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13 <div class="chapter" lang="en" id="chap02_basic"> |
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14 <div class="article" title="Basic Python"> |
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15 <div class="titlepage"> |
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16 <div><div><h2 class="title"> |
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17 <a name="id2957059"></a>Basic Python</h2></div></div> |
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18 <hr> |
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19 </div> |
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20 <div class="toc"> |
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21 <p><b>Table of Contents</b></p> |
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22 <dl> |
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23 <dt><span class="section"><a href="#introduction">1. Introduction</a></span></dt> |
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24 <dt><span class="section"><a href="#the-python-interpreter">2. The Python Interpreter</a></span></dt> |
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25 <dd><dl> |
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26 <dt><span class="section"><a href="#the-interactive-interpreter">2.1. The Interactive Interpreter</a></span></dt> |
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27 <dt><span class="section"><a href="#ipython-an-enhanced-interactive-python-interpreter">2.2. |
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28 <span class="emphasis"><em>ipython</em></span> - An enhanced interactive Python interpreter |
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29 </a></span></dt> |
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30 </dl></dd> |
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31 <dt><span class="section"><a href="#editing-and-running-a-python-file">3. Editing and running a python file</a></span></dt> |
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32 <dt><span class="section"><a href="#basic-datatypes-and-operators-in-python">4. Basic Datatypes and operators in Python</a></span></dt> |
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33 <dd><dl> |
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34 <dt><span class="section"><a href="#numbers">4.1. Numbers</a></span></dt> |
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35 <dt><span class="section"><a href="#variables">4.2. Variables</a></span></dt> |
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36 <dt><span class="section"><a href="#strings">4.3. Strings</a></span></dt> |
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37 <dt><span class="section"><a href="#boolean">4.4. Boolean</a></span></dt> |
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38 </dl></dd> |
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39 <dt><span class="section"><a href="#the-while-loop">5. The |
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40 <span class="strong"><strong>while</strong></span> loop |
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41 </a></span></dt> |
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42 <dt><span class="section"><a href="#the-if-conditional">6. The |
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43 <span class="strong"><strong>if</strong></span> conditional |
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44 </a></span></dt> |
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45 <dt><span class="section"><a href="#raw_input">7. <span class="strong"><strong>raw_input()</strong></span> |
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46 </a></span></dt> |
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47 <dt><span class="section"><a href="#int-method">8. |
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48 <span class="strong"><strong>int()</strong></span> method |
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49 </a></span></dt> |
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50 </dl> |
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51 </div> |
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52 <p id="x_197"></a>This document is intended to be handed out at the end of the workshop. It has been designed for Engineering students who are Python beginners and have basic programming skills. The focus is on basic numerics and plotting using Python.</p> |
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53 <p id="x_198"></a>The system requirements:</p> |
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54 <div class="itemizedlist"><ul class="itemizedlist" type="*"> |
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55 <li class="listitem" style="list-style-type: *"><p id="x_199"></a>Python - version 2.5.x or newer.</p></li> |
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56 <li class="listitem" style="list-style-type: *"><p id="x_19a"></a>IPython</p></li> |
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57 <li class="listitem" style="list-style-type: *"><p id="x_19b"></a>Text editor - scite, vim, emacs or whatever you are comfortable with.</p></li> |
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58 </ul></div> |
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59 <div class="section" title="1. Introduction"> |
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60 <div class="titlepage"><div><div><h2 class="title" style="clear: both"> |
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61 <a name="introduction"></a>1. Introduction</h2></div></div></div> |
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62 <p id="x_19c"></a>The Python programming language was created by a dutch named Guido van Rossum. The idea of Python was conceived in December 1989. The name Python has nothing to do with the reptilian, but its been named after the 70s comedy series "Monty Python's Flying Circus", since it happens to be Guido's favourite TV series.</p> |
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63 <p id="x_19d"></a>Current stable version of Python is 2.6.x, although Python 3.0 is also the stable version, it is not backwards compatible with the previous versions and is hence not entirely popular at the moment. This material will focus on the 2.6.x series.</p> |
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64 <p id="x_19e"></a>Python is licensed under the Python Software Foundation License (PSF License) which is GPL compatible Free Software license (excepting license version 1.6 and 2.0) It is a no strings attached license, which means the source code is free to modify and redistribute.</p> |
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65 <p id="x_19f"></a>The Python docs define Python as "Python is an interpreted, object-oriented, high-level programming language with dynamic semantics." A more detailed summary can be found at |
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66 <a class="ulink" href="http://www.python.org/doc/essays/blurb.html" target="_top">http://www.python.org/doc/essays/blurb.html</a>. Python is a language that has been designed to help the programmer concentrate on solving the problem at hand and not worry about the programming language idiosyncrasies. |
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67 </p> |
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68 <p id="x_1a0"></a>Python is a highly cross platform compatible language on account of it being an interpreted language. It is highly scalable and hence has been adapted to run on the Nokia 60 series phones. Python has been designed to be readable and easy to use</p> |
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69 <p id="x_1a1"></a> |
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70 <span class="strong"><strong>Resources available for reference</strong></span> |
