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15 <div class="chapter" id="ch2intro"> |
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16 <div class="titlepage"></div> |
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17 <div class="toc"> |
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18 <p><b>Table of Contents</b></p> |
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19 <dl> |
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20 <dt><span class="article"><a href="#id2706129">Basic Python</a></span></dt> |
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21 <dd><dl> |
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22 <dt><span class="section"><a href="#id2705187">1. Introduction</a></span></dt> |
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23 <dt><span class="section"><a href="#id2757271">2. The Python Interpreter</a></span></dt> |
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24 <dd><dl> |
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25 <dt><span class="section"><a href="#id2757279">2.1. The Interactive Interpreter</a></span></dt> |
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26 <dt><span class="section"><a href="#id2757464">2.2. <span class="emphasis"><em>ipython</em></span> - An enhanced interactive Python interpreter</a></span></dt> |
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27 </dl></dd> |
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28 <dt><span class="section"><a href="#id2757589">3. Editing and running a python file</a></span></dt> |
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29 <dt><span class="section"><a href="#id2757696">4. Basic Datatypes and operators in Python</a></span></dt> |
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30 <dd><dl> |
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31 <dt><span class="section"><a href="#id2757731">4.1. Numbers</a></span></dt> |
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32 <dt><span class="section"><a href="#id2757854">4.2. Variables</a></span></dt> |
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33 <dt><span class="section"><a href="#id2757903">4.3. Strings</a></span></dt> |
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34 <dt><span class="section"><a href="#id2758097">4.4. Boolean</a></span></dt> |
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35 </dl></dd> |
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36 <dt><span class="section"><a href="#id2758126">5. The <span class="strong"><strong>while</strong></span> loop</a></span></dt> |
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37 <dt><span class="section"><a href="#id2758163">6. The <span class="strong"><strong>if</strong></span> conditional</a></span></dt> |
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38 <dt><span class="section"><a href="#id2758232">7. <span class="strong"><strong>raw_input()</strong></span></a></span></dt> |
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39 <dt><span class="section"><a href="#id2758387">8. <span class="strong"><strong>int()</strong></span> method</a></span></dt> |
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40 </dl></dd> |
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41 </dl> |
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42 </div> |
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43 <div class="article" title="Basic Python"> |
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44 <div class="titlepage"> |
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45 <div><div><h2 class="title"> |
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46 <a name="id2706129"></a>Basic Python</h2></div></div> |
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47 <hr> |
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48 </div> |
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49 <div class="toc"> |
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50 <p><b>Table of Contents</b></p> |
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51 <dl> |
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52 <dt><span class="section"><a href="#id2705187">1. Introduction</a></span></dt> |
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53 <dt><span class="section"><a href="#id2757271">2. The Python Interpreter</a></span></dt> |
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54 <dd><dl> |
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55 <dt><span class="section"><a href="#id2757279">2.1. The Interactive Interpreter</a></span></dt> |
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56 <dt><span class="section"><a href="#id2757464">2.2. <span class="emphasis"><em>ipython</em></span> - An enhanced interactive Python interpreter</a></span></dt> |
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57 </dl></dd> |
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58 <dt><span class="section"><a href="#id2757589">3. Editing and running a python file</a></span></dt> |
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59 <dt><span class="section"><a href="#id2757696">4. Basic Datatypes and operators in Python</a></span></dt> |
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60 <dd><dl> |
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61 <dt><span class="section"><a href="#id2757731">4.1. Numbers</a></span></dt> |
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62 <dt><span class="section"><a href="#id2757854">4.2. Variables</a></span></dt> |
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63 <dt><span class="section"><a href="#id2757903">4.3. Strings</a></span></dt> |
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64 <dt><span class="section"><a href="#id2758097">4.4. Boolean</a></span></dt> |
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65 </dl></dd> |
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66 <dt><span class="section"><a href="#id2758126">5. The <span class="strong"><strong>while</strong></span> loop</a></span></dt> |
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67 <dt><span class="section"><a href="#id2758163">6. The <span class="strong"><strong>if</strong></span> conditional</a></span></dt> |
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68 <dt><span class="section"><a href="#id2758232">7. <span class="strong"><strong>raw_input()</strong></span></a></span></dt> |
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69 <dt><span class="section"><a href="#id2758387">8. <span class="strong"><strong>int()</strong></span> method</a></span></dt> |
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70 </dl> |
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71 </div> |
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72 <p id="ch2intro_1"></a>This document is intended to be handed out at the end of the workshop. It has |
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73 been designed for Engineering students who are Python beginners and have basic |
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74 programming skills. The focus is on basic numerics and plotting using Python.</p> |
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75 <span style="color: black"><span style="color: black"><span style="color: black">The system requirements:</span><span style="color: black"><span style="color: black"><span style="color: black"><p id="ch2intro_2"></a>Python - version 2.5.x or newer.</p></span><span style="color: black"><p id="ch2intro_3"></a>IPython</p></span><span style="color: black"><p id="ch2intro_4"></a>Text editor - scite, vim, emacs or whatever you are comfortable with.</p></span></span></span></span></span><div class="section" title="1. Introduction"> |
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76 <div class="titlepage"><div><div><h2 class="title" style="clear: both"> |
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77 <a name="id2705187"></a>1. Introduction</h2></div></div></div> |
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78 <p id="ch2intro_5"></a>The Python programming language was created by a dutch named Guido van Rossum. |
