| author | amit@thunder |
| Thu, 25 Feb 2010 00:45:20 +0530 | |
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<head><meta http-equiv="Content-Type" content="text/html; charset=utf-8" /><title>Chapter. list_tuples</title><link rel="stylesheet" href="/review/support/styles.css" type="text/css" /><meta name="generator" content="DocBook XSL Stylesheets V1.74.3" /><link rel="shortcut icon" type="image/png" href="/review/support/figs/favicon.png" /><script type="text/javascript" src="/review/support/jquery-min.js"></script><script type="text/javascript" src="/review/support/form.js"></script><script type="text/javascript" src="/review/support/hsbook.js"></script></head> |
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<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"><div class="chapter" id="ch3list_tuples"> |
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<div class="titlepage"></div> |
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<div class="toc"> |
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<p><b>Table of Contents</b></p> |
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<dl> |
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<dt><span class="article"><a href="#id2896388">Lists and Tuples</a></span></dt> |
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<dd><dl> |
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<dt><span class="section"><a href="#id2706433">1. Lists</a></span></dt> |
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<dd><dl> |
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<dt><span class="section"><a href="#id2896192">1.1. Common List Operations</a></span></dt> |
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<dt><span class="section"><a href="#id2947658">1.2. None, Empty Lists, and Initialization</a></span></dt> |
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<dt><span class="section"><a href="#id2947698">1.3. Nested Lists</a></span></dt> |
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<dt><span class="section"><a href="#id2947732">1.4. List Methods</a></span></dt> |
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</dl></dd> |
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<dt><span class="section"><a href="#id2948048">2. Tuples</a></span></dt> |
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<dd><dl><dt><span class="section"><a href="#id2948145">2.1. Common Tuple Operations</a></span></dt></dl></dd> |
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<dt><span class="section"><a href="#id2948295">3. Additional Syntax</a></span></dt> |
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<dd><dl> |
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<dt><span class="section"><a href="#id2948312">3.1. range()</a></span></dt> |
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<dt><span class="section"><a href="#id2948344">3.2. for</a></span></dt> |
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</dl></dd> |
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<dt><span class="section"><a href="#id2948403">4. Conclusion</a></span></dt> |
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</dl></dd> |
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</dl> |
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</div> |
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<div class="article" title="Lists and Tuples"> |
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<div class="titlepage"> |
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<div><div><h2 class="title"> |
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<a name="id2896388"></a>Lists and Tuples</h2></div></div> |
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<hr /> |
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</div> |
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<div class="toc"> |
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<p><b>Table of Contents</b></p> |
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<dl> |
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<dt><span class="section"><a href="#id2706433">1. Lists</a></span></dt> |
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<dd><dl> |
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<dt><span class="section"><a href="#id2896192">1.1. Common List Operations</a></span></dt> |
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<dt><span class="section"><a href="#id2947658">1.2. None, Empty Lists, and Initialization</a></span></dt> |
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<dt><span class="section"><a href="#id2947698">1.3. Nested Lists</a></span></dt> |
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<dt><span class="section"><a href="#id2947732">1.4. List Methods</a></span></dt> |
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</dl></dd> |
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<dt><span class="section"><a href="#id2948048">2. Tuples</a></span></dt> |
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<dd><dl><dt><span class="section"><a href="#id2948145">2.1. Common Tuple Operations</a></span></dt></dl></dd> |
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<dt><span class="section"><a href="#id2948295">3. Additional Syntax</a></span></dt> |
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<dd><dl> |
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<dt><span class="section"><a href="#id2948312">3.1. range()</a></span></dt> |
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<dt><span class="section"><a href="#id2948344">3.2. for</a></span></dt> |
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</dl></dd> |
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<dt><span class="section"><a href="#id2948403">4. Conclusion</a></span></dt> |
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</dl> |
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</div> |
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<div class="section" title="1.Lists"> |
