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<head><meta http-equiv="Content-Type" content="text/html; charset=utf-8" /><title>Chapter. intro</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="ch2intro"> |
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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="#id2915639">Basic Python</a></span></dt> |
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<dd><dl> |
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<dt><span class="section"><a href="#id2723402">1. Introduction</a></span></dt> |
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<dt><span class="section"><a href="#id2966769">2. The Python Interpreter</a></span></dt> |
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<dd><dl> |
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<dt><span class="section"><a href="#id2966777">2.1. The Interactive Interpreter</a></span></dt> |
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<dt><span class="section"><a href="#id2966950">2.2. <span class="emphasis"><em>ipython</em></span> - An enhanced interactive Python interpreter</a></span></dt> |
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</dl></dd> |
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<dt><span class="section"><a href="#id2967075">3. Editing and running a python file</a></span></dt> |
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<dt><span class="section"><a href="#id2967182">4. Basic Datatypes and operators in Python</a></span></dt> |
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<dd><dl> |
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<dt><span class="section"><a href="#id2967216">4.1. Numbers</a></span></dt> |
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<dt><span class="section"><a href="#id2967346">4.2. Variables</a></span></dt> |
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<dt><span class="section"><a href="#id2967394">4.3. Strings</a></span></dt> |
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<dt><span class="section"><a href="#id2967588">4.4. Boolean</a></span></dt> |
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</dl></dd> |
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<dt><span class="section"><a href="#id2967617">5. The <span class="strong"><strong>while</strong></span> loop</a></span></dt> |
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<dt><span class="section"><a href="#id2967654">6. The <span class="strong"><strong>if</strong></span> conditional</a></span></dt> |
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<dt><span class="section"><a href="#id2967723">7. <span class="strong"><strong>raw_input()</strong></span></a></span></dt> |
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<dt><span class="section"><a href="#id2967877">8. <span class="strong"><strong>int()</strong></span> method</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="Basic Python"> |
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<div class="titlepage"> |
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<div><div><h2 class="title"> |
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<a name="id2915639"></a>Basic Python</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="#id2723402">1. Introduction</a></span></dt> |
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<dt><span class="section"><a href="#id2966769">2. The Python Interpreter</a></span></dt> |
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<dd><dl> |
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<dt><span class="section"><a href="#id2966777">2.1. The Interactive Interpreter</a></span></dt> |
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<dt><span class="section"><a href="#id2966950">2.2. <span class="emphasis"><em>ipython</em></span> - An enhanced interactive Python interpreter</a></span></dt> |
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</dl></dd> |
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<dt><span class="section"><a href="#id2967075">3. Editing and running a python file</a></span></dt> |
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<dt><span class="section"><a href="#id2967182">4. Basic Datatypes and operators in Python</a></span></dt> |
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<dd><dl> |
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<dt><span class="section"><a href="#id2967216">4.1. Numbers</a></span></dt> |
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<dt><span class="section"><a href="#id2967346">4.2. Variables</a></span></dt> |
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<dt><span class="section"><a href="#id2967394">4.3. Strings</a></span></dt> |
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<dt><span class="section"><a href="#id2967588">4.4. Boolean</a></span></dt> |
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</dl></dd> |
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<dt><span class="section"><a href="#id2967617">5. The <span class="strong"><strong>while</strong></span> loop</a></span></dt> |
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<dt><span class="section"><a href="#id2967654">6. The <span class="strong"><strong>if</strong></span> conditional</a></span></dt> |
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<dt><span class="section"><a href="#id2967723">7. <span class="strong"><strong>raw_input()</strong></span></a></span></dt> |
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<dt><span class="section"><a href="#id2967877">8. <span class="strong"><strong>int()</strong></span> method</a></span></dt> |
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</dl> |
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</div> |
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<p id="ch2intro_1">This document is intended to be handed out at the end of the workshop. It has |
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been designed for Engineering students who are Python beginners and have basic |
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programming skills. The focus is on basic numerics and plotting using Python.</p>The system requirements:<div class="itemizedlist"><ul class="itemizedlist" type="*"> |
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<li class="listitem" style="list-style-type: *"><p id="ch2intro_2">Python - version 2.5.x or newer.</p></li> |
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<li class="listitem" style="list-style-type: *"><p id="ch2intro_3">IPython</p></li> |
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<li class="listitem" style="list-style-type: *"><p id="ch2intro_4">Text editor - scite, vim, emacs or whatever you are comfortable with.</p></li> |
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</ul></div> |
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<div class="section" title="1.Introduction"> |
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<div class="titlepage"><div><div><h2 class="title" style="clear: both"> |
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<a name="id2723402"></a>1.Introduction</h2></div></div></div> |
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<p id="ch2intro_5">The Python programming language was created by a dutch named Guido van Rossum. |
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The idea of Python was conceived in December 1989. The name Python has nothing |
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to do with the reptilian, but its been named after the 70s comedy series |
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"Monty Python's Flying Circus", since it happens to be Guido's favourite |
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TV series.</p> |
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<p id="ch2intro_6">Current stable version of Python is 2.6.x, although Python 3.0 is also the stable |
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version, it is not backwards compatible with the previous versions and is hence |
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not entirely popular at the moment. This material will focus on the 2.6.x series.</p> |