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71 </p> |
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72 <div class="itemizedlist"><ul class="itemizedlist" type="*"> |
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73 <li class="listitem" style="list-style-type: *"><p id="x_1a2"></a>Web: |
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74 <a class="ulink" href="http://www.python.org" target="_top">http://www.python.org</a> |
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75 </p></li> |
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76 <li class="listitem" style="list-style-type: *"><p id="x_1a3"></a>Doc: |
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77 <a class="ulink" href="http://www.python.org/doc" target="_top">http://www.python.org/doc</a> |
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78 </p></li> |
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79 <li class="listitem" style="list-style-type: *"><p id="x_1a4"></a>Free Tutorials: |
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80 <span class="emphasis"><em> Official Python Tutorial: |
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81 <a class="ulink" href="http://docs.python.org/tut/tut.html" target="_top">http://docs.python.org/tut/tut.html</a> |
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82 </em></span> Byte of Python: |
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83 <a class="ulink" href="http://www.byteofpython.info" target="_top">http://www.byteofpython.info</a>/ * Dive into Python: |
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84 <a class="ulink" href="http://diveintopython.org" target="_top">http://diveintopython.org</a>/ |
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85 </p></li> |
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86 </ul></div> |
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87 <p id="x_1a5"></a> |
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88 <span class="strong"><strong>Advantages of Python - Why Python??</strong></span> |
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89 </p> |
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90 <div class="itemizedlist"><ul class="itemizedlist" type="*"> |
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91 <li class="listitem" style="list-style-type: *"><p id="x_1a6"></a>Python has been designed for readability and ease of use. Its been designed in such a fashion that it imposes readability on the programmer. Python does away with the braces and the semicolons and instead implements code blocks based on indentation, thus enhancing readability.</p></li> |
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92 <li class="listitem" style="list-style-type: *"><p id="x_1a7"></a>Python is a high level, interpreted, modular and object oriented language. Python performs memory management on its own, thus the programmer need not bother about allocating and deallocating memory to variables. Python provides extensibility by providing modules which can be easily imported similar to headers in C and packages in Java. Python is object oriented and hence provides all the object oriented characteristics such as inheritance, encapsulation and polymorphism.</p></li> |
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93 <li class="listitem" style="list-style-type: *"><p id="x_1a8"></a>Python offers a highly powerful interactive programming interface in the form of the 'Interactive Interpreter' which will be discussed in more detail in the following sections.</p></li> |
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94 <li class="listitem" style="list-style-type: *"><p id="x_1a9"></a>Python provides a rich standard library and an extensive set of modules. The power of Python modules can be seen in this slightly exaggerated cartoon |
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95 <a class="ulink" href="http://xkcd.com/353" target="_top">http://xkcd.com/353</a>/ |
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96 </p></li> |
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97 <li class="listitem" style="list-style-type: *"><p id="x_1aa"></a>Python interfaces well with most other programming languages such as C, C++ and FORTRAN.</p></li> |
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98 </ul></div> |
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99 <p id="x_1ab"></a>Although, Python has one setback. Python is not fast as some of the compiled languages like C or C++. Yet, the amount of flexibility and power more than make up for this setback.</p> |
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100 </div> |
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101 <div class="section" title="2. The Python Interpreter"> |
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102 <div class="titlepage"><div><div><h2 class="title" style="clear: both"> |
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103 <a name="the-python-interpreter"></a>2. The Python Interpreter</h2></div></div></div> |
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104 <div class="section" title="2.1. The Interactive Interpreter"> |
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105 <div class="titlepage"><div><div><h3 class="title"> |
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106 <a name="the-interactive-interpreter"></a>2.1. The Interactive Interpreter</h3></div></div></div> |
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107 <p id="x_1ac"></a>Typing |
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108 <span class="emphasis"><em>python</em></span> at the shell prompt on any standard Unix/Gnu-Linux system and hitting the enter key fires up the Python 'Interactive Interpreter'. The Python interpreter is one of the most integral features of Python. The prompt obtained when the interactive interpreter is similar to what is shown below. The exact appearance might differ based on the version of Python being used. The |
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109 <code class="literal">>>></code> thing shown is the python prompt. When something is typed at the prompt and the enter key is hit, the python interpreter interprets the command entered and performs the appropriate action. All the examples presented in this document are to be tried hands on, on the interactive interpreter. |
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110 </p> |
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111 <pre class="programlisting">Python 2.5.2 (r252:60911, Oct 5 2008, 19:24:49) |
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112 [GCC 4.3.2] on linux2 |
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113 Type "help", "copyright", "credits" or "license" for more information. |
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114 >>> |
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115 |
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116 </pre> |
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117 <p id="x_1ad"></a>Lets try with an example, type |
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118 <code class="literal">print 'Hello, World!'</code> at the prompt and hit the enter key. |
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119 </p> |
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120 <pre class="programlisting">>>> print 'Hello, World!' |
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121 Hello, World! |
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122 |
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123 </pre> |
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124 <p id="x_1ae"></a>This example was quite straight forward, and thus we have written our first line of Python code. Now let us try typing something arbitrary at the prompt. For example:</p> |