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<div class="titlepage"><div><div><h2 class="title" style="clear: both"> |
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<a name="id2706433"></a>1.Lists</h2></div></div></div> |
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<p id="ch3list_tuples_1">Python provides an intuitive way to represent a group items, called <span class="emphasis"><em>Lists</em></span>. The |
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items of a <span class="emphasis"><em>List</em></span> are called its elements. Unlike C/C++, elements can be of any |
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type. A <span class="emphasis"><em>List</em></span> is represented as a list of comma-sepated elements with square |
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brackets around them:</p> |
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<pre class="programlisting"> |
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>>> a = [10, 'Python programming', 20.3523, 23, 3534534L] |
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>>> a |
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[10, 'Python programming', 20.3523, 23, 3534534L]</pre> |
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<div class="section" title="1.1.Common List Operations"> |
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<div class="titlepage"><div><div><h3 class="title"> |
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<a name="id2896192"></a>1.1.Common List Operations</h3></div></div></div> |
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<p id="ch3list_tuples_2">The following are some of the most commonly used operations on <span class="emphasis"><em>Lists</em></span>.</p> |
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<div class="section" title="1.1.1.Indexing"> |
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<div class="titlepage"><div><div><h4 class="title"> |
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<a name="id2896236"></a>1.1.1.Indexing</h4></div></div></div> |
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<p id="ch3list_tuples_3">Individual elements of a <span class="emphasis"><em>List</em></span> can be accessed using an index to the element. |
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The indices start at 0. One can also access the elements of the <span class="emphasis"><em>List</em></span> in reverse |
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using negative indices.:</p> |
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<pre class="programlisting"> |
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>>> a[1] |
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'Python programming' |
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>>> a[-1] |
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3534534L</pre> |
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<p id="ch3list_tuples_4">It is important to note here that the last element of the <span class="emphasis"><em>List</em></span> has an index of |
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-1.</p> |
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</div> |
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<div class="section" title="1.1.2.Concatenating"> |
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<div class="titlepage"><div><div><h4 class="title"> |
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<a name="id2947372"></a>1.1.2.Concatenating</h4></div></div></div> |
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<p id="ch3list_tuples_5">Two or more <span class="emphasis"><em>Lists</em></span> can be concatenated using the + operator:</p> |
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<pre class="programlisting"> |
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>>> a + ['foo', 12, 23.3432, 54] |
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[10, 'Python programming', 20.3523, 'foo', 12, 23.3432, 54] |
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>>> [54, 75, 23] + ['write', 67, 'read'] |
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[54, 75, 23, 'write', 67, 'read']</pre> |
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</div> |
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<div class="section" title="1.1.3.Slicing"> |
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<div class="titlepage"><div><div><h4 class="title"> |
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<a name="id2947393"></a>1.1.3.Slicing</h4></div></div></div> |
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<p id="ch3list_tuples_6">A <span class="emphasis"><em>List</em></span> can be sliced off to contain a subset of elements of the <span class="emphasis"><em>List</em></span>. Slicing |
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can be done by using two indices separated by a colon, where the first index is |
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inclusive and the second index is exclusive. The resulting slice is also a <span class="emphasis"><em>List</em></span>.:</p> |
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<pre class="programlisting"> |
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>>> num = [1, 2, 3, 4, 5, 6, 7, 8, 9] |
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>>> num[3:6] |
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[4, 5, 6] |
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>>> num[0:1] |
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[1] |
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>>> num[7:10] |
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[7, 8, 9]</pre> |
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<p id="ch3list_tuples_7">The last example showed how to access last 3 elements of the <span class="emphasis"><em>List</em></span>. There is a |
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small catch here. The second index 10 actually refers to the 11th element of the |