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<p id="ch2intro_7">Python is licensed under the Python Software Foundation License (PSF License) |
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which is GPL compatible Free Software license (excepting license version 1.6 and 2.0) |
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It is a no strings attached license, which means the source code is free to modify |
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and redistribute.</p> |
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<p id="ch2intro_8">The Python docs define Python as "Python is an interpreted, object-oriented, |
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high-level programming language with dynamic semantics." A more detailed summary |
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can be found at </p> |
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<div class="reference"> |
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<div class="titlepage"><hr /></div>http://www.python.org/doc/essays/blurb.html</div> |
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<p>. Python is a language that |
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has been designed to help the programmer concentrate on solving the problem at hand |
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and not worry about the programming language idiosyncrasies.</p> |
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<p id="ch2intro_9">Python is a highly cross platform compatible language on account of it being an |
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interpreted language. It is highly scalable and hence has been adapted to run on |
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the Nokia 60 series phones. Python has been designed to be readable and easy to use</p> |
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<p id="ch2intro_a"><span class="strong"><strong>Resources available for reference</strong></span></p> |
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<div class="itemizedlist"><ul class="itemizedlist" type="*"> |
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<li class="listitem" style="list-style-type: *"> |
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<p id="ch2intro_b">Web: </p> |
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<div class="reference"> |
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<div class="titlepage"><hr /></div>http://www.python.org</div> |
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</li> |
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<li class="listitem" style="list-style-type: *"> |
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<p id="ch2intro_c">Doc: </p> |
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<div class="reference"> |
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<div class="titlepage"><hr /></div>http://www.python.org/doc</div> |
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</li> |
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<li class="listitem" style="list-style-type: *">Free Tutorials:<div class="itemizedlist"><ul class="itemizedlist" type="*"> |
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<li class="listitem" style="list-style-type: *"> |
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<p id="ch2intro_d">Official Python Tutorial: </p> |
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<div class="reference"> |
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<div class="titlepage"><hr /></div>http://docs.python.org/tut/tut.html</div> |
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</li> |
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<li class="listitem" style="list-style-type: *"> |
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<p id="ch2intro_e">Byte of Python: </p> |
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<div class="reference"> |
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<div class="titlepage"><hr /></div>http://www.byteofpython.info/</div> |
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</li> |
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<li class="listitem" style="list-style-type: *"> |
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<p id="ch2intro_f">Dive into Python: </p> |
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<div class="reference"> |
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<div class="titlepage"><hr /></div>http://diveintopython.org/</div> |
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</li> |
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</ul></div> |
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</li> |
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</ul></div> |
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<p id="ch2intro_10"><span class="strong"><strong>Advantages of Python - Why Python??</strong></span></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="ch2intro_11">Python has been designed for readability and ease of use. Its been designed in |
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such a fashion that it imposes readability on the programmer. Python does away |
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with the braces and the semicolons and instead implements code blocks based on |
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indentation, thus enhancing readability.</p></li> |
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<li class="listitem" style="list-style-type: *"><p id="ch2intro_12">Python is a high level, interpreted, modular and object oriented language. |
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Python performs memory management on its own, thus the programmer need not bother |
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about allocating and deallocating memory to variables. Python provides extensibility |
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by providing modules which can be easily imported similar to headers in C and |
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packages in Java. Python is object oriented and hence provides all the object oriented |
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characteristics such as inheritance, encapsulation and polymorphism.</p></li> |
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<li class="listitem" style="list-style-type: *"><p id="ch2intro_13">Python offers a highly powerful interactive programming interface in the form |
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of the 'Interactive Interpreter' which will be discussed in more detail in the |
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following sections.</p></li> |
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<li class="listitem" style="list-style-type: *"> |
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<p id="ch2intro_14">Python provides a rich standard library and an extensive set of modules. The |
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power of Python modules can be seen in this slightly exaggerated cartoon |
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</p> |
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<div class="reference"> |
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<div class="titlepage"><hr /></div>http://xkcd.com/353/</div> |
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</li> |
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<li class="listitem" style="list-style-type: *"><p id="ch2intro_15">Python interfaces well with most other programming languages such as C, C++ |
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and FORTRAN.</p></li> |
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</ul></div> |
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<p id="ch2intro_16">Although, Python has one setback. Python is not fast as some of the compiled |
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languages like C or C++. Yet, the amount of flexibility and power more than make |
