”</p>
<p>But there is no real consensus about what “functional” means.</p>
<p>But Python does support these “classic” functional methods.</p>
<div class="section" id="map">
<h2>map<a class="headerlink" href="#map" title="Permalink to this headline"></a></h2>
<p><code class="docutils literal notranslate"><span class="pre">map</span></code> “maps” a function onto a sequence of objects – It applies the function to each item in the sequence, returning an “iterable” “map object”.</p>
<p>The map object delays evaluation until you iterate over it. That way you can pass it to another map, or use it in a for loop, without creating an unnecessary copy of the list.</p>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [4]: </span><span class="n">l</span> <span class="o">=</span> <span class="p">[</span><span class="mi">2</span><span class="p">,</span> <span class="mi">5</span><span class="p">,</span> <span class="mi">7</span><span class="p">,</span> <span class="mi">12</span><span class="p">,</span> <span class="mi">6</span><span class="p">,</span> <span class="mi">4</span><span class="p">]</span>
<span class="gp">In [5]: </span><span class="k">def</span> <span class="nf">fun</span><span class="p">(</span><span class="n">x</span><span class="p">):</span>
<span class="gp"> ...: </span> <span class="k">return</span> <span class="n">x</span><span class="o">*</span><span class="mi">2</span> <span class="o">+</span> <span class="mi">10</span>
<span class="gp"> ...:</span>
<span class="gp">In [6]: </span><span class="nb">map</span><span class="p">(</span><span class="n">fun</span><span class="p">,</span> <span class="n">l</span><span class="p">)</span>
<span class="gh">Out[6]: </span><span class="go"><map at 0x1046f2ac8></span>
<span class="gp">In [7]: </span><span class="nb">list</span><span class="p">(</span><span class="nb">map</span><span class="p">(</span><span class="n">fun</span><span class="p">,</span> <span class="n">l</span><span class="p">))</span>
<span class="gh">Out[7]: </span><span class="go">[14, 20, 24, 34, 22, 18]</span>
</pre></div>
</div>
<p>And if it’s a small function, and you only need it once, this is a great use for <code class="docutils literal notranslate"><span class="pre">lambda</span></code></p>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [26]: </span><span class="nb">list</span><span class="p">(</span><span class="nb">map</span><span class="p">(</span><span class="k">lambda</span> <span class="n">x</span><span class="p">:</span> <span class="n">x</span><span class="o">*</span><span class="mi">2</span> <span class="o">+</span> <span class="mi">10</span><span class="p">,</span> <span class="n">l</span><span class="p">))</span>
<span class="gh">Out[26]: </span><span class="go">[14, 20, 24, 34, 22, 18]</span>
</pre></div>
</div>
</div>
<div class="section" id="filter">
<h2>filter<a class="headerlink" href="#filter" title="Permalink to this headline"></a></h2>
<p><code class="docutils literal notranslate"><span class="pre">filter</span></code> “filters” a sequence of objects with a boolean function –
It keeps only those for which the function is True – filtering out the rest.</p>
<p>It similarly returns an iterable object.</p>
<p>To get only the even numbers:</p>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [27]: </span><span class="n">l</span> <span class="o">=</span> <span class="p">[</span><span class="mi">2</span><span class="p">,</span> <span class="mi">5</span><span class="p">,</span> <span class="mi">7</span><span class="p">,</span> <span class="mi">12</span><span class="p">,</span> <span class="mi">6</span><span class="p">,</span> <span class="mi">4</span><span class="p">]</span>
<span class="gp">In [28]: </span><span class="nb">list</span><span class="p">(</span><span class="nb">filter</span><span class="p">(</span><span class="k">lambda</span> <span class="n">x</span><span class="p">:</span> <span class="ow">not</span> <span class="n">x</span><span class="o">%</span><span class="k">2</span>, l))
<span class="gh">Out[28]: </span><span class="go">[2, 12, 6, 4]</span>
</pre></div>
</div>
<p>If you pass <code class="docutils literal notranslate"><span class="pre">None</span></code> to <code class="docutils literal notranslate"><span class="pre">filter()</span></code>, you get only items that evaluate to true:</p>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [1]: </span><span class="n">l</span> <span class="o">=</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="mf">2.3</span><span class="p">,</span> <span class="mf">0.0</span><span class="p">,</span> <span class="s1">'text'</span><span class="p">,</span> <span class="s1">''</span><span class="p">,</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="p">[],</span> <span class="kc">False</span><span class="p">,</span> <span class="kc">True</span><span class="p">,</span> <span class="kc">None</span> <span class="p">]</span>
<span class="gp">In [2]: </span><span class="nb">list</span><span class="p">(</span><span class="nb">filter</span><span class="p">(</span><span class="kc">None</span><span class="p">,</span> <span class="n">l</span><span class="p">))</span>
<span class="gh">Out[2]: </span><span class="go">[1, 2.3, 'text', [1, 2], True]</span>