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<span class="emphasis"><em>List</em></span> which is still valid, even though it doesn't exist because the second |
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index is exclusive and tells the Python interpreter to get the last element of |
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the <span class="emphasis"><em>List</em></span>. But this can also be done in a much easier way using negative indices:</p> |
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<pre class="programlisting"> |
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>>> num[-3:-1] |
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[7, 8, 9]</pre> |
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<p id="ch3list_tuples_8">Excluding the first index implies that the slice must start at the beginning of |
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the <span class="emphasis"><em>List</em></span>, while excluding the second index includes all the elements till the |
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end of the <span class="emphasis"><em>List</em></span>. A third parameter to a slice, which is implicitly taken as 1 |
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is the step of the slice. It is specified as a value which follows a colon after |
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the second index:</p> |
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<pre class="programlisting"> |
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>>> num[:4] |
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[1, 2, 3, 4] |
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>>> num[7:] |
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[8, 9] |
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>>> num[-3:] |
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[7, 8, 9] |
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>>> num[:] |
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[1, 2, 3, 4, 5, 6, 7, 8, 9] |
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>>> num[4:9:3] |
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[5, 8] |
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>>> num[3::2] |
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[4, 6, 8] |
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>>> num[::4] |
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[1, 5, 9]</pre> |
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</div> |
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<div class="section" title="1.1.4.Multiplication"> |
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<div class="titlepage"><div><div><h4 class="title"> |
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<a name="id2947472"></a>1.1.4.Multiplication</h4></div></div></div> |
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<p id="ch3list_tuples_9">A <span class="emphasis"><em>List</em></span> can be multiplied with an integer to repeat itself:</p> |
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<pre class="programlisting"> |
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>>> [20] * 5 |
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[20, 20, 20, 20, 20] |
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>>> [42, 'Python', 54] * 3 |
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[42, 'Python', 54, 42, 'Python', 54, 42, 'Python', 54]</pre> |
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</div> |
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<div class="section" title="1.1.5.Membership"> |
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<div class="titlepage"><div><div><h4 class="title"> |
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<a name="id2947497"></a>1.1.5.Membership</h4></div></div></div> |
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<p id="ch3list_tuples_a"><span class="strong"><strong>in</strong></span> operator is used to find whether an element is part of the <span class="emphasis"><em>List</em></span>. It |
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returns <span class="strong"><strong>True</strong></span> if the element is present in the <span class="emphasis"><em>List</em></span> or <span class="strong"><strong>False</strong></span> if it is not |
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present. Since this operator returns a Boolean value it is called a Boolean |
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operator:</p> |
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<pre class="programlisting"> |
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>>> names = ['Guido', 'Alex', 'Tim'] |
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>>> 'Tim' in names |
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True |
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>>> 'Adam' in names |
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False</pre> |
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</div> |
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<div class="section" title="1.1.6.Length, Maximum and Minimum"> |
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<div class="titlepage"><div><div><h4 class="title"> |
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<a name="id2947539"></a>1.1.6.Length, Maximum and Minimum</h4></div></div></div> |
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<p id="ch3list_tuples_b">Length of a <span class="emphasis"><em>List</em></span> can be found out using the len function. The max function |
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returns the element with the largest value and the min function returns the |
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element with the smallest value:</p> |
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<pre class="programlisting"> |
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>>> num = [4, 1, 32, 12, 67, 34, 65] |
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>>> len(num) |
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7 |