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up for this setback.</p> |
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</div> |
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<div class="section" title="2.The Python Interpreter"> |
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<div class="titlepage"><div><div><h2 class="title" style="clear: both"> |
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<a name="id2966769"></a>2.The Python Interpreter</h2></div></div></div> |
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<div class="section" title="2.1.The Interactive Interpreter"> |
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<div class="titlepage"><div><div><h3 class="title"> |
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<a name="id2966777"></a>2.1.The Interactive Interpreter</h3></div></div></div> |
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<p id="ch2intro_17">Typing <span class="emphasis"><em>python</em></span> at the shell prompt on any standard Unix/Gnu-Linux system and |
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hitting the enter key fires up the Python 'Interactive Interpreter'. The Python |
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interpreter is one of the most integral features of Python. The prompt obtained |
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when the interactive interpreter is similar to what is shown below. The exact |
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appearance might differ based on the version of Python being used. The <code class="literal">>>></code> |
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thing shown is the python prompt. When something is typed at the prompt and the |
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enter key is hit, the python interpreter interprets the command entered and |
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performs the appropriate action. All the examples presented in this document are |
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to be tried hands on, on the interactive interpreter.</p> |
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<pre class="programlisting"> |
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Python 2.5.2 (r252:60911, Oct 5 2008, 19:24:49) |
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[GCC 4.3.2] on linux2 |
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Type "help", "copyright", "credits" or "license" for more information. |
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>>></pre> |
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<p id="ch2intro_18">Lets try with an example, type <code class="literal">print 'Hello, World!'</code> at the prompt and hit |
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the enter key.</p> |
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<pre class="programlisting"> |
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>>> print 'Hello, World!' |
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Hello, World!</pre> |
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<p id="ch2intro_19">This example was quite straight forward, and thus we have written our first |
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line of Python code. Now let us try typing something arbitrary at the prompt. |
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For example:</p> |
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<pre class="programlisting"> |
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>>> arbit word |
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File "<stdin>", line 1 |
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arbit word |
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^ |
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SyntaxError: invalid syntax |
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>>></pre> |
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<p id="ch2intro_1a">The interpreter gave an error message saying that 'arbit word' was invalid |
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syntax which is valid. The interpreter is an amazing tool when learning to |
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program in Python. The interpreter provides a help function that provides the |
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necessary documentation regarding all Python syntax, constructs, modules and |
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objects. Typing <span class="emphasis"><em>help()</em></span> at the prompt gives the following output:</p> |
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<pre class="programlisting"> |
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>>> help() |
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Welcome to Python 2.5! This is the online help utility. |
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If this is your first time using Python, you should definitely check out |
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the tutorial on the Internet at http://www.python.org/doc/tut/. |
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Enter the name of any module, keyword, or topic to get help on writing |
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Python programs and using Python modules. To quit this help utility and |
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return to the interpreter, just type "quit". |
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To get a list of available modules, keywords, or topics, type "modules", |
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"keywords", or "topics". Each module also comes with a one-line summary |
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of what it does; to list the modules whose summaries contain a given word |
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such as "spam", type "modules spam". |
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help></pre> |
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<p id="ch2intro_1b">As mentioned in the output, entering the name of any module, keyword or topic |
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will provide the documentation and help regarding the same through the online |
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help utility. Pressing <span class="emphasis"><em>Ctrl+d</em></span> exits the help prompt and returns to the |
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python prompt.</p> |
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<p id="ch2intro_1c">Let us now try a few examples at the python interpreter.</p> |
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<p id="ch2intro_1d">Eg 1:</p> |
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<pre class="programlisting"> |
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>>> print 'Hello, python!' |
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Hello, python! |
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>>></pre> |
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<p id="ch2intro_1e">Eg 2:</p> |
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<pre class="programlisting"> |
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>>> print 4321*567890 |
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2453852690 |
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>>></pre> |
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<p id="ch2intro_1f">Eg 3:</p> |
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<pre class="programlisting"> |
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>>> 4321*567890 |
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2453852690L |
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>>></pre> |
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<pre class="programlisting"> |
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Note: Notice the 'L' at the end of the output. The 'L' signifies that the |
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output of the operation is of type *long*. It was absent in the previous |