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125 <pre class="programlisting">>>> arbit word |
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126 File "<stdin>", line 1 |
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127 arbit word |
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128 ^ |
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129 SyntaxError: invalid syntax |
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130 >>> |
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131 |
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132 </pre> |
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133 <p id="x_1af"></a>The interpreter gave an error message saying that 'arbit word' was invalid syntax which is valid. The interpreter is an amazing tool when learning to program in Python. The interpreter provides a help function that provides the necessary documentation regarding all Python syntax, constructs, modules and objects. Typing |
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134 <span class="emphasis"><em>help()</em></span> at the prompt gives the following output: |
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135 </p> |
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136 <pre class="programlisting">>>> help() |
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137 |
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138 Welcome to Python 2.5! This is the online help utility. |
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139 |
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140 If this is your first time using Python, you should definitely check out |
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141 the tutorial on the Internet at http://www.python.org/doc/tut/. |
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142 |
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143 Enter the name of any module, keyword, or topic to get help on writing |
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144 Python programs and using Python modules. To quit this help utility and |
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145 return to the interpreter, just type "quit". |
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146 |
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147 To get a list of available modules, keywords, or topics, type "modules", |
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148 "keywords", or "topics". Each module also comes with a one-line summary |
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149 of what it does; to list the modules whose summaries contain a given word |
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150 such as "spam", type "modules spam". |
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151 |
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152 help> |
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153 |
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154 |
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155 </pre> |
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156 <p id="x_1b0"></a>As mentioned in the output, entering the name of any module, keyword or topic will provide the documentation and help regarding the same through the online help utility. Pressing |
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157 <span class="emphasis"><em>Ctrl+d</em></span> exits the help prompt and returns to the python prompt. |
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158 </p> |
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159 <p id="x_1b1"></a>Let us now try a few examples at the python interpreter.</p> |
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160 <p id="x_1b2"></a>Eg 1:</p> |
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161 <pre class="programlisting">>>> print 'Hello, python!' |
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162 Hello, python! |
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163 >>> |
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164 |
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165 </pre> |
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166 <p id="x_1b3"></a>Eg 2:</p> |
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167 <pre class="programlisting">>>> print 4321*567890 |
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168 2453852690 |
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169 >>> |
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170 |
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171 </pre> |
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172 <p id="x_1b4"></a>Eg 3:</p> |
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173 <pre class="programlisting">>>> 4321*567890 |
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174 2453852690L |
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175 >>> |
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176 |
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177 </pre> |
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178 <pre class="programlisting">Note: Notice the 'L' at the end of the output. The 'L' signifies that the |
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179 output of the operation is of type *long*. It was absent in the previous |
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180 example because we used the print statement. This is because *print* formats |
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181 the output before displaying. |
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182 |
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183 </pre> |
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184 <p id="x_1b5"></a>Eg 4:</p> |
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185 <pre class="programlisting">>>> big = 12345678901234567890 ** 3 |
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186 >>> print big |
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187 1881676372353657772490265749424677022198701224860897069000 |
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188 >>> |
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189 |
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190 </pre> |
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191 <pre class="programlisting">This example is to show that unlike in C or C++ there is no limit on the |
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192 value of an integer. |
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193 |
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194 </pre> |
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195 <p id="x_1b6"></a>Try this on the interactive interpreter: |
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196 <code class="literal">import this</code> |
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197 </p> |
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198 <p id="x_1b7"></a> |
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199 <span class="emphasis"><em>Hint: The output gives an idea of Power of Python</em></span> |
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200 </p> |
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201 </div> |
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202 <div class="section" title="2.2. ipython - An enhanced interactive Python interpreter"> |
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203 <div class="titlepage"><div><div><h3 class="title"> |
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204 <a name="ipython-an-enhanced-interactive-python-interpreter"></a>2.2. |
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205 <span class="emphasis"><em>ipython</em></span> - An enhanced interactive Python interpreter |
