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python-for-data-part-1-data-structures.html
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python-for-data-part-1-data-structures.html
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<h1>
<a href="./python-for-data-part-1-data-structures.html"
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title="Permalink to Python for Data Part 1: Data Structures">
Python for Data Part 1: Data Structures
</a>
</h1>
<i><time datetime="2016-09-24T00:00:00-04:00"> Sat 24 September 2016</time></i>
<div class="entry-content">
<div class="panel">
<br/>
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<br/>
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<hr class="featurette-divider">
<h1>Data Structures</h1>
<ul>
<li>tuple</li>
<li>list</li>
<li>dict</li>
<li>set</li>
</ul>
<h2>tuple</h2>
<p>A tuple is a one dimensional, fixed-length, immutable sequence.</p>
<p>Create a tuple:</p>
<div class="highlight"><pre><span class="n">tup</span> <span class="o">=</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">3</span><span class="p">)</span>
<span class="n">tup</span>
</pre></div>
<p><code>(1, 2, 3)</code></p>
<p>Convert to a tuple:</p>
<div class="highlight"><pre><span class="n">list_1</span> <span class="o">=</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">3</span><span class="p">]</span>
<span class="nb">type</span><span class="p">(</span><span class="nb">tuple</span><span class="p">(</span><span class="n">list_1</span><span class="p">))</span>
</pre></div>
<p><code>tuple</code></p>
<p>Create a nested tuple:</p>
<div class="highlight"><pre><span class="n">nested_tup</span> <span class="o">=</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">3</span><span class="p">],</span> <span class="p">(</span><span class="mi">4</span><span class="p">,</span> <span class="mi">5</span><span class="p">))</span>
<span class="n">nested_tup</span>
</pre></div>
<p><code>([1, 2, 3], (4, 5))</code></p>
<p>Access a tuple's elements by index O(1):</p>
<div class="highlight"><pre><span class="n">nested_tup</span><span class="p">[</span><span class="mi">0</span><span class="p">]</span>
</pre></div>
<p><code>[1, 2, 3]</code></p>
<p>Although tuples are immutable, their contents can contain mutable objects.</p>
<p>Modify a tuple's contents:</p>
<div class="highlight"><pre><span class="n">nested_tup</span><span class="p">[</span><span class="mi">0</span><span class="p">]</span><span class="o">.</span><span class="n">append</span><span class="p">(</span><span class="mi">4</span><span class="p">)</span>
<span class="n">nested_tup</span><span class="p">[</span><span class="mi">0</span><span class="p">]</span>
</pre></div>
<p><code>[1, 2, 3, 4]</code></p>
<p>Concatenate tuples by creating a new tuple and copying objects:</p>
<div class="highlight"><pre><span class="p">(</span><span class="mi">1</span><span class="p">,</span> <span class="mi">3</span><span class="p">,</span> <span class="mi">2</span><span class="p">)</span> <span class="o">+</span> <span class="p">(</span><span class="mi">4</span><span class="p">,</span> <span class="mi">5</span><span class="p">,</span> <span class="mi">6</span><span class="p">)</span>
</pre></div>
<p><code>(1, 3, 2, 4, 5, 6)</code></p>
<p>Multiply tuples to copy references to objects (objects themselves are not copied):</p>
<div class="highlight"><pre><span class="p">(</span><span class="s">'foo'</span><span class="p">,</span> <span class="s">'bar'</span><span class="p">)</span> <span class="o">*</span> <span class="mi">2</span>
</pre></div>
<p><code>('foo', 'bar', 'foo', 'bar')</code></p>
<p>Unpack tuples:</p>
<div class="highlight"><pre><span class="n">a</span><span class="p">,</span> <span class="n">b</span> <span class="o">=</span> <span class="n">nested_tup</span>
<span class="n">a</span><span class="p">,</span> <span class="n">b</span>
</pre></div>
<p><code>([1, 2, 3, 4], (4, 5))</code></p>
<p>Unpack nested tuples:</p>
<div class="highlight"><pre><span class="p">(</span><span class="n">a</span><span class="p">,</span> <span class="n">b</span><span class="p">,</span> <span class="n">c</span><span class="p">,</span> <span class="n">d</span><span class="p">),</span> <span class="p">(</span><span class="n">e</span><span class="p">,</span> <span class="n">f</span><span class="p">)</span> <span class="o">=</span> <span class="n">nested_tup</span>