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>>> max(num) |
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>>> min(num) |
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1</pre> |
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</div> |
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<div class="section" title="1.1.7.Changing Elements"> |
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<div class="titlepage"><div><div><h4 class="title"> |
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<a name="id2947564"></a>1.1.7.Changing Elements</h4></div></div></div> |
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<p id="ch3list_tuples_c">Unlike Strings <span class="emphasis"><em>Lists</em></span> are mutable, i.e. elements of a <span class="emphasis"><em>List</em></span> can be manipulated:</p> |
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<pre class="programlisting"> |
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>>> a = [1, 3, 5, 7] |
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>>> a[2] = 9 |
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>>> a |
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[1, 3, 9, 7]</pre> |
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</div> |
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<div class="section" title="1.1.8.Deleting Elements"> |
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<div class="titlepage"><div><div><h4 class="title"> |
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<a name="id2947590"></a>1.1.8.Deleting Elements</h4></div></div></div> |
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<p id="ch3list_tuples_d">An element or a slice of a <span class="emphasis"><em>List</em></span> can be deleted by using the <span class="strong"><strong>del</strong></span> statement:</p> |
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<pre class="programlisting"> |
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>>> a = [1, 3, 5, 7, 9, 11] |
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>>> del a[-2:] |
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>>> a |
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[1, 3, 5, 7] |
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>>> del a[1] |
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>>> a |
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[1, 5, 7]</pre> |
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</div> |
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<div class="section" title="1.1.9.Assign to Slices"> |
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<div class="titlepage"><div><div><h4 class="title"> |
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<a name="id2947617"></a>1.1.9.Assign to Slices</h4></div></div></div> |
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<p id="ch3list_tuples_e">In the same way, values can be assigned to individual elements of the <span class="emphasis"><em>List</em></span>, |
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a <span class="emphasis"><em>List</em></span> of elements can be assigned to a slice:</p> |
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<pre class="programlisting"> |
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>>> a = [2, 3, 4, 5] |
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>>> a[:2] = [0, 1] |
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[0, 1, 4, 5] |
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>>> a[2:2] = [2, 3] |
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>>> a |
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[0, 1, 2, 3, 4, 5] |
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>>> a[2:4] = [] |
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>>> a |
| 28 | 212 |
[0, 1, 4, 5]</pre> |
213 |
<p id="ch3list_tuples_f">The last two examples should be particularly noted carefully. The last but one |
|
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example insert elements or a list of elements into a <span class="emphasis"><em>List</em></span> and the last example |
|
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deletes a list of elements from the <span class="emphasis"><em>List</em></span>.</p> |
|
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</div> |
|
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</div> |
|
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<div class="section" title="1.2.None, Empty Lists, and Initialization"> |
|
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<div class="titlepage"><div><div><h3 class="title"> |
|
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<a name="id2947658"></a>1.2.None, Empty Lists, and Initialization</h3></div></div></div> |
|
221 |
<p id="ch3list_tuples_10">An <span class="emphasis"><em>Empty List</em></span> is a <span class="emphasis"><em>List</em></span> with no elements and is simply represented as |
|
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[]. A <span class="emphasis"><em>None List</em></span> is one with all elements in it being <span class="strong"><strong>None</strong></span>. It serves |
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the purpose having a container list of some fixed number of elements with |
| 28 | 224 |
no value:</p> |
225 |
<pre class="programlisting"> |
|
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>>> a = [] |
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>>> a |
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[] |
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>>> n = [None] * 10 |
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>>> n |
| 28 | 231 |
[None, None, None, None, None, None, None, None, None, None]</pre> |
232 |
</div> |
|
233 |
<div class="section" title="1.3.Nested Lists"> |
|
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<div class="titlepage"><div><div><h3 class="title"> |
|
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<a name="id2947698"></a>1.3.Nested Lists</h3></div></div></div> |
|