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79 The idea of Python was conceived in December 1989. The name Python has nothing |
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80 to do with the reptilian, but its been named after the 70s comedy series |
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81 "Monty Python's Flying Circus", since it happens to be Guido's favourite |
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82 TV series.</p> |
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83 <p id="ch2intro_6"></a>Current stable version of Python is 2.6.x, although Python 3.0 is also the stable |
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84 version, it is not backwards compatible with the previous versions and is hence |
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85 not entirely popular at the moment. This material will focus on the 2.6.x series.</p> |
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86 <p id="ch2intro_7"></a>Python is licensed under the Python Software Foundation License (PSF License) |
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87 which is GPL compatible Free Software license (excepting license version 1.6 and 2.0) |
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88 It is a no strings attached license, which means the source code is free to modify |
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89 and redistribute.</p> |
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90 <p id="ch2intro_8"></a>The Python docs define Python as "Python is an interpreted, object-oriented, |
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91 high-level programming language with dynamic semantics." A more detailed summary |
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92 can be found at </p> |
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93 <div class="reference"> |
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94 <div class="titlepage"><hr></div>http://www.python.org/doc/essays/blurb.html</div> |
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95 <p>. Python is a language that |
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96 has been designed to help the programmer concentrate on solving the problem at hand |
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97 and not worry about the programming language idiosyncrasies.</p> |
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98 <p id="ch2intro_9"></a>Python is a highly cross platform compatible language on account of it being an |
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99 interpreted language. It is highly scalable and hence has been adapted to run on |
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100 the Nokia 60 series phones. Python has been designed to be readable and easy to use</p> |
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101 <p id="ch2intro_a"></a><span class="strong"><strong>Resources available for reference</strong></span></p> |
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102 <span style="color: black"><span style="color: black"><p id="ch2intro_b"></a>Web: </p> |
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103 <div class="reference"> |
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104 <div class="titlepage"><hr></div>http://www.python.org</div></span><span style="color: black"><p id="ch2intro_c"></a>Doc: </p> |
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105 <div class="reference"> |
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106 <div class="titlepage"><hr></div>http://www.python.org/doc</div></span><span style="color: black"><span style="color: black"><span style="color: black"><span style="color: black">Free Tutorials:</span><span style="color: black"><span style="color: black"><span style="color: black"><p id="ch2intro_d"></a>Official Python Tutorial: </p> |
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107 <div class="reference"> |
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108 <div class="titlepage"><hr></div>http://docs.python.org/tut/tut.html</div></span><span style="color: black"><p id="ch2intro_e"></a>Byte of Python: </p> |
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109 <div class="reference"> |
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110 <div class="titlepage"><hr></div>http://www.byteofpython.info/</div></span><span style="color: black"><p id="ch2intro_f"></a>Dive into Python: </p> |
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111 <div class="reference"> |
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112 <div class="titlepage"><hr></div>http://diveintopython.org/</div></span></span></span></span></span></span></span><p id="ch2intro_10"></a><span class="strong"><strong>Advantages of Python - Why Python??</strong></span></p> |
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113 <span style="color: black"><span style="color: black"><p id="ch2intro_11"></a>Python has been designed for readability and ease of use. Its been designed in |
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114 such a fashion that it imposes readability on the programmer. Python does away |
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115 with the braces and the semicolons and instead implements code blocks based on |
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116 indentation, thus enhancing readability.</p></span><span style="color: black"><p id="ch2intro_12"></a>Python is a high level, interpreted, modular and object oriented language. |
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117 Python performs memory management on its own, thus the programmer need not bother |
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118 about allocating and deallocating memory to variables. Python provides extensibility |
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119 by providing modules which can be easily imported similar to headers in C and |
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120 packages in Java. Python is object oriented and hence provides all the object oriented |
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121 characteristics such as inheritance, encapsulation and polymorphism.</p></span><span style="color: black"><p id="ch2intro_13"></a>Python offers a highly powerful interactive programming interface in the form |
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122 of the 'Interactive Interpreter' which will be discussed in more detail in the |
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123 following sections.</p></span><span style="color: black"><p id="ch2intro_14"></a>Python provides a rich standard library and an extensive set of modules. The |
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124 power of Python modules can be seen in this slightly exaggerated cartoon |
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125 </p> |
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126 <div class="reference"> |
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127 <div class="titlepage"><hr></div>http://xkcd.com/353/</div></span><span style="color: black"><p id="ch2intro_15"></a>Python interfaces well with most other programming languages such as C, C++ |
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128 and FORTRAN.</p></span></span><p id="ch2intro_16"></a>Although, Python has one setback. Python is not fast as some of the compiled |
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129 languages like C or C++. Yet, the amount of flexibility and power more than make |
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130 up for this setback.</p> |
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131 </div> |
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132 <div class="section" title="2. The Python Interpreter"> |
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133 <div class="titlepage"><div><div><h2 class="title" style="clear: both"> |
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134 <a name="id2757271"></a>2. The Python Interpreter</h2></div></div></div> |