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206 </h3></div></div></div> |
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207 <p id="x_1b8"></a>The power and the importance of the interactive interpreter was the highlight of the previous section. This section provides insight into the enhanced interpreter with more advanced set of features called |
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208 <span class="strong"><strong>ipython</strong></span>. Entering |
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209 <span class="emphasis"><em>ipython</em></span> at the shell prompt fires up the interactive interpreter. |
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210 </p> |
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211 <pre class="programlisting">$ ipython |
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212 Python 2.5.2 (r252:60911, Oct 5 2008, 19:24:49) |
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213 Type "copyright", "credits" or "license" for more information. |
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214 |
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215 IPython 0.8.4 -- An enhanced Interactive Python. |
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216 ? -> Introduction and overview of IPython's features. |
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217 %quickref -> Quick reference. |
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218 help -> Python's own help system. |
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219 object? -> Details about 'object'. ?object also works, ?? prints more. |
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220 |
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221 In [1]: |
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222 |
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223 </pre> |
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224 <p id="x_1b9"></a>This is the output obtained upon firing ipython. The exact appearance may change based on the Python version installed. The following are some of the various features provided by |
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225 <span class="strong"><strong>ipython</strong></span>: |
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226 </p> |
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227 <div class="variablelist"><dl> |
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228 <dt><span class="term"></span></dt> |
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229 <dd> |
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230 <p></p> |
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231 <p id="x_1ba"></a>Suggestions - ipython provides suggestions of the possible methods and operations available for the given python object.</p> |
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232 </dd> |
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233 </dl></div> |
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234 <p id="x_1bb"></a>Eg 5:</p> |
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235 <pre class="programlisting">In [4]: a = 6 |
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236 |
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237 In [5]: a. |
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238 a.__abs__ a.__divmod__ a.__index__ a.__neg__ a.__rand__ a.__rmod__ a.__rxor__ |
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239 a.__add__ a.__doc__ a.__init__ a.__new__ a.__rdiv__ a.__rmul__ a.__setattr__ |
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240 a.__and__ a.__float__ a.__int__ a.__nonzero__ a.__rdivmod__ a.__ror__ a.__str__ |
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241 a.__class__ a.__floordiv__ a.__invert__ a.__oct__ a.__reduce__ a.__rpow__ a.__sub__ |
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242 a.__cmp__ a.__getattribute__ a.__long__ a.__or__ a.__reduce_ex__ a.__rrshift__ a.__truediv__ |
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243 a.__coerce__ a.__getnewargs__ a.__lshift__ a.__pos__ a.__repr__ a.__rshift__ a.__xor__ |
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244 a.__delattr__ a.__hash__ a.__mod__ a.__pow__ a.__rfloordiv__ a.__rsub__ |
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245 a.__div__ a.__hex__ a.__mul__ a.__radd__ a.__rlshift__ a.__rtruediv__ |
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246 |
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247 </pre> |
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248 <p id="x_1bc"></a>In this example, we initialized 'a' (a variable - a concept that will be discussed in the subsequent sections.) to 6. In the next line when the |
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249 <span class="emphasis"><em>tab</em></span> key is pressed after typing ' |
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250 <span class="emphasis"><em>a.</em></span>' ipython displays the set of all possible methods that are applicable on the object 'a' (an integer in this context). Ipython provides many such datatype specific features which will be presented in the further sections as and when the datatypes are introduced. |
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251 </p> |
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252 </div> |
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253 </div> |
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254 <div class="section" title="3. Editing and running a python file"> |
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255 <div class="titlepage"><div><div><h2 class="title" style="clear: both"> |
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256 <a name="editing-and-running-a-python-file"></a>3. Editing and running a python file</h2></div></div></div> |
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257 <p id="x_1bd"></a>The previous sections focused on the use of the interpreter to run python code. While the interpeter is an excellent tool to test simple solutions and experiment with small code snippets, its main disadvantage is that everything written in the interpreter is lost once its quit. Most of the times a program is used by people other than the author. So the programs have to be available in some form suitable for distribution, and hence they are written in files. This section will focus on editing and running python files. Start by opening a text editor ( it is recommended you choose one from the list at the top of this page ). In the editor type down python code and save the file with an extension |
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258 <span class="strong"><strong>.py</strong></span> (python files have an extension of .py). Once done with the editing, save the file and exit the editor. |
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259 </p> |
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260 <p id="x_1be"></a>Let us look at a simple example of calculating the gcd of 2 numbers using Python:</p> |
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261 <p id="x_1bf"></a> |
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262 <span class="strong"><strong>Creating the first python script(file)</strong></span> : |
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263 </p> |
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264 <pre class="programlisting">$ emacs gcd.py |
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265 def gcd(x,y): |
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266 if x % y == 0: |
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267 return y |
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268 return gcd(y, x%y) |
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269 |