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<p id="ch3list_tuples_11">As mentioned earlier, a List can contain elements of any data type. This also |
|
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implies a <span class="emphasis"><em>List</em></span> can have a <span class="emphasis"><em>Lists</em></span> themselves as its elements. These are |
|
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called as <span class="emphasis"><em>Nested Lists</em></span>. There is no limit on the depth of the <span class="emphasis"><em>Nested Lists</em></span>:</p> |
|
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<pre class="programlisting"> |
|
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>>> a = [1, [1, 2, 3], 3, [1, [1, 2, 3]], 7]</pre> |
|
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</div> |
|
242 |
<div class="section" title="1.4.List Methods"> |
|
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<div class="titlepage"><div><div><h3 class="title"> |
|
244 |
<a name="id2947732"></a>1.4.List Methods</h3></div></div></div> |
|
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<p id="ch3list_tuples_12">A method is a function that is coupled to an object. More about objects |
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and its methods are discussed in Advanced Python module. In general, a |
| 28 | 247 |
method is called like:</p> |
248 |
<pre class="programlisting"> |
|
249 |
object.method(arguments)</pre> |
|
250 |
<p id="ch3list_tuples_13">For now, it is enough to know that a list of elements is an object and |
|
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so <span class="emphasis"><em>List</em></span> methods can be called upon them. Also some of the methods change |
|
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the <span class="emphasis"><em>List</em></span> in-place, meaning it modifies the existing list instead of creating |
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a new one, while other methods don't. It must be noted as we run through |
| 28 | 254 |
the <span class="emphasis"><em>List</em></span> methods.</p> |
255 |
<p id="ch3list_tuples_14">Some of the most commonly used <span class="emphasis"><em>List</em></span> methods are as follows:</p> |
|
256 |
<div class="section" title="1.4.1.append"> |
|
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<div class="titlepage"><div><div><h4 class="title"> |
|
258 |
<a name="id2947779"></a>1.4.1.append</h4></div></div></div> |
|
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<p id="ch3list_tuples_15">The <span class="emphasis"><em>append</em></span> method is used to append an object at the end of the list:</p> |
|
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<pre class="programlisting"> |
|
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>>> prime = [2, 3, 5] |
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>>> prime.append(7) |
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>>> prime |
| 28 | 264 |
[2, 3, 5, 7]</pre> |
265 |
<p id="ch3list_tuples_16">It is important to note that append changes the <span class="emphasis"><em>List</em></span> in-place.</p> |
|
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</div> |
|
267 |
<div class="section" title="1.4.2.count"> |
|
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<div class="titlepage"><div><div><h4 class="title"> |
|
269 |
<a name="id2947808"></a>1.4.2.count</h4></div></div></div> |
|
270 |
<p id="ch3list_tuples_17">The <span class="emphasis"><em>count</em></span> method returns the number of occurences of a particular element |
|
271 |
in a list:</p> |
|
272 |
<pre class="programlisting"> |
|
273 |
>>> [1, 4, 4, 9, 9, 9].count(9) |
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3 |
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>>> tlst = ['Python', 'is', 'a', 'beautiful', 'language'] |
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>>> tlst.count('Python')
|
| 28 | 277 |
1</pre> |
278 |
</div> |
|
279 |
<div class="section" title="1.4.3.extend"> |
|
280 |
<div class="titlepage"><div><div><h4 class="title"> |
|
281 |
<a name="id2947831"></a>1.4.3.extend</h4></div></div></div> |
|
282 |
<p id="ch3list_tuples_18">The <span class="emphasis"><em>extend</em></span> method extends the list on which it is called by the list supplied |
|
283 |
as argument to it:</p> |
|
284 |
<pre class="programlisting"> |
|
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>>> a = [1, 2, 3] |
|
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>>> b = [4, 5, 6] |
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>>> a.extend(b) |
| 28 | 288 |
[1, 2, 3, 4, 5, 6]</pre> |
289 |
<p id="ch3list_tuples_19">This is an in-place method. This method is equivalent to using the + operator, but |
|
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using the + operator returns a new list.</p> |
|
291 |
</div> |
|
292 |
<div class="section" title="1.4.4.index"> |
|
293 |
<div class="titlepage"><div><div><h4 class="title"> |
|
294 |
<a name="id2947859"></a>1.4.4.index</h4></div></div></div> |
|
295 |
<p id="ch3list_tuples_1a">The <span class="emphasis"><em>index</em></span> method returns the index position of the element in the list |
|
296 |
specified as argument:</p> |
|
297 |
<pre class="programlisting"> |
|
298 |
>>> a = [1, 2, 3, ,4, 5] |
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>>> a.index(4) |
| 28 | 300 |
3</pre> |
301 |
</div> |
|
302 |
<div class="section" title="1.4.5.insert"> |
|
303 |
<div class="titlepage"><div><div><h4 class="title"> |
|
304 |
<a name="id2947879"></a>1.4.5.insert</h4></div></div></div> |
|
305 |
<p id="ch3list_tuples_1b">The <span class="emphasis"><em>insert</em></span> method is used to insert an element specified as the second |
|
306 |
argument to the list at the position specified by the first argument:</p> |
|
307 |
<pre class="programlisting"> |
|
308 |
>>> a = ['Python', 'is', 'cool'] |
|
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>>> a.insert(2, 'so') |
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>>> a |
| 28 | 311 |