<span class="n">a</span><span class="p">,</span> <span class="n">b</span><span class="p">,</span> <span class="n">c</span><span class="p">,</span> <span class="n">d</span><span class="p">,</span> <span class="n">e</span><span class="p">,</span> <span class="n">f</span>
</pre></div>
<p><code>(1, 2, 3, 4, 4, 5)</code></p>
<p>A common use of variable unpacking is when iterating over sequences of tuples or lists:</p>
<div class="highlight"><pre><span class="n">seq</span> <span class="o">=</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">3</span><span class="p">),</span> <span class="p">(</span><span class="mi">4</span><span class="p">,</span> <span class="mi">5</span><span class="p">,</span> <span class="mi">6</span><span class="p">),</span> <span class="p">(</span><span class="mi">7</span><span class="p">,</span> <span class="mi">8</span><span class="p">,</span> <span class="mi">9</span><span class="p">)]</span>
<span class="k">for</span> <span class="n">a</span><span class="p">,</span> <span class="n">b</span><span class="p">,</span> <span class="n">c</span> <span class="ow">in</span> <span class="n">seq</span><span class="p">:</span>
<span class="k">print</span><span class="p">(</span><span class="n">a</span><span class="p">,</span> <span class="n">b</span><span class="p">,</span> <span class="n">c</span><span class="p">)</span>
</pre></div>
<p><code>(1, 2, 3)</code></p>
<p><code>(4, 5, 6)</code></p>
<p><code>(7, 8, 9)</code></p>
<h2>list</h2>
<p>A list is a one dimensional, variable-length, mutable sequence.</p>
<p>Create a list:</p>
<div class="highlight"><pre><span class="n">list_1</span> <span class="o">=</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">3</span><span class="p">]</span>
<span class="n">list_1</span>
</pre></div>
<p><code>[1, 2, 3]</code></p>
<p>Convert to a list:</p>
<div class="highlight"><pre><span class="nb">type</span><span class="p">(</span><span class="nb">list</span><span class="p">(</span><span class="n">tup</span><span class="p">))</span>
</pre></div>
<p><code>list</code></p>
<p>Create a nested list:</p>
<div class="highlight"><pre><span class="n">nested_list</span> <span class="o">=</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">3</span><span class="p">),</span> <span class="p">[</span><span class="mi">4</span><span class="p">,</span> <span class="mi">5</span><span class="p">]]</span>
<span class="n">nested_list</span>
</pre></div>
<p><code>[(1, 2, 3), [4, 5]]</code></p>
<p>Access a list's elements by index O(1):</p>
<div class="highlight"><pre><span class="n">nested_list</span><span class="p">[</span><span class="mi">1</span><span class="p">]</span>
</pre></div>
<p><code>[4, 5]</code></p>
<p>Append an element to a list O(1):</p>
<div class="highlight"><pre><span class="n">nested_list</span><span class="o">.</span><span class="n">append</span><span class="p">(</span><span class="mi">6</span><span class="p">)</span>
<span class="n">nested_list</span>
</pre></div>
<p><code>[(1, 2, 3), [4, 5], 6]</code></p>
<p>Insert an element to a list at a specific index (note that insert is expensive as it has to shift subsequent elements O(n)):</p>
<div class="highlight"><pre><span class="n">nested_list</span><span class="o">.</span><span class="n">insert</span><span class="p">(</span><span class="mi">0</span><span class="p">,</span> <span class="s">'start'</span><span class="p">)</span>
<span class="n">nested_list</span>
</pre></div>
<p><code>['start', (1, 2, 3), [4, 5], 6]</code></p>
<p>Pop is expensive as it has to shift subsequent elements O(n). The operation is O(1) if pop is used for the last element.</p>
<p>Remove and return an element from a specified index:</p>
<div class="highlight"><pre><span class="n">nested_list</span><span class="o">.</span><span class="n">pop</span><span class="p">(</span><span class="mi">0</span><span class="p">)</span>
<span class="n">nested_list</span>
</pre></div>
<p><code>[(1, 2, 3), [4, 5], 6]</code></p>
<p>Locates the first such value and remove it O(n):</p>