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example because we used the print statement. This is because *print* formats |
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the output before displaying.</pre> |
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<p id="ch2intro_20">Eg 4:</p> |
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<pre class="programlisting"> |
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>>> big = 12345678901234567890 ** 3 |
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>>> print big |
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1881676372353657772490265749424677022198701224860897069000 |
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>>></pre> |
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<pre class="programlisting"> |
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This example is to show that unlike in C or C++ there is no limit on the |
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value of an integer.</pre> |
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<p id="ch2intro_21">Try this on the interactive interpreter: |
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<code class="literal">import this</code></p> |
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<p id="ch2intro_22"><span class="emphasis"><em>Hint: The output gives an idea of Power of Python</em></span></p> |
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</div> |
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<div class="section" title="2.2.ipython - An enhanced interactive Python interpreter"> |
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<div class="titlepage"><div><div><h3 class="title"> |
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<a name="id2966950"></a>2.2.<span class="emphasis"><em>ipython</em></span> - An enhanced interactive Python interpreter</h3></div></div></div> |
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<p id="ch2intro_23">The power and the importance of the interactive interpreter was the highlight |
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of the previous section. This section provides insight into the enhanced |
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interpreter with more advanced set of features called <span class="strong"><strong>ipython</strong></span>. Entering |
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<span class="emphasis"><em>ipython</em></span> at the shell prompt fires up the interactive interpreter.</p> |
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<pre class="programlisting"> |
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$ ipython |
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Python 2.5.2 (r252:60911, Oct 5 2008, 19:24:49) |
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Type "copyright", "credits" or "license" for more information. |
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IPython 0.8.4 -- An enhanced Interactive Python. |
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? -> Introduction and overview of IPython's features. |
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%quickref -> Quick reference. |
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help -> Python's own help system. |
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object? -> Details about 'object'. ?object also works, ?? prints more. |
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In [1]:</pre> |
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<p id="ch2intro_24">This is the output obtained upon firing ipython. The exact appearance may |
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change based on the Python version installed. The following are some of the |
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various features provided by <span class="strong"><strong>ipython</strong></span>:</p> |
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<p id="ch2intro_25">Suggestions - ipython provides suggestions of the possible methods and |
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operations available for the given python object.</p> |
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<p id="ch2intro_26">Eg 5:</p> |
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<pre class="programlisting"> |
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In [4]: a = 6 |
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In [5]: a. |
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a.__abs__ a.__divmod__ a.__index__ a.__neg__ a.__rand__ a.__rmod__ a.__rxor__ |
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a.__add__ a.__doc__ a.__init__ a.__new__ a.__rdiv__ a.__rmul__ a.__setattr__ |
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a.__and__ a.__float__ a.__int__ a.__nonzero__ a.__rdivmod__ a.__ror__ a.__str__ |
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a.__class__ a.__floordiv__ a.__invert__ a.__oct__ a.__reduce__ a.__rpow__ a.__sub__ |
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a.__cmp__ a.__getattribute__ a.__long__ a.__or__ a.__reduce_ex__ a.__rrshift__ a.__truediv__ |
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a.__coerce__ a.__getnewargs__ a.__lshift__ a.__pos__ a.__repr__ a.__rshift__ a.__xor__ |
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a.__delattr__ a.__hash__ a.__mod__ a.__pow__ a.__rfloordiv__ a.__rsub__ |
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a.__div__ a.__hex__ a.__mul__ a.__radd__ a.__rlshift__ a.__rtruediv__</pre> |
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<p id="ch2intro_27">In this example, we initialized 'a' (a variable - a concept that will be |
|
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discussed in the subsequent sections.) to 6. In the next line when the <span class="emphasis"><em>tab</em></span> key |
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is pressed after typing '<span class="emphasis"><em>a.</em></span>' ipython displays the set of all possible methods |
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that are applicable on the object 'a' (an integer in this context). Ipython |
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provides many such datatype specific features which will be presented in the |
| 28 | 291 |
further sections as and when the datatypes are introduced.</p> |
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</div> |
|
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</div> |
|
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<div class="section" title="3.Editing and running a python file"> |
|
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<div class="titlepage"><div><div><h2 class="title" style="clear: both"> |
|
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<a name="id2967075"></a>3.Editing and running a python file</h2></div></div></div> |
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<p id="ch2intro_28">The previous sections focused on the use of the interpreter to run python code. |
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While the interpeter is an excellent tool to test simple solutions and |
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experiment with small code snippets, its main disadvantage is that everything |
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written in the interpreter is lost once its quit. Most of the times a program is |
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used by people other than the author. So the programs have to be available in |
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some form suitable for distribution, and hence they are written in files. This |
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section will focus on editing and running python files. Start by opening a text |
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editor ( it is recommended you choose one from the list at the top of this page ). |
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In the editor type down python code and save the file with an extension <span class="strong"><strong>.py</strong></span> |