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270 print gcd(72, 92) |
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271 |
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272 </pre> |
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273 <p id="x_1c0"></a>To run the script, open the shell prompt, navigate to the directory that contains the python file and run |
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274 <code class="literal">python <filename.py></code> at the prompt ( in this case filename is gcd.py ) |
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275 </p> |
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276 <p id="x_1c1"></a> |
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277 <span class="strong"><strong>Running the python script</strong></span> : |
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278 </p> |
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279 <pre class="programlisting">$ python gcd.py |
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280 4 |
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281 $ |
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282 |
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283 </pre> |
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284 <p id="x_1c2"></a>Another method to run a python script would be to include the line</p> |
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285 <p id="x_1c3"></a> |
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286 <code class="literal">#! /usr/bin/python</code> |
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287 </p> |
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288 <p id="x_1c4"></a>at the beginning of the python file and then make the file executable by</p> |
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289 <p id="x_1c5"></a>$ chmod a+x |
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290 <span class="emphasis"><em>filename.py</em></span> |
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291 </p> |
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292 <p id="x_1c6"></a>Once this is done, the script can be run as a standalone program as follows:</p> |
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293 <p id="x_1c7"></a>$ ./ |
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294 <span class="emphasis"><em>filename.py</em></span> |
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295 </p> |
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296 </div> |
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297 <div class="section" title="4. Basic Datatypes and operators in Python"> |
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298 <div class="titlepage"><div><div><h2 class="title" style="clear: both"> |
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299 <a name="basic-datatypes-and-operators-in-python"></a>4. Basic Datatypes and operators in Python</h2></div></div></div> |
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300 <p id="x_1c8"></a>Python provides the following set of basic datatypes.</p> |
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301 <div class="blockquote"><blockquote class="blockquote"><div class="itemizedlist"><ul class="itemizedlist" type="*"> |
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302 <li class="listitem" style="list-style-type: *"><p id="x_1c9"></a>Numbers: int, float, long, complex</p></li> |
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303 <li class="listitem" style="list-style-type: *"><p id="x_1ca"></a>Strings</p></li> |
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304 <li class="listitem" style="list-style-type: *"><p id="x_1cb"></a>Boolean</p></li> |
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305 </ul></div></blockquote></div> |
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306 <div class="section" title="4.1. Numbers"> |
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307 <div class="titlepage"><div><div><h3 class="title"> |
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308 <a name="numbers"></a>4.1. Numbers</h3></div></div></div> |
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309 <p id="x_1cc"></a>Numbers were introduced in the examples presented in the interactive interpreter section. Numbers include types as mentioned earlier viz., int (integers), float (floating point numbers), long (large integers), complex (complex numbers with real and imaginary parts). Python is not a strongly typed language, which means the type of a variable need not mentioned during its initialization. Let us look at a few examples.</p> |
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310 <p id="x_1cd"></a>Eg 6:</p> |
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311 <pre class="programlisting">>>> a = 1 #here a is an integer variable |
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312 |
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313 </pre> |
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314 <p id="x_1ce"></a>Eg 7:</p> |
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315 <pre class="programlisting">>>> lng = 122333444455555666666777777788888888999999999 #here lng is a variable of type long |
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316 >>> lng |
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317 122333444455555666666777777788888888999999999L #notice the trailing 'L' |
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318 >>> print lng |
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319 122333444455555666666777777788888888999999999 #notice the absence of the trailing 'L' |
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320 >>> lng+1 |
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321 122333444455555666666777777788888889000000000L |
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322 |
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323 |
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324 </pre> |
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325 <p id="x_1cf"></a>Long numbers are the same as integers in almost all aspects. They can be used in operations just like integers and along with integers without any distinction. The only distinction comes during type checking (which is not a healthy practice). Long numbers are tucked with a trailing 'L' just to signify that they are long. Notice that in the example just lng at the prompt displays the value of the variable with the 'L' whereas |
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326 <code class="literal">print lng</code> displays without the 'L'. This is because print formats the output before printing. Also in the example, notice that adding an integer to a long does not give any errors and the result is as expected. So for all practical purposes longs can be treated as ints. |
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327 </p> |
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328 <p id="x_1d0"></a>Eg 8:</p> |
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329 <pre class="programlisting">>>> fl = 3.14159 #fl is a float variable |
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330 >>> e = 1.234e-4 #e is also a float variable, specified in the exponential form |
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331 >>> a = 1 |
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332 >>> b = 2 |
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333 >>> a/b #integer division |
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334 0 |
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335 >>> a/fl #floating point division |
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336 0.31831015504887655 |
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337 >>> e/fl |