['Python', 'is', 'so', 'cool']</pre> |
312 |
<p id="ch3list_tuples_1c">The <span class="emphasis"><em>insert</em></span> method changes the <span class="emphasis"><em>List</em></span> in-place.</p> |
|
313 |
</div> |
|
314 |
<div class="section" title="1.4.6.pop"> |
|
315 |
<div class="titlepage"><div><div><h4 class="title"> |
|
316 |
<a name="id2947913"></a>1.4.6.pop</h4></div></div></div> |
|
317 |
<p id="ch3list_tuples_1d">The <span class="emphasis"><em>pop</em></span> method removes an element from the list. The index position |
|
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of the element to be removed can be specified as an argument to the |
| 28 | 319 |
<span class="emphasis"><em>pop</em></span> method, if not it removes the last element by default:</p> |
320 |
<pre class="programlisting"> |
|
321 |
>>> a = [1, 2, 3, 4, 5] |
|
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>>> a.pop() |
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>>> a |
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5 |
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>>> a.pop(2) |
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326 |
>>> a |
| 28 | 327 |
3</pre> |
328 |
<p id="ch3list_tuples_1e">The <span class="emphasis"><em>pop</em></span> method changes the <span class="emphasis"><em>List</em></span> in-place.</p> |
|
329 |
</div> |
|
330 |
<div class="section" title="1.4.7.remove"> |
|
331 |
<div class="titlepage"><div><div><h4 class="title"> |
|
332 |
<a name="id2947951"></a>1.4.7.remove</h4></div></div></div> |
|
333 |
<p id="ch3list_tuples_1f">The <span class="emphasis"><em>remove</em></span> method removes the first occurence of an element supplied as a |
|
334 |
parameter:</p> |
|
335 |
<pre class="programlisting"> |
|
336 |
>>> a = [1, 2, 3, 4, 2, 5, 2] |
|
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>>> a.remove(2) |
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338 |
>>> a |
| 28 | 339 |
[1, 3, 4, 2, 5, 2]</pre> |
340 |
</div> |
|
341 |
<div class="section" title="1.4.8.reverse"> |
|
342 |
<div class="titlepage"><div><div><h4 class="title"> |
|
343 |
<a name="id2947972"></a>1.4.8.reverse</h4></div></div></div> |
|
344 |
<p id="ch3list_tuples_20">The <span class="emphasis"><em>reverse</em></span> method reverses elements in the list. It is important to note |
|
345 |
here that <span class="emphasis"><em>reverse</em></span> method changes the list in-place and doesn't return any |
|
346 |
thing:</p> |
|
347 |
<pre class="programlisting"> |
|
348 |
>>> a = ['guido', 'alex', 'tim'] |
|
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>>> a.reverse() |
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350 |
>>> a |
| 28 | 351 |
['tim', 'alex', 'guido']</pre> |
352 |
</div> |
|
353 |
<div class="section" title="1.4.9.sort"> |
|
354 |
<div class="titlepage"><div><div><h4 class="title"> |
|
355 |
<a name="id2947998"></a>1.4.9.sort</h4></div></div></div> |
|
356 |
<p id="ch3list_tuples_21">The <span class="emphasis"><em>sort</em></span> method is used to sort the elements of the list. The <span class="emphasis"><em>sort</em></span> method |
|
357 |
also sorts in-place and does not return anything:</p> |
|
358 |
<pre class="programlisting"> |
|
359 |
>>> a = [5, 1, 3, 7, 4] |
|
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>>> a.sort() |
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361 |
>>> a |
| 28 | 362 |
[1, 3, 4, 5, 7]</pre> |
363 |
<p id="ch3list_tuples_22">In addition to the sort method on a <span class="emphasis"><em>List</em></span> object we can also use the built-in |
|
364 |
<span class="strong"><strong>sorted</strong></span> function. This function takes the <span class="emphasis"><em>List</em></span> as a parameter and returns |
|
365 |
a sorted copy of the list. However the original list is left intact:</p> |
|
366 |
<pre class="programlisting"> |
|
367 |
>>> a = [5, 1, 3, 7, 4] |
|
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>>> b = sorted(a) |
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>>> b |
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[1, 3, 4, 5, 7] |
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371 |
>>> a |
| 28 | 372 |
[5, 1, 3, 7, 4]</pre> |
373 |
</div> |
|
374 |
</div> |
|
375 |
</div> |
|
376 |
<div class="section" title="2.Tuples"> |
|
377 |
<div class="titlepage"><div><div><h2 class="title" style="clear: both"> |
|
378 |
<a name="id2948048"></a>2.Tuples</h2></div></div></div> |
|
379 |
<p id="ch3list_tuples_23"><span class="emphasis"><em>Tuples</em></span> are sequences just like <span class="emphasis"><em>Lists</em></span>, but they are immutable. In other |
|
380 |
words <span class="emphasis"><em>Tuples</em></span> provides a way to represent a group of items, where the group |
|
381 |
of items cannot be changed in any way. The syntax of a <span class="emphasis"><em>Tuple</em></span> is also very |
|
382 |
similar to <span class="emphasis"><em>List</em></span>. A <span class="emphasis"><em>Tuple</em></span> is represented with the list of items, called |
|
383 |
elements of the <span class="emphasis"><em>Tuple</em></span> separated by comma, with the entire list being enclosed |
|
384 |
in parenthesis. It is not compulsory to use parenthesis around a <span class="emphasis"><em>Tuple</em></span> but |
|
385 |
it may be necessary in some of the cases:</p> |
|
386 |
<pre class="programlisting"> |
|
387 |
>>> a = 1, 2, 3 |
|
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388 |
>>> a |
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389 |
(1, 2, 3) |
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390 |
>>> b = 1, |
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391 |
>>> b |
| 28 | 392 |
(1,)</pre> |
393 |
<p id="ch3list_tuples_24">It is interesting to note the second example. Just a value followed by a comma |
|
394 |
automatically makes that an element of a <span class="emphasis"><em>Tuple</em></span> with only one element. It is |
|
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also important to note that, irrespective of input having a parenthesis, the |