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(python files have an extension of .py). Once done with the editing, save the |
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file and exit the editor.</p> |
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<p id="ch2intro_29">Let us look at a simple example of calculating the gcd of 2 numbers using Python:</p> |
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<p id="ch2intro_2a"><span class="strong"><strong>Creating the first python script(file)</strong></span></p> |
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<pre class="programlisting"> |
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$ emacs gcd.py |
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def gcd(x,y): |
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if x % y == 0: |
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return y |
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return gcd(y, x%y) |
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print gcd(72, 92)</pre> |
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<p id="ch2intro_2b">To run the script, open the shell prompt, navigate to the directory that |
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contains the python file and run <code class="literal">python <filename.py></code> at the prompt ( in this |
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case filename is gcd.py )</p> |
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<p id="ch2intro_2c"><span class="strong"><strong>Running the python script</strong></span></p> |
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<pre class="programlisting"> |
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$ python gcd.py |
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4 |
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$</pre> |
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<p id="ch2intro_2d">Another method to run a python script would be to include the line</p> |
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<p id="ch2intro_2e"><code class="literal">#! /usr/bin/python</code></p> |
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<p id="ch2intro_2f">at the beginning of the python file and then make the file executable by</p> |
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<p id="ch2intro_30">$ chmod a+x <span class="emphasis"><em>filename.py</em></span></p> |
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<p id="ch2intro_31">Once this is done, the script can be run as a standalone program as follows:</p> |
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<p id="ch2intro_32">$ ./<span class="emphasis"><em>filename.py</em></span></p> |
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</div> |
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<div class="section" title="4.Basic Datatypes and operators in Python"> |
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<div class="titlepage"><div><div><h2 class="title" style="clear: both"> |
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<a name="id2967182"></a>4.Basic Datatypes and operators in Python</h2></div></div></div> |
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<p id="ch2intro_33">Python provides the following set of basic datatypes.</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="ch2intro_34">Numbers: int, float, long, complex</p></li> |
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<li class="listitem" style="list-style-type: *"><p id="ch2intro_35">Strings</p></li> |
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<li class="listitem" style="list-style-type: *"><p id="ch2intro_36">Boolean</p></li> |
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</ul></div> |
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<div class="section" title="4.1.Numbers"> |
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<div class="titlepage"><div><div><h3 class="title"> |
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<a name="id2967216"></a>4.1.Numbers</h3></div></div></div> |
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<p id="ch2intro_37">Numbers were introduced in the examples presented in the interactive interpreter |
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section. Numbers include types as mentioned earlier viz., int (integers), float |
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(floating point numbers), long (large integers), complex (complex numbers with |
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real and imaginary parts). Python is not a strongly typed language, which means |
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the type of a variable need not mentioned during its initialization. Let us look |
| 28 | 350 |
at a few examples.</p> |
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<p id="ch2intro_38">Eg 6:</p> |
|
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<pre class="programlisting"> |
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>>> a = 1 #here a is an integer variable</pre> |
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<p id="ch2intro_39">Eg 7:</p> |
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<pre class="programlisting"> |
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>>> lng = 122333444455555666666777777788888888999999999 #here lng is a variable of type long |
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>>> lng |
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122333444455555666666777777788888888999999999L #notice the trailing 'L' |
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>>> print lng |
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122333444455555666666777777788888888999999999 #notice the absence of the trailing 'L' |
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>>> lng+1 |
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122333444455555666666777777788888889000000000L</pre> |
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<p id="ch2intro_3a">Long numbers are the same as integers in almost all aspects. They can be used in |
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operations just like integers and along with integers without any distinction. |
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The only distinction comes during type checking (which is not a healthy practice). |
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Long numbers are tucked with a trailing 'L' just to signify that they are long. |
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Notice that in the example just lng at the prompt displays the value of the variable |
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with the 'L' whereas <code class="literal">print lng</code> displays without the 'L'. This is because print |
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formats the output before printing. Also in the example, notice that adding an |
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integer to a long does not give any errors and the result is as expected. So for |
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all practical purposes longs can be treated as ints.</p> |
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<p id="ch2intro_3b">Eg 8:</p> |
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<pre class="programlisting"> |
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>>> fl = 3.14159 #fl is a float variable |
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>>> e = 1.234e-4 #e is also a float variable, specified in the exponential form |
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>>> a = 1 |
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>>> b = 2 |
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>>> a/b #integer division |
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0 |
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>>> a/fl #floating point division |
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0.31831015504887655 |
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>>> e/fl |