<div class="highlight"><pre><span class="n">nested_list</span><span class="o">.</span><span class="n">remove</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">3</span><span class="p">))</span>
<span class="n">nested_list</span>
</pre></div>
<p><code>[[4, 5], 6]</code></p>
<p>Check if a list contains a value O(n):</p>
<div class="highlight"><pre><span class="mi">6</span> <span class="ow">in</span> <span class="n">nested_list</span>
</pre></div>
<p><code>True</code></p>
<p>Concatenate lists by creating a new list and copying objects:</p>
<div class="highlight"><pre><span class="p">[</span><span class="mi">1</span><span class="p">,</span> <span class="mi">3</span><span class="p">,</span> <span class="mi">2</span><span class="p">]</span> <span class="o">+</span> <span class="p">[</span><span class="mi">4</span><span class="p">,</span> <span class="mi">5</span><span class="p">,</span> <span class="mi">6</span><span class="p">]</span>
</pre></div>
<p><code>[1, 3, 2, 4, 5, 6]</code></p>
<p>Extend a list by appending elements (faster than concatenating lists, as it does not have to create a new list):</p>
<div class="highlight"><pre><span class="n">nested_list</span><span class="o">.</span><span class="n">extend</span><span class="p">([</span><span class="mi">7</span><span class="p">,</span> <span class="mi">8</span><span class="p">,</span> <span class="mi">9</span><span class="p">])</span>
<span class="n">nested_list</span>
</pre></div>
<p><code>[[4, 5], 6, 7, 8, 9]</code></p>
<h2>dict</h2>
<p>A dict is also known as a hash map or associative array. A dict is a mutable collection of key-value pairs.</p>
<p>Note: Big O complexities are listed as average case, with most worst case complexities being O(n).</p>
<p>Create a dict:</p>
<div class="highlight"><pre><span class="n">dict_1</span> <span class="o">=</span> <span class="p">{</span> <span class="s">'a'</span> <span class="p">:</span> <span class="s">'foo'</span><span class="p">,</span> <span class="s">'b'</span> <span class="p">:</span> <span class="p">[</span><span class="mi">0</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">3</span><span class="p">]</span> <span class="p">}</span>
<span class="n">dict_1</span>
</pre></div>
<p><code>{'a': 'foo', 'b': [0, 1, 2, 3]}</code></p>
<p>Access a dict's elements by index O(1)</p>
<div class="highlight"><pre><span class="n">dict_1</span><span class="p">[</span><span class="s">'b'</span><span class="p">]</span>
</pre></div>
<p><code>[0, 1, 2, 3]</code></p>
<p>Insert or set a dict's elements by index O(1):</p>
<div class="highlight"><pre><span class="n">dict_1</span><span class="p">[</span><span class="mi">5</span><span class="p">]</span> <span class="o">=</span> <span class="s">'bar'</span>
<span class="n">dict_1</span>
</pre></div>
<p><code>{5: 'bar', 'a': 'foo', 'b': [0, 1, 2, 3]}</code></p>
<p>Check if a dict contains a key O(1):</p>
<div class="highlight"><pre><span class="mi">5</span> <span class="ow">in</span> <span class="n">dict_1</span>
</pre></div>
<p><code>True</code></p>
<p>Delete a value from a dict O(1):</p>
<div class="highlight"><pre><span class="n">dict_2</span> <span class="o">=</span> <span class="nb">dict</span><span class="p">(</span><span class="n">dict_1</span><span class="p">)</span>
<span class="k">del</span> <span class="n">dict_2</span><span class="p">[</span><span class="mi">5</span><span class="p">]</span>
<span class="n">dict_2</span>
</pre></div>
<p><code>{'a': 'foo', 'b': [0, 1, 2, 3]}</code></p>
<p>Remove and return an element from a specified index O(1):</p>
<div class="highlight"><pre><span class="n">value</span> <span class="o">=</span> <span class="n">dict_2</span><span class="o">.</span><span class="n">pop</span><span class="p">(</span><span class="s">'b'</span><span class="p">)</span>
<span class="k">print</span><span class="p">(</span><span class="n">value</span><span class="p">)</span>
<span class="k">print</span><span class="p">(</span><span class="n">dict_2</span><span class="p">)</span>
</pre></div>
<p><code>[0, 1, 2, 3]</code>
<code>{'a': 'foo'}</code></p>
<p>Get or pop can be called with a default value if the key is not found. By default, get() will return None and pop() will throw an exception if the key is not found.</p>