| 28 | 396 |
output always has a parenthesis.</p> |
397 |
<p id="ch3list_tuples_25">The first example is also known as <span class="emphasis"><em>Tuple packing</em></span>, because values are being |
|
398 |
packed into a tuple. It is also possible to do <span class="emphasis"><em>Tuple unpacking</em></span> which is more |
|
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399 |
interesting. It is better to understand that by example. Say we have a |
| 28 | 400 |
co-ordinate pair from which we need to separate x and y co-ordinates:</p> |
401 |
<pre class="programlisting"> |
|
402 |
>>> a = (1, 2) |
|
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403 |
>>> x, y = a |
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404 |
>>> x |
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405 |
1 |
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406 |
>>> y |
| 28 | 407 |
2</pre> |
408 |
<p id="ch3list_tuples_26"><span class="emphasis"><em>Tuple unpacking</em></span> also has several other use-cases of which the most interesting |
|
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one is to swap the values of two variables. Using programming languages like C |
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would require anywhere around 10 lines of code and an extra temporary variable |
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411 |
to do this (including all the #include stuff). Python does it in the most |
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412 |
intuitive way in just one line. Say we want to swap the co-ordinates in the |
| 28 | 413 |
above example:</p> |
414 |
<pre class="programlisting"> |
|
415 |
>>> x, y = y, x |
|
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416 |
>>> x |
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2 |
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>>> y |
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1</pre> |
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<div class="section" title="2.1.Common Tuple Operations"> |
|
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<div class="titlepage"><div><div><h3 class="title"> |
|
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<a name="id2948145"></a>2.1.Common Tuple Operations</h3></div></div></div> |
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<p id="ch3list_tuples_27">There is no need to introduce all the <span class="emphasis"><em>Tuple</em></span> operations again, since <span class="emphasis"><em>Tuples</em></span> |
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support the following operations that <span class="emphasis"><em>List</em></span> supports in exactly the same way:</p> |
|
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<div class="itemizedlist"><ul class="itemizedlist" type="*"> |
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<li class="listitem" style="list-style-type: *"><p id="ch3list_tuples_28">Indexing</p></li> |
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<li class="listitem" style="list-style-type: *"><p id="ch3list_tuples_29">Concatenating</p></li> |
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<li class="listitem" style="list-style-type: *"><p id="ch3list_tuples_2a">Slicing</p></li> |
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<li class="listitem" style="list-style-type: *"><p id="ch3list_tuples_2b">Membership</p></li> |
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<li class="listitem" style="list-style-type: *"><p id="ch3list_tuples_2c">Multiplication</p></li> |
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<li class="listitem" style="list-style-type: *"><p id="ch3list_tuples_2d">Length, Maximum, Minimum</p></li> |
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</ul></div> |
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<p id="ch3list_tuples_2e">The following examples illustrate the above operations:</p> |
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<pre class="programlisting"> |
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>>> a = (1, 2, 3, 4, 5, 6) |
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>>> a[5] |
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6 |
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>>> b = (7, 8, 9) |
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>>> a + b |
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(1, 2, 3, 4, 5, 6, 7, 8, 9) |
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>>> a[3:5] |
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(4, 5) |
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>>> 5 in a |
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True |
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>>> c = (1,) |
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>>> c * 5 |
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(1, 1, 1, 1, 1) |
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>>> len(a) |
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6 |
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>>> max(a) |
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6 |
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>>> min(a) |
| 28 | 453 |
1</pre> |
454 |
<p id="ch3list_tuples_2f">However the following <span class="emphasis"><em>List</em></span> operations are not supported by <span class="emphasis"><em>Tuples</em></span> because |
|
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<span class="emphasis"><em>Tuples</em></span> cannot be changed once they are created:</p> |
|
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<div class="itemizedlist"><ul class="itemizedlist" type="*"> |
|
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<li class="listitem" style="list-style-type: *"><p id="ch3list_tuples_30">Changing elements</p></li> |
|
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<li class="listitem" style="list-style-type: *"><p id="ch3list_tuples_31">Deleting elements</p></li> |
|