| 28 | 383 |
3.9279473133031364e-05</pre> |
384 |
<p id="ch2intro_3c">Floating point numbers, simply called floats are real numbers with a decimal point. |
|
|
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The example above shows the initialization of a float variable. Shown also in this |
|
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example is the difference between integer division and floating point division. |
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'a' and 'b' here are integer variables and hence the division gives 0 as the quotient. |
|
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When either of the operands is a float, the operation is a floating point division, |
| 28 | 389 |
and the result is also a float as illustrated.</p> |
390 |
<p id="ch2intro_3d">Eg 9:</p> |
|
391 |
<pre class="programlisting"> |
|
392 |
>>> cplx = 3 + 4j #cplx is a complex variable |
|
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|
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>>> cplx |
|
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(3+4j) |
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|
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>>> print cplx.real #prints the real part of the complex number |
|
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3.0 |
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>>> print cplx.imag #prints the imaginary part of the complex number |
|
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4.0 |
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>>> print cplx*fl #multiplies the real and imag parts of the complex number with the multiplier |
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(9.42477+12.56636j) |
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>>> abs(cplx) #returns the absolute value of the complex number |
| 28 | 402 |
5.0</pre> |
403 |
<p id="ch2intro_3e">Python provides a datatype for complex numbers. Complex numbers are initialized |
|
404 |
as shown in the example above. The <span class="emphasis"><em>real</em></span> and <span class="emphasis"><em>imag</em></span> operators return the real and |
|
405 |
imaginary parts of the complex number as shown. The <span class="emphasis"><em>abs()</em></span> returns the absolute |
|
406 |
value of the complex number.</p> |
|
407 |
</div> |
|
408 |
<div class="section" title="4.2.Variables"> |
|
409 |
<div class="titlepage"><div><div><h3 class="title"> |
|
410 |
<a name="id2967346"></a>4.2.Variables</h3></div></div></div> |
|
411 |
<p id="ch2intro_3f">Variables are just names that represent a value. Variables have already been |
|
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introduced in the various examples from the previous sections. Certain rules about |
| 28 | 413 |
using variables:</p> |
414 |
<div class="itemizedlist"><ul class="itemizedlist" type="*"> |
|
415 |
<li class="listitem" style="list-style-type: *"><p id="ch2intro_40">Variables have to be initialized or assigned a value before being used.</p></li> |
|
416 |
<li class="listitem" style="list-style-type: *"><p id="ch2intro_41">Variable names can consist of letters, digits and underscores(_).</p></li> |
|
417 |
<li class="listitem" style="list-style-type: *"><p id="ch2intro_42">Variable names cannot begin with digits, but can contain digits in them.</p></li> |
|
418 |
</ul></div> |
|
419 |
<p id="ch2intro_43">In reference to the previous section examples, 'a', 'b', 'lng', 'fl', 'e' and 'cplx' |
|
420 |
are all variables of various datatypes.</p> |
|
421 |
<pre class="programlisting"> |
|
422 |
Note: Python is not a strongly typed language and hence an integer variable can at a |
|
423 |
later stage be used as a float variable as well.</pre> |
|
424 |
</div> |
|
425 |
<div class="section" title="4.3.Strings"> |
|
426 |
<div class="titlepage"><div><div><h3 class="title"> |
|
427 |
<a name="id2967394"></a>4.3.Strings</h3></div></div></div> |
|
428 |
<p id="ch2intro_44">Strings are one of the essential data structures of any programming language. |
|
429 |
The <code class="literal">print "Hello, World!"</code> program was introduced in the earlier section, and |
|
430 |
the <span class="emphasis"><em>"Hello, World!"</em></span> in the print statement is a string. A string is basically |
|
431 |
a set of characters. Strings can be represented in various ways shown below:</p> |
|
432 |
<pre class="programlisting"> |
|
433 |
s = 'this is a string' # a string variable can be represented using single quotes |
|
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s = 'This one has "quotes" inside!' # The string can have quotes inside it as shown |
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s = "I have 'single-quotes' inside!" |
| 25 | 436 |
l = "A string spanning many lines |
437 |
one more line |
|
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yet another" # a string can span more than a single line. |
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t = """A triple quoted string does # another way of representing multiline strings. |
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|
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not need to be escaped at the end and |
| 28 | 441 |
"can have nested quotes" etc."""</pre> |
442 |
<p id="ch2intro_45">Try the following on the interpreter: |
|
443 |
<code class="literal">s = 'this is a string with 'quotes' of similar kind'</code></p> |
|
444 |
<p id="ch2intro_46"><span class="strong"><strong>Exercise: How to use single quotes within single quotes in a string as shown |
|
445 |
in the above example without getting an error?</strong></span></p> |
|
446 |
<div class="section" title="4.3.1.String operations"> |
|
447 |
<div class="titlepage"><div><div><h4 class="title"> |
|
448 |
<a name="id2967445"></a>4.3.1.String operations</h4></div></div></div> |
|
449 |
<p id="ch2intro_47">A few basic string operations are presented here.</p> |
|
450 |
<p id="ch2intro_48"><span class="strong"><strong>String concatenation</strong></span> |
|
451 |
String concatenation is done by simple addition of two strings.</p> |
|
452 |
<pre class="programlisting"> |
|
453 |
>>> x = 'Hello' |
|
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>>> y = ' Python' |
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|
455 |
>>> print x+y |
| 28 | 456 |
Hello Python</pre> |
457 |
<p id="ch2intro_49"><span class="emphasis"><em>Try this yourself:</em></span></p> |
|
458 |
<pre class="programlisting"> |
|
459 |
>>> somenum = 13 |
|
460 |
>>> print x+somenum</pre> |
|
461 |
<p id="ch2intro_4a">The problem with the above example is that here a string variable and an integer |
|
|
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|
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variable are trying to be concantenated. To obtain the desired result from the |
| 28 | 463 |
above example the str(), repr() and the `` can be used.</p> |
464 |
<p id="ch2intro_4b"><span class="strong"><strong>str()</strong></span> simply converts a value to a string in a reasonable form. |
|
465 |
<span class="strong"><strong>repr()</strong></span> creates a string that is a representation of the value.</p> |
|
466 |
<p id="ch2intro_4c">The difference can be seen in the example shown below:</p> |
|
467 |
<pre class="programlisting"> |
|
468 |
>>> str(1000000000000000000000000000000000000000000000000L) |
|
|
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'1000000000000000000000000000000000000000000000000' |
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|
470 |
>>> repr(1000000000000000000000000000000000000000000000000L) |
| 28 | 471 |
'1000000000000000000000000000000000000000000000000L'</pre> |
472 |
<p id="ch2intro_4d">It can be observed that the 'L' in the long value shown was omitted by str(), |
|
|
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whereas repr() converted that into a string too. An alternative way of using |
| 28 | 474 |
repr(value) is <code class="literal">`value`</code>.</p> |
475 |
<p id="ch2intro_4e">A few more examples:</p> |
|
476 |
<pre class="programlisting"> |
|
477 |