</pre></div>
</div>
</div>
<div class="section" id="reduce">
<h2>reduce<a class="headerlink" href="#reduce" title="Permalink to this headline"></a></h2>
<p><code class="docutils literal notranslate"><span class="pre">reduce</span></code> “reduces” a sequence of objects to a single object with a function that combines two arguments.</p>
<p>In python3, reduce has been a bit hidden in the “functools” module:</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">functools</span> <span class="kn">import</span> <span class="n">reduce</span>
</pre></div>
</div>
<p>To get the sum:</p>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [11]: </span><span class="kn">from</span> <span class="nn">functools</span> <span class="kn">import</span> <span class="n">reduce</span>
<span class="gp">In [12]: </span><span class="n">l</span> <span class="o">=</span> <span class="p">[</span><span class="mi">2</span><span class="p">,</span> <span class="mi">5</span><span class="p">,</span> <span class="mi">7</span><span class="p">,</span> <span class="mi">12</span><span class="p">,</span> <span class="mi">6</span><span class="p">,</span> <span class="mi">4</span><span class="p">]</span>
<span class="gp">In [13]: </span><span class="n">reduce</span><span class="p">(</span><span class="k">lambda</span> <span class="n">x</span><span class="p">,</span><span class="n">y</span><span class="p">:</span> <span class="n">x</span><span class="o">+</span><span class="n">y</span><span class="p">,</span> <span class="n">l</span><span class="p">)</span>
<span class="gh">Out[13]: </span><span class="go">36</span>
</pre></div>
</div>
<p>To get the product:</p>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [32]: </span><span class="n">reduce</span><span class="p">(</span><span class="k">lambda</span> <span class="n">x</span><span class="p">,</span><span class="n">y</span><span class="p">:</span> <span class="n">x</span><span class="o">*</span><span class="n">y</span><span class="p">,</span> <span class="n">l</span><span class="p">)</span>
<span class="gh">Out[32]: </span><span class="go">20160</span>
</pre></div>
</div>
<p>or</p>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [13]: </span><span class="kn">import</span> <span class="nn">operator</span>
<span class="gp">In [14]: </span><span class="n">reduce</span><span class="p">(</span><span class="n">operator</span><span class="o">.</span><span class="n">mul</span><span class="p">,</span> <span class="n">l</span><span class="p">)</span>
<span class="gh">Out[14]: </span><span class="go">20160</span>
</pre></div>
</div>
</div>
<div class="section" id="comprehensions">
<h2>Comprehensions<a class="headerlink" href="#comprehensions" title="Permalink to this headline"></a></h2>
<p>Couldn’t you do all this with comprehensions?</p>
<p>Yes:</p>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [33]: </span><span class="p">[</span><span class="n">x</span><span class="o">+</span><span class="mi">2</span> <span class="o">+</span> <span class="mi">10</span> <span class="k">for</span> <span class="n">x</span> <span class="ow">in</span> <span class="n">l</span><span class="p">]</span>
<span class="gh">Out[33]: </span><span class="go">[14, 17, 19, 24, 18, 16]</span>
<span class="gp">In [34]: </span><span class="p">[</span><span class="n">x</span> <span class="k">for</span> <span class="n">x</span> <span class="ow">in</span> <span class="n">l</span> <span class="k">if</span> <span class="ow">not</span> <span class="n">x</span><span class="o">%</span><span class="k">2</span>]
<span class="gh">Out[34]: </span><span class="go">[2, 12, 6, 4]</span>
<span class="gp">In [6]: </span><span class="n">l</span>
<span class="gh">Out[6]: </span><span class="go">[1, 0, 2.3, 0.0, 'text', '', [1, 2], [], False, True, None]</span>
<span class="gp">In [7]: </span><span class="p">[</span><span class="n">i</span> <span class="k">for</span> <span class="n">i</span> <span class="ow">in</span> <span class="n">l</span> <span class="k">if</span> <span class="n">i</span><span class="p">]</span>
<span class="gh">Out[7]: </span><span class="go">[1, 2.3, 'text', [1, 2], True]</span>
</pre></div>
</div>
<p>(Except Reduce)</p>
<p>But Guido thinks almost all uses of reduce are really <code class="docutils literal notranslate"><span class="pre">sum()</span></code>. In fact, that’s why it got moved out of built-ins into the <code class="docutils literal notranslate"><span class="pre">functools</span></code> module.</p>
</div>
<div class="section" id="functional-programming">
<h2>Functional Programming<a class="headerlink" href="#functional-programming" title="Permalink to this headline"></a></h2>
<p>Comprehensions and map, filter, reduce are all “functional programming” approaches.</p>
<p><code class="docutils literal notranslate"><span class="pre">map,</span> <span class="pre">filter</span></code> and <code class="docutils literal notranslate"><span class="pre">reduce</span></code> pre-date comprehensions in Python’s history</p>
<p>Some people like that syntax better.</p>
<p>And “map-reduce” is a big concept these days for parallel processing of “Big Data” in NoSQL databases.</p>
<p>(Hadoop, MongoDB, etc.)</p>
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