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<li class="listitem" style="list-style-type: *"><p id="ch3list_tuples_32">Assigning to slices</p></li> |
|
460 |
</ul></div> |
|
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<p id="ch3list_tuples_33">Similarity to <span class="emphasis"><em>Lists</em></span> leads to the questions like, why not <span class="emphasis"><em>Lists</em></span> only? Why do |
|
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we even want <span class="emphasis"><em>Tuples</em></span>? Can we do the same with <span class="emphasis"><em>Lists</em></span>? And the answer is <span class="strong"><strong>Yes</strong></span> |
|
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we can do it, but <span class="emphasis"><em>Tuples</em></span> are helpful at times, like we can return Tuples from |
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functions. They are also returned by some built-in functions and methods. And |
| 28 | 465 |
also there are some use cases like co-ordinate among other things. So <span class="emphasis"><em>Tuples</em></span> |
466 |
are helpful.</p> |
|
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</div> |
|
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</div> |
|
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<div class="section" title="3.Additional Syntax"> |
|
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<div class="titlepage"><div><div><h2 class="title" style="clear: both"> |
|
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<a name="id2948295"></a>3.Additional Syntax</h2></div></div></div> |
|
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<p id="ch3list_tuples_34">The following additional syntax are introduced to make it easier to operate on |
|
473 |
<span class="emphasis"><em>Lists</em></span>.</p> |
|
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<div class="section" title="3.1.range()"> |
|
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<div class="titlepage"><div><div><h3 class="title"> |
|
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<a name="id2948312"></a>3.1.range()</h3></div></div></div> |
|
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<p id="ch3list_tuples_35">The <span class="emphasis"><em>range</em></span> function takes at least one argument and 2 additional optional |
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arguments. If two or more arguments are specified, the range function returns |
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a list of natural numbers starting from the first argument passed to it to the |
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second argument. The third argument, if specified is used as a step. Suppose |
| 28 | 481 |
only one argument is specified, then <span class="emphasis"><em>range</em></span> function returns a list of natural |
482 |
numbers starting from 0 upto the argument specified:</p> |
|
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<pre class="programlisting"> |
|
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>>> range(5, 10, 2) |
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[5, 7, 9] |
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>>> range(2, 15) |
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[2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14] |
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>>> range(12) |
| 28 | 489 |
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11]</pre> |
490 |
</div> |
|
491 |
<div class="section" title="3.2.for"> |
|
492 |
<div class="titlepage"><div><div><h3 class="title"> |
|
493 |
<a name="id2948344"></a>3.2.for</h3></div></div></div> |
|
494 |
<p id="ch3list_tuples_36">The <span class="strong"><strong>for</strong></span> keyword is used as a part of the looping construct. Unlike for loops |
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in other languages, Python's for is used to iterate through the elements of |
| 28 | 496 |
sequences like <span class="emphasis"><em>Lists</em></span>, <span class="emphasis"><em>Tuples</em></span>, <span class="emphasis"><em>Dictionaries</em></span>, etc. The syntax of the for loop |
497 |
consists of <span class="strong"><strong>for</strong></span>, followed by a variable to hold the individual or the current |
|
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element of the list during iteration and <span class="strong"><strong>in</strong></span>, followed by the sequence and a |
|
499 |
semicolon(':') The next line which is part of the <span class="strong"><strong>for</strong></span> loop, i.e the statements
|
|
500 |
that are part of the loop should start with a new intend:</p> |
|
501 |
<pre class="programlisting"> |
|
502 |
>>> names = ['Guido', 'Alex', 'Tim'] |
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>>> for name in names: |
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... print "Name =", name |
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... |
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Name = Guido |
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Name = Alex |
| 28 | 508 |
Name = Tim</pre> |
509 |
</div> |
|
510 |
</div> |
|
511 |
<div class="section" title="4.Conclusion"> |
|
512 |
<div class="titlepage"><div><div><h2 class="title" style="clear: both"> |
|
513 |
<a name="id2948403"></a>4.Conclusion</h2></div></div></div> |
|
514 |
<p id="ch3list_tuples_37">This section on <span class="emphasis"><em>Lists</em></span> and <span class="emphasis"><em>Tuples</em></span> introduces almost all the necessary |
|
515 |
machinary required to work on <span class="emphasis"><em>Lists</em></span> and <span class="emphasis"><em>Tuples</em></span>. Topics like how to |
|
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use these data structures in bigger more useful programs will be introduced |
| 28 | 517 |
in the subsequent chapters.</p> |
518 |
</div> |
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519 |
</div> |
|
520 |
</div></body> |
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</html> |
| 28 | 522 |