>>> x = "Let's go nto Pycon" |
|
|
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|
478 |
>>> print x |
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|
479 |
Let's go |
| 28 | 480 |
to Pycon</pre> |
481 |
<p id="ch2intro_4f">In the above example, notice that the 'n'(newline) character is formatted and |
|
|
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|
482 |
the string is printed on two lines. The strings discussed until now were normal |
|
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|
483 |
strings. Other than these there are two other types of strings namely, raw strings |
| 28 | 484 |
and unicode strings.</p> |
485 |
<p id="ch2intro_50"><span class="strong"><strong>Raw strings</strong></span> are strings which are unformatted, that is the backslashes() are |
|
|
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486 |
not parsed and are left as it is in the string. Raw strings are represented with |
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|
487 |
an 'r' at the start of a string. |
| 28 | 488 |
Let us look at an example</p> |
489 |
<pre class="programlisting"> |
|
490 |
>>> x = r"Let's go nto Pycon" |
|
|
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|
491 |
>>> print x |
| 28 | 492 |
Let's go nto Pycon</pre> |
493 |
<p id="ch2intro_51">Note: The 'n' is not being parsed into a new line and is left as it is.</p> |
|
494 |
<p id="ch2intro_52"><span class="emphasis"><em>Try this yourself:</em></span></p> |
|
495 |
<pre class="programlisting"> |
|
496 |
>>> x = r"Let's go to Pycon "</pre> |
|
497 |
<p id="ch2intro_53"><span class="strong"><strong>Unicode strings</strong></span> are strings where the characters are Unicode characters as |
|
|
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498 |
opposed to ASCII characters. Unicode strings are represented with a 'u' at the |
|
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|
499 |
start of the string. |
| 28 | 500 |
Let us look at an example:</p> |
501 |
<pre class="programlisting"> |
|
502 |
>>> x = u"Let's go to Pycon!" |
|
|
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|
503 |
>>> print x |
| 28 | 504 |
Let's go to Pycon!</pre> |
505 |
</div> |
|
506 |
</div> |
|
507 |
<div class="section" title="4.4.Boolean"> |
|
508 |
<div class="titlepage"><div><div><h3 class="title"> |
|
509 |
<a name="id2967588"></a>4.4.Boolean</h3></div></div></div> |
|
510 |
<p id="ch2intro_54">Python also provides special Boolean datatype. A boolean variable can assume a |
|
511 |
value of either <span class="emphasis"><em>True</em></span> or <span class="emphasis"><em>False</em></span> (Note the capitalizations).</p> |
|
512 |
<p id="ch2intro_55">Let us look at examples:</p> |
|
513 |
<pre class="programlisting"> |
|
514 |
>>> t = True |
|
|
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|
515 |
>>> f = not t |
|
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|
516 |
>>> print f |
|
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|
517 |
False |
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|
518 |
>>> f or t |
|
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|
519 |
True |
|
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|
520 |
>>> f and t |
| 28 | 521 |
False</pre> |
522 |
</div> |
|
523 |
</div> |
|
524 |
<div class="section" title="5.The while loop"> |
|
525 |
<div class="titlepage"><div><div><h2 class="title" style="clear: both"> |
|
526 |
<a name="id2967617"></a>5.The <span class="strong"><strong>while</strong></span> loop</h2></div></div></div> |
|
527 |
<p id="ch2intro_56">The Python <span class="strong"><strong>while</strong></span> loop is similar to the C/C++ while loop. The syntax is as |
|
528 |
follows:</p> |
|
529 |
<pre class="programlisting"> |
|
530 |
statement 0 |
|
|
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|
531 |
while condition: |
|
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|
532 |
statement 1 #while block |
|
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|
533 |
statement 2 #while block |
| 28 | 534 |
statement 3 #outside the while block.</pre> |
535 |
<p id="ch2intro_57">Let us look at an example:</p> |
|
536 |
<pre class="programlisting"> |
|
537 |
>>> x = 1 |
|
|
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|
538 |
>>> while x <= 5: |
|
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|
539 |
... print x |
|
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|
540 |
... x += 1 |
|
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|
541 |
... |
|
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|
542 |
1 |
|
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|
543 |
2 |
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|
544 |
3 |
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|
545 |
4 |
| 28 | 546 |
5</pre> |
547 |
</div> |
|
548 |
<div class="section" title="6.The if conditional"> |
|
549 |
<div class="titlepage"><div><div><h2 class="title" style="clear: both"> |
|
550 |
<a name="id2967654"></a>6.The <span class="strong"><strong>if</strong></span> conditional</h2></div></div></div> |
|
551 |
<p id="ch2intro_58">The Python <span class="strong"><strong>if</strong></span> block provides the conditional execution of statements. |
|
|
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|
552 |
If the condition evaluates as true the block of statements defined under the if |
| 28 | 553 |
block are executed.</p> |
554 |
<p id="ch2intro_59">If the first block is not executed on account of the condition not being satisfied, |
|
555 |
the set of statements in the <span class="strong"><strong>else</strong></span> block are executed.</p> |
|
556 |
<p id="ch2intro_5a">The <span class="strong"><strong>elif</strong></span> block provides the functionality of evaluation of multiple conditions |
|
557 |
as shown in the example.</p> |
|
558 |
<p id="ch2intro_5b">The syntax is as follows:</p> |
|
559 |
<pre class="programlisting"> |
|
560 |
if condition : |
|
|
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|
561 |
statement_1 |
|
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|
562 |
statement_2 |
|
1
672eaaab9204
Added some new html files that failed earlier during change from rst
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0
diff
changeset
|
563 |
|
|
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|
564 |
elif condition: |
|
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|
565 |
statement_3 |
|
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|
566 |
statement_4 |
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|
567 |
else: |
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changeset
|
568 |
statement_5 |
| 28 | 569 |
statement_6</pre> |
570 |
<p id="ch2intro_5c">Let us look at an example:</p> |
|
571 |
<pre class="programlisting"> |
|
572 |
>>> n = raw_input("Input a number:")
|
|
|
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|
573 |
>>> if n < 0: |
|
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|
574 |
print n," is negative" |
|
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changeset
|
575 |
elif n > 0: |
|
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changeset
|
576 |
print n," is positive" |
|
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changeset
|
577 |
else: |
| 28 | 578 |
print n, " is 0"</pre> |
579 |
</div> |
|
580 |
<div class="section" title="7.raw_input()"> |
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<div class="titlepage"><div><div><h2 class="title" style="clear: both"> |
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<a name="id2967723"></a>7.<span class="strong"><strong>raw_input()</strong></span> |
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</h2></div></div></div> |
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<p id="ch2intro_5d">In the previous example we saw the call to the raw_input() subroutine. |
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The <span class="strong"><strong>raw_input()</strong></span> method is used to take user inputs through the console. |
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Unlike <span class="strong"><strong>input()</strong></span> which assumes the data entered by the user as a standard python |