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338 3.9279473133031364e-05 |
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339 |
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340 |
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341 </pre> |
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342 <p id="x_1d1"></a>Floating point numbers, simply called floats are real numbers with a decimal point. The example above shows the initialization of a float variable. Shown also in this example is the difference between integer division and floating point division. 'a' and 'b' here are integer variables and hence the division gives 0 as the quotient. When either of the operands is a float, the operation is a floating point division, and the result is also a float as illustrated.</p> |
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343 <p id="x_1d2"></a>Eg 9:</p> |
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344 <pre class="programlisting">>>> cplx = 3 + 4j #cplx is a complex variable |
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345 >>> cplx |
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346 (3+4j) |
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347 >>> print cplx.real #prints the real part of the complex number |
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348 3.0 |
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349 >>> print cplx.imag #prints the imaginary part of the complex number |
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350 4.0 |
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351 >>> print cplx*fl #multiplies the real and imag parts of the complex number with the multiplier |
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352 (9.42477+12.56636j) |
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353 >>> abs(cplx) #returns the absolute value of the complex number |
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354 5.0 |
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355 |
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356 </pre> |
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357 <p id="x_1d3"></a>Python provides a datatype for complex numbers. Complex numbers are initialized as shown in the example above. The |
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358 <span class="emphasis"><em>real</em></span> and |
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359 <span class="emphasis"><em>imag</em></span> operators return the real and imaginary parts of the complex number as shown. The |
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360 <span class="emphasis"><em>abs()</em></span> returns the absolute value of the complex number. |
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361 </p> |
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362 </div> |
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363 <div class="section" title="4.2. Variables"> |
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364 <div class="titlepage"><div><div><h3 class="title"> |
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365 <a name="variables"></a>4.2. Variables</h3></div></div></div> |
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366 <p id="x_1d4"></a>Variables are just names that represent a value. Variables have already been introduced in the various examples from the previous sections. Certain rules about using variables:</p> |
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367 <div class="blockquote"><blockquote class="blockquote"><div class="itemizedlist"><ul class="itemizedlist" type="*"> |
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368 <li class="listitem" style="list-style-type: *"><p id="x_1d5"></a>Variables have to be initialized or assigned a value before being used.</p></li> |
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369 <li class="listitem" style="list-style-type: *"><p id="x_1d6"></a>Variable names can consist of letters, digits and |
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370 <a class="link" href="#">underscores</a> . |
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371 </p></li> |
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372 <li class="listitem" style="list-style-type: *"><p id="x_1d7"></a>Variable names cannot begin with digits, but can contain digits in them.</p></li> |
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373 </ul></div></blockquote></div> |
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374 <p id="x_1d8"></a>In reference to the previous section examples, 'a', 'b', 'lng', 'fl', 'e' and 'cplx' are all variables of various datatypes.</p> |
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375 <pre class="programlisting">Note: Python is not a strongly typed language and hence an integer variable can at a |
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376 later stage be used as a float variable as well. |
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377 |
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378 </pre> |
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379 </div> |
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380 <div class="section" title="4.3. Strings"> |
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381 <div class="titlepage"><div><div><h3 class="title"> |
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382 <a name="strings"></a>4.3. Strings</h3></div></div></div> |
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383 <p id="x_1d9"></a>Strings are one of the essential data structures of any programming language. The |
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384 <code class="literal">print "Hello, World!"</code> program was introduced in the earlier section, and the |
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385 <span class="emphasis"><em>"Hello, World!"</em></span> in the print statement is a string. A string is basically a set of characters. Strings can be represented in various ways shown below: |
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386 </p> |
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387 <pre class="programlisting">s = 'this is a string' # a string variable can be represented using single quotes |
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388 s = 'This one has "quotes" inside!' # The string can have quotes inside it as shown |
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389 s = "I have 'single-quotes' inside!" |
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390 l = "A string spanning many lines\ |
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391 one more line\ |
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392 yet another" # a string can span more than a single line. |
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393 t = """A triple quoted string does # another way of representing multiline strings. |
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394 not need to be escaped at the end and |
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395 "can have nested quotes" etc.""" |
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396 |
|
397 </pre> |
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398 <p id="x_1da"></a>Try the following on the interpreter: |
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399 <code class="literal">s = 'this is a string with 'quotes' of similar kind'</code> |
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400 </p> |
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401 <p id="x_1db"></a> |
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402 <span class="strong"><strong>Exercise: How to use single quotes within single quotes in a string as shown in the above example without getting an error?</strong></span> |