<div class="highlight"><pre><span class="n">value</span> <span class="o">=</span> <span class="n">dict_1</span><span class="o">.</span><span class="n">get</span><span class="p">(</span><span class="s">'z'</span><span class="p">,</span> <span class="mi">0</span><span class="p">)</span>
<span class="n">value</span>
</pre></div>
<p><code>0</code></p>
<p>Return a default value if the key is not found:</p>
<div class="highlight"><pre><span class="k">print</span><span class="p">(</span><span class="n">dict_1</span><span class="o">.</span><span class="n">setdefault</span><span class="p">(</span><span class="s">'b'</span><span class="p">,</span> <span class="bp">None</span><span class="p">))</span>
<span class="k">print</span><span class="p">(</span><span class="n">dict_1</span><span class="o">.</span><span class="n">setdefault</span><span class="p">(</span><span class="s">'z'</span><span class="p">,</span> <span class="bp">None</span><span class="p">))</span>
</pre></div>
<p><code>[0, 1, 2, 3]</code>
<code>None</code></p>
<p>By contrast to setdefault(), defaultdict lets you specify the default when the container is initialized, which works well if the default is appropriate for all keys:</p>
<div class="highlight"><pre><span class="kn">from</span> <span class="nn">collections</span> <span class="kn">import</span> <span class="n">defaultdict</span>
<span class="n">seq</span> <span class="o">=</span> <span class="p">[</span><span class="s">'foo'</span><span class="p">,</span> <span class="s">'bar'</span><span class="p">,</span> <span class="s">'baz'</span><span class="p">]</span>
<span class="n">first_letter</span> <span class="o">=</span> <span class="n">defaultdict</span><span class="p">(</span><span class="nb">list</span><span class="p">)</span>
<span class="k">for</span> <span class="n">elem</span> <span class="ow">in</span> <span class="n">seq</span><span class="p">:</span>
<span class="n">first_letter</span><span class="p">[</span><span class="n">elem</span><span class="p">[</span><span class="mi">0</span><span class="p">]]</span><span class="o">.</span><span class="n">append</span><span class="p">(</span><span class="n">elem</span><span class="p">)</span>
<span class="n">first_letter</span>
</pre></div>
<p><code>defaultdict(<type 'list'>, {'b': ['bar', 'baz'], 'f': ['foo']})</code></p>
<p>dict keys must be "hashable", i.e. they must be immutable objects like scalars (int, float, string) or tuples whose objects are all immutable. Lists are mutable and therefore are not hashable, although you can convert the list portion to a tuple as a quick fix.</p>
<div class="highlight"><pre><span class="k">print</span><span class="p">(</span><span class="nb">hash</span><span class="p">(</span><span class="s">'string'</span><span class="p">))</span>
<span class="k">print</span><span class="p">(</span><span class="nb">hash</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="mi">3</span><span class="p">,</span> <span class="mi">4</span><span class="p">))))</span>
</pre></div>
<p><code>-9167918882415130555</code>
<code>-2725224101759650258</code></p>
<p>Get the list of keys in no particular order (although keys() outputs the keys in the same order). In Python 3, keys() returns an iterator instead of a list.</p>
<div class="highlight"><pre><span class="n">dict_1</span><span class="o">.</span><span class="n">keys</span><span class="p">()</span>
</pre></div>
<p><code>['a', 'b', 5, 'z']</code></p>
<p>Get the list of values in no particular order (although values() outputs the keys in the same order). In Python 3, keys() returns an iterator instead of a list.</p>
<div class="highlight"><pre><span class="n">dict_1</span><span class="o">.</span><span class="n">values</span><span class="p">()</span>
</pre></div>
<p><code>['foo', [0, 1, 2, 3], 'bar', None]</code></p>
<p>Iterate through a dictionary's keys and values:</p>
<div class="highlight"><pre><span class="k">for</span> <span class="n">key</span><span class="p">,</span> <span class="n">value</span> <span class="ow">in</span> <span class="n">dict_1</span><span class="o">.</span><span class="n">items</span><span class="p">():</span>
<span class="k">print</span> <span class="n">key</span><span class="p">,</span> <span class="n">value</span>