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expression, <span class="strong"><strong>raw_input()</strong></span> treats all the input data as raw data and converts |
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everything into a string. To illustrate this let us look at an example.</p> |
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<pre class="programlisting"> |
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>>> input("Enter a number thats a palindrome:")
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Enter a number thats a palindrome:121 |
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121 |
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>>> input("Enter your name:")
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Enter your name:PythonFreak |
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Traceback (most recent call last): |
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File "<stdin>", line 1, in <module> |
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File "<string>", line 1, in <module> |
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NameError: name 'PythonFreak' is not defined</pre> |
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<p id="ch2intro_5e">As shown above the <span class="strong"><strong>input()</strong></span> assumes that the data entered is a valid Python |
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expression. In the first call it prompts for an integer input and when entered |
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it accepts the integer as an integer, whereas in the second call, when the string |
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is entered without the quotes, <span class="strong"><strong>input()</strong></span> assumes that the entered data is a valid |
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Python expression and hence it raises and exception saying PythonFreak is not |
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defined.</p> |
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<pre class="programlisting"> |
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>>> input("Enter your name:")
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Enter your name:'PythonFreak' |
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'PythonFreak' |
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>>></pre> |
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<p id="ch2intro_5f">Here the name is accepted because its entered as a string (within quotes). But |
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its unreasonable to go on using quotes each time a string is entered. Hence the |
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alternative is to use <span class="strong"><strong>raw_input()</strong></span>.</p> |
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<p id="ch2intro_60">Let us now look at how <span class="strong"><strong>raw_input()</strong></span> operates with an example.</p> |
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<pre class="programlisting"> |
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>>> raw_input("Enter your name:")
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Enter your name:PythonFreak |
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'PythonFreak'</pre> |
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<p id="ch2intro_61">Observe that the <span class="strong"><strong>raw_input()</strong></span> is converting it into a string all by itself.</p> |
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<pre class="programlisting"> |
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>>> pal = raw_input("Enter a number thats a palindrome:")
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Enter a number thats a palindrome:121 |
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'121'</pre> |
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<p id="ch2intro_62">Observe that <span class="strong"><strong>raw_input()</strong></span> is converting the integer 121 also to a string as |
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'121'. Let us look at another example:</p> |
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<pre class="programlisting"> |
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>>> pal = raw_input("Enter a number thats a palindrome:")
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Enter a number thats a palindrome:121 |
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>>> pal + 2 |
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Traceback (most recent call last): |
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File "<stdin>", line 1, in <module> |
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TypeError: cannot concatenate 'str' and 'int' objects |
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>>> pal |
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'121'</pre> |
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<p id="ch2intro_63">Observe here that the variable <span class="emphasis"><em>pal</em></span> is a string and hence integer operations |
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cannot be performed on it. Hence the exception is raised.</p> |
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</div> |
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<div class="section" title="8.int() method"> |
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<div class="titlepage"><div><div><h2 class="title" style="clear: both"> |
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<a name="id2967877"></a>8.<span class="strong"><strong>int()</strong></span> method</h2></div></div></div> |
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<p id="ch2intro_64">Generally for computing purposes, the data used is not strings or raw data but |
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on integers, floats and similar mathematical data structures. The data obtained |
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from <span class="strong"><strong>raw_input()</strong></span> is raw data in the form of strings. In order to obtain integers |
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from strings we use the method <span class="strong"><strong>int()</strong></span>.</p> |
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<p id="ch2intro_65">Let us look at an example.</p> |
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<pre class="programlisting"> |
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>>> intpal = int(pal) |
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>>> intpal |
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121</pre> |
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<p id="ch2intro_66">In the previous example it was observed that <span class="emphasis"><em>pal</em></span> was a string variable. Here |
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using the <span class="strong"><strong>int()</strong></span> method the string <span class="emphasis"><em>pal</em></span> was converted to an integer variable.</p> |
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<p id="ch2intro_67"><span class="emphasis"><em>Try This Yourself:</em></span></p> |
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<pre class="programlisting"> |
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>>> stringvar = raw_input("Enter a name:")
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Enter a name:Guido Van Rossum |
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>>> stringvar |
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'Guido Van Rossum' |
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>>> numvar = int(stringvar)</pre> |
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</div> |
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</div> |
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</div></body> |
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</html> |
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