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403 </p> |
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404 <div class="section" title="4.3.1. String operations"> |
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405 <div class="titlepage"><div><div><h4 class="title"> |
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406 <a name="string-operations"></a>4.3.1. String operations</h4></div></div></div> |
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407 <p id="x_1dc"></a>A few basic string operations are presented here.</p> |
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408 <p id="x_1dd"></a> |
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409 <span class="strong"><strong>String concatenation</strong></span> String concatenation is done by simple addition of two strings. |
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410 </p> |
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411 <pre class="programlisting">>>> x = 'Hello' |
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412 >>> y = ' Python' |
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413 >>> print x+y |
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414 Hello Python |
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415 |
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416 </pre> |
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417 <p id="x_1de"></a> <span class="emphasis"><em>Try this yourself:</em></span> |
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418 </p> |
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419 <pre class="programlisting">>>> somenum = 13 |
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420 >>> print x+somenum |
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421 |
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422 </pre> |
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423 <p id="x_1df"></a>The problem with the above example is that here a string variable and an integer variable are trying to be concantenated. To obtain the desired result from the above example the str(), repr() and the `` can be used.</p> |
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424 <p id="x_1e0"></a> |
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425 <span class="strong"><strong>str()</strong></span> simply converts a value to a string in a reasonable form. |
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426 <span class="strong"><strong>repr()</strong></span> creates a string that is a representation of the value. |
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427 </p> |
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428 <p id="x_1e1"></a>The difference can be seen in the example shown below:</p> |
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429 <pre class="programlisting">>>> str(1000000000000000000000000000000000000000000000000L) |
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430 '1000000000000000000000000000000000000000000000000' |
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431 >>> repr(1000000000000000000000000000000000000000000000000L) |
|
432 '1000000000000000000000000000000000000000000000000L' |
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433 |
|
434 </pre> |
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435 <p id="x_1e2"></a>It can be observed that the 'L' in the long value shown was omitted by str(), whereas repr() converted that into a string too. An alternative way of using repr(value) is ` |
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436 <code class="literal">value</code>`. |
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437 </p> |
|
438 <p id="x_1e3"></a>A few more examples:</p> |
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439 <pre class="programlisting">>>> x = "Let's go \nto Pycon" |
|
440 >>> print x |
|
441 Let's go |
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442 to Pycon |
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443 |
|
444 </pre> |
|
445 <p id="x_1e4"></a>In the above example, notice that the '\n'(newline) character is formatted and the string is printed on two lines. The strings discussed until now were normal strings. Other than these there are two other types of strings namely, raw strings and unicode strings.</p> |
|
446 <p id="x_1e5"></a> |
|
447 <span class="strong"><strong>Raw strings</strong></span> are strings which are unformatted, that is the backslashes(\) are not parsed and are left as it is in the string. Raw strings are represented with an 'r' at the start of a string. Let us look at an example |
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448 </p> |
|
449 <pre class="programlisting">>>> x = r"Let's go \nto Pycon" |
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450 >>> print x |
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451 Let's go \nto Pycon |
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452 |
|
453 </pre> |
|
454 <p id="x_1e6"></a>Note: The '\n' is not being parsed into a new line and is left as it is.</p> |
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455 <p id="x_1e7"></a> |
|
456 <span class="emphasis"><em>Try this yourself:</em></span> |
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457 </p> |
|
458 <pre class="programlisting">>>> x = r"Let's go to Pycon\" |
|
459 |
|
460 </pre> |
|
461 <p id="x_1e8"></a> <span class="strong"><strong>Unicode strings</strong></span> are strings where the characters are Unicode characters as opposed to ASCII characters. Unicode strings are represented with a 'u' at the start of the string. Let us look at an example: |
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462 </p> |
|
463 <pre class="programlisting">>>> x = u"Let's go to Pycon!" |
|
464 >>> print x |
|
465 Let's go to Pycon! |
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466 |
|
467 </pre> |
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468 </div> |
|
469 </div> |
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470 <div class="section" title="4.4. Boolean"> |
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471 <div class="titlepage"><div><div><h3 class="title"> |
|
472 <a name="boolean"></a>4.4. Boolean</h3></div></div></div> |
|
473 <p id="x_1e9"></a>Python also provides special Boolean datatype. A boolean variable can assume a value of either |
|
474 <span class="emphasis"><em>True</em></span> or |
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475 <span class="emphasis"><em>False</em></span> (Note the capitalizations). |
|
476 </p> |
|
477 <p id="x_1ea"></a>Let us look at examples:</p> |
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478 <pre class="programlisting">>>> t = True |
|
479 >>> f = not t |
|
480 >>> print f |
|
481 False |
|
482 >>> f or t |
|
483 True |
|
484 >>> f and t |
|
485 False |
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486 |
|
487 </pre> |
|
488 </div> |
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489 </div> |
|
490 <div class="section" title="5. The while loop"> |
|
491 <div class="titlepage"><div><div><h2 class="title" style="clear: both"> |
|
492 <a name="the-while-loop"></a>5. The |
|
493 <span class="strong"><strong>while</strong></span> loop |
|
494 </h2></div></div></div> |
|
495 <p id="x_1eb"></a>The Python |
|
496 <span class="strong"><strong>while</strong></span> loop is similar to the C/C++ while loop. The syntax is as follows: |
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497 </p> |
|
498 <pre class="programlisting">statement 0 |
|
499 while condition: |
|
500 statement 1 #while block |
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501 statement 2 #while block |
|
502 statement 3 #outside the while block. |
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503 |
|
504 </pre> |
|
505 <p id="x_1ec"></a>Let us look at an example:</p> |
|