</pre></div>
<p><code>a foo</code></p>
<p><code>b [0, 1, 2, 3]</code></p>
<p><code>5 bar</code></p>
<p><code>z None</code></p>
<p>Merge one dict into another:</p>
<div class="highlight"><pre><span class="n">dict_1</span><span class="o">.</span><span class="n">update</span><span class="p">({</span><span class="s">'e'</span> <span class="p">:</span> <span class="s">'elephant'</span><span class="p">,</span> <span class="s">'f'</span> <span class="p">:</span> <span class="s">'fish'</span><span class="p">})</span>
<span class="n">dict_1</span>
</pre></div>
<p><code>{5: 'bar',
'a': 'foo',
'b': [0, 1, 2, 3],
'e': 'elephant',
'f': 'fish',
'z': None}</code></p>
<p>Pair up two sequences element-wise in a dict:</p>
<div class="highlight"><pre><span class="n">mapping</span> <span class="o">=</span> <span class="nb">dict</span><span class="p">(</span><span class="nb">zip</span><span class="p">(</span><span class="nb">range</span><span class="p">(</span><span class="mi">7</span><span class="p">),</span> <span class="nb">reversed</span><span class="p">(</span><span class="nb">range</span><span class="p">(</span><span class="mi">7</span><span class="p">))))</span>
<span class="n">mapping</span>
</pre></div>
<p><code>{0: 6, 1: 5, 2: 4, 3: 3, 4: 2, 5: 1, 6: 0}</code></p>
<h2>set</h2>
<p>A set is an unordered sequence of unique elements.</p>
<p>Create a set:</p>
<div class="highlight"><pre><span class="n">set_1</span> <span class="o">=</span> <span class="nb">set</span><span class="p">([</span><span class="mi">0</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">3</span><span class="p">,</span> <span class="mi">4</span><span class="p">,</span> <span class="mi">5</span><span class="p">])</span>
<span class="n">set_1</span>
</pre></div>
<p><code>{0, 1, 2, 3, 4, 5}</code></p>
<div class="highlight"><pre><span class="n">set_2</span> <span class="o">=</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">3</span><span class="p">,</span> <span class="mi">5</span><span class="p">,</span> <span class="mi">8</span><span class="p">,</span> <span class="mi">13</span><span class="p">}</span>
<span class="n">set_2</span>
</pre></div>
<p><code>{1, 2, 3, 5, 8, 13}</code></p>
<p>Sets support set operations like union, intersection, difference, and symmetric difference.</p>
<p>Union O(len(set_1) + len(set_2)):</p>
<div class="highlight"><pre><span class="n">set_1</span> <span class="o">|</span> <span class="n">set_2</span>
</pre></div>
<p><code>{0, 1, 2, 3, 4, 5, 8, 13}</code></p>
<p>Intersection O(min(len(set_1), len(set_2)):</p>
<div class="highlight"><pre><span class="n">set_1</span> <span class="o">&</span> <span class="n">set_2</span>
</pre></div>
<p><code>{1, 2, 3, 5}</code></p>
<p>Difference O(len(set_1)):</p>
<div class="highlight"><pre><span class="n">set_1</span> <span class="o">-</span> <span class="n">set_2</span>
</pre></div>
<p><code>{0, 4}</code></p>
<p>Symmetric Difference O(len(set_1)):</p>
<div class="highlight"><pre><span class="n">set_1</span> <span class="o">^</span> <span class="n">set_2</span>
</pre></div>
<p><code>{0, 4, 8, 13}</code></p>
<p>Subset O(len(set_3)):</p>
<div class="highlight"><pre><span class="n">set_3</span> <span class="o">=</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">3</span><span class="p">}</span>
<span class="n">set_3</span><span class="o">.</span><span class="n">issubset</span><span class="p">(</span><span class="n">set_2</span><span class="p">)</span>
</pre></div>
<p><code>True</code></p>
<p>Superset O(len(set_3)):</p>
<div class="highlight"><pre><span class="n">set_2</span><span class="o">.</span><span class="n">issuperset</span><span class="p">(</span><span class="n">set_3</span><span class="p">)</span>
</pre></div>
<p><code>True</code></p>
<p>Equal O(min(len(set_1), len(set_2)):</p>
<div class="highlight"><pre><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">3</span><span class="p">}</span> <span class="o">==</span> <span class="p">{</span><span class="mi">3</span><span class="p">,</span> <span class="mi">2</span><span class="p">,</span> <span class="mi">1</span><span class="p">}</span>
</pre></div>
<p><code>True</code></p>
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