506 <pre class="programlisting">>>> x = 1 |
|
507 >>> while x <= 5: |
|
508 ... print x |
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509 ... x += 1 |
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510 ... |
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511 1 |
|
512 2 |
|
513 3 |
|
514 4 |
|
515 5 |
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516 |
|
517 </pre> |
|
518 </div> |
|
519 <div class="section" title="6. The if conditional"> |
|
520 <div class="titlepage"><div><div><h2 class="title" style="clear: both"> |
|
521 <a name="the-if-conditional"></a>6. The |
|
522 <span class="strong"><strong>if</strong></span> conditional |
|
523 </h2></div></div></div> |
|
524 <p id="x_1ed"></a>The Python |
|
525 <span class="strong"><strong>if</strong></span> block provides the conditional execution of statements. If the condition evaluates as true the block of statements defined under the if block are executed. |
|
526 </p> |
|
527 <p id="x_1ee"></a>If the first block is not executed on account of the condition not being satisfied, the set of statements in the |
|
528 <span class="strong"><strong>else</strong></span> block are executed. |
|
529 </p> |
|
530 <p id="x_1ef"></a>The |
|
531 <span class="strong"><strong>elif</strong></span> block provides the functionality of evaluation of multiple conditions as shown in the example. |
|
532 </p> |
|
533 <p id="x_1f0"></a>The syntax is as follows:</p> |
|
534 <pre class="programlisting">if condition : |
|
535 statement_1 |
|
536 statement_2 |
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537 |
|
538 elif condition: |
|
539 statement_3 |
|
540 statement_4 |
|
541 else: |
|
542 statement_5 |
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543 statement_6 |
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544 |
|
545 </pre> |
|
546 <p id="x_1f1"></a>Let us look at an example:</p> |
|
547 <pre class="programlisting">>>> n = raw_input("Input a number:") |
|
548 >>> if n < 0: |
|
549 print n," is negative" |
|
550 elif n > 0: |
|
551 print n," is positive" |
|
552 else: |
|
553 print n, " is 0" |
|
554 |
|
555 </pre> |
|
556 </div> |
|
557 <div class="section" title="7. raw_input()"> |
|
558 <div class="titlepage"><div><div><h2 class="title" style="clear: both"> |
|
559 <a name="raw_input"></a>7. <span class="strong"><strong>raw_input()</strong></span> |
|
560 </h2></div></div></div> |
|
561 <p id="x_1f2"></a>In the previous example we saw the call to the raw_input() subroutine. The |
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562 <span class="strong"><strong>raw_input()</strong></span> method is used to take user inputs through the console. Unlike |
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563 <span class="strong"><strong>input()</strong></span> which assumes the data entered by the user as a standard python expression, |
|
564 <span class="strong"><strong>raw_input()</strong></span> treats all the input data as raw data and converts everything into a string. To illustrate this let us look at an example. |
|
565 </p> |
|
566 <pre class="programlisting">>>> input("Enter a number thats a palindrome:") |
|
567 Enter a number thats a palindrome:121 |
|
568 121 |
|
569 |
|
570 >>> input("Enter your name:") |
|
571 Enter your name:PythonFreak |
|
572 Traceback (most recent call last): |
|
573 File "<stdin>", line 1, in <module> |
|
574 File "<string>", line 1, in <module> |
|
575 NameError: name 'PythonFreak' is not defined |
|
576 |
|
577 </pre> |
|
578 <p id="x_1f3"></a>As shown above the |
|
579 <span class="strong"><strong>input()</strong></span> assumes that the data entered is a valid Python expression. In the first call it prompts for an integer input and when entered it accepts the integer as an integer, whereas in the second call, when the string is entered without the quotes, |
|
580 <span class="strong"><strong>input()</strong></span> assumes that the entered data is a valid Python expression and hence it raises and exception saying PythonFreak is not defined. |
|
581 </p> |
|
582 <pre class="programlisting">>>> input("Enter your name:") |
|
583 Enter your name:'PythonFreak' |
|
584 'PythonFreak' |
|
585 >>> |
|
586 |
|
587 </pre> |
|
588 <p id="x_1f4"></a>Here the name is accepted because its entered as a string (within quotes). But its unreasonable to go on using quotes each time a string is entered. Hence the alternative is to use |
|
589 <span class="strong"><strong>raw_input()</strong></span>. |
|
590 </p> |
|
591 <p id="x_1f5"></a>Let us now look at how |
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592 <span class="strong"><strong>raw_input()</strong></span> operates with an example. |
|
593 </p> |
|
594 <pre class="programlisting">>>> raw_input("Enter your name:") |
|
595 Enter your name:PythonFreak |
|
596 'PythonFreak' |
|
597 |
|
598 </pre> |
|
599 <p id="x_1f6"></a>Observe that the |
|
600 <span class="strong"><strong>raw_input()</strong></span> is converting it into a string all by itself. |
|
601 </p> |
|
602 <pre class="programlisting">>>> pal = raw_input("Enter a number thats a palindrome:") |
|
603 Enter a number thats a palindrome:121 |
|
604 '121' |
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605 |
|
606 </pre> |
|
607 <p id="x_1f7"></a>Observe that |
|
608 <span class="strong"><strong>raw_input()</strong></span> is converting the integer 121 also to a string as '121'. Let us look at another example: |
|
609 </p> |
|
610 <pre class="programlisting">>>> pal = raw_input("Enter a number thats a palindrome:") |
|
611 Enter a number thats a palindrome:121 |
|
612 >>> pal + 2 |
|
613 Traceback (most recent call last): |
|
614 File "<stdin>", line 1, in <module> |
|
615 TypeError: cannot concatenate 'str' and 'int' objects |
|
616 >>> pal |
|
617 '121' |
|
618 |
|
619 </pre> |
|
620 <p id="x_1f8"></a>Observe here that the variable |
|
621 <span class="emphasis"><em>pal</em></span> is a string and hence integer operations cannot be performed on it. Hence the exception is raised. |
|
622 </p> |
|
623 </div> |
|
624 <div class="section" title="8. int() method"> |
|
625 <div class="titlepage"><div><div><h2 class="title" style="clear: both"> |
|
626 <a name="int-method"></a>8. |
|
627 <span class="strong"><strong>int()</strong></span> method |
|
628 </h2></div></div></div> |
|
629 <p id="x_1f9"></a>Generally for computing purposes, the data used is not strings or raw data but on integers, floats and similar mathematical data structures. The data obtained from |
|
630 <span class="strong"><strong>raw_input()</strong></span> is raw data in the form of strings. In order to obtain integers from strings we use the method |
|
631 <span class="strong"><strong>int()</strong></span>. |
|
632 </p> |
|
633 <p id="x_1fa"></a>Let us look at an example.</p> |
|
634 <pre class="programlisting">>>> intpal = int(pal) |
|
635 >>> intpal |
|
636 121 |
|
637 |
|
638 </pre> |
|
639 <p id="x_1fb"></a>In the previous example it was observed that |
|
640 <span class="emphasis"><em>pal</em></span> was a string variable. Here using the |
|
641 <span class="strong"><strong>int()</strong></span> method the string |
|
642 <span class="emphasis"><em>pal</em></span> was converted to an integer variable. |
|
643 </p> |
|
644 <p id="x_1fc"></a> |
|
645 <span class="emphasis"><em>Try This Yourself:</em></span> |
|
646 </p> |
|
647 <pre class="programlisting">>>> stringvar = raw_input("Enter a name:") |
|
648 Enter a name:Guido Van Rossum |
|
649 >>> stringvar |
|
650 'Guido Van Rossum' |
|
651 >>> numvar = int(stringvar) |
|
652 |
|
653 </pre> |
|
654 </div> |
|
655 </div> |
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656 </div> |
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657 </body> |
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658 </html> |
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