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135 <div class="section" title="2.1. The Interactive Interpreter"> |
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136 <div class="titlepage"><div><div><h3 class="title"> |
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137 <a name="id2757279"></a>2.1. The Interactive Interpreter</h3></div></div></div> |
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138 <p id="ch2intro_17"></a>Typing <span class="emphasis"><em>python</em></span> at the shell prompt on any standard Unix/Gnu-Linux system and |
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139 hitting the enter key fires up the Python 'Interactive Interpreter'. The Python |
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140 interpreter is one of the most integral features of Python. The prompt obtained |
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141 when the interactive interpreter is similar to what is shown below. The exact |
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142 appearance might differ based on the version of Python being used. The <code class="literal">>>></code> |
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143 thing shown is the python prompt. When something is typed at the prompt and the |
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144 enter key is hit, the python interpreter interprets the command entered and |
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145 performs the appropriate action. All the examples presented in this document are |
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146 to be tried hands on, on the interactive interpreter.</p> |
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147 <pre class="programlisting"> Python 2.5.2 (r252:60911, Oct 5 2008, 19:24:49) |
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148 [GCC 4.3.2] on linux2 |
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149 Type "help", "copyright", "credits" or "license" for more information. |
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150 >>></pre> |
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151 <p id="ch2intro_18"></a>Lets try with an example, type <code class="literal">print 'Hello, World!'</code> at the prompt and hit |
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152 the enter key.</p> |
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153 <pre class="programlisting"> >>> print 'Hello, World!' |
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154 Hello, World!</pre> |
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155 <p id="ch2intro_19"></a>This example was quite straight forward, and thus we have written our first |
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156 line of Python code. Now let us try typing something arbitrary at the prompt. |
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157 For example:</p> |
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158 <pre class="programlisting"> >>> arbit word |
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159 File "<stdin>", line 1 |
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160 arbit word |
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161 ^ |
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162 SyntaxError: invalid syntax |
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163 >>></pre> |
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164 <p id="ch2intro_1a"></a>The interpreter gave an error message saying that 'arbit word' was invalid |
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165 syntax which is valid. The interpreter is an amazing tool when learning to |
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166 program in Python. The interpreter provides a help function that provides the |
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167 necessary documentation regarding all Python syntax, constructs, modules and |
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168 objects. Typing <span class="emphasis"><em>help()</em></span> at the prompt gives the following output:</p> |
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169 <pre class="programlisting"> >>> help() |
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170 |
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171 Welcome to Python 2.5! This is the online help utility. |
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172 |
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173 If this is your first time using Python, you should definitely check out |
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174 the tutorial on the Internet at http://www.python.org/doc/tut/. |
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175 |
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176 Enter the name of any module, keyword, or topic to get help on writing |
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177 Python programs and using Python modules. To quit this help utility and |
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178 return to the interpreter, just type "quit". |
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179 |
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180 To get a list of available modules, keywords, or topics, type "modules", |
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181 "keywords", or "topics". Each module also comes with a one-line summary |
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182 of what it does; to list the modules whose summaries contain a given word |
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183 such as "spam", type "modules spam". |
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184 |
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185 help></pre> |
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186 <p id="ch2intro_1b"></a>As mentioned in the output, entering the name of any module, keyword or topic |
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187 will provide the documentation and help regarding the same through the online |
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188 help utility. Pressing <span class="emphasis"><em>Ctrl+d</em></span> exits the help prompt and returns to the |
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189 python prompt.</p> |
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190 <p id="ch2intro_1c"></a>Let us now try a few examples at the python interpreter.</p> |
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191 <p id="ch2intro_1d"></a>Eg 1:</p> |
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192 <pre class="programlisting"> >>> print 'Hello, python!' |
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193 Hello, python! |
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194 >>></pre> |
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195 <p id="ch2intro_1e"></a>Eg 2:</p> |
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196 <pre class="programlisting"> >>> print 4321*567890 |
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197 2453852690 |
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198 >>></pre> |
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199 <p id="ch2intro_1f"></a>Eg 3:</p> |
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200 <pre class="programlisting"> >>> 4321*567890 |
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201 2453852690L |
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202 >>></pre> |
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203 <pre class="programlisting"> Note: Notice the 'L' at the end of the output. The 'L' signifies that the |
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204 output of the operation is of type *long*. It was absent in the previous |
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205 example because we used the print statement. This is because *print* formats |
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206 the output before displaying.</pre> |
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207 <p id="ch2intro_20"></a>Eg 4:</p> |
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208 <pre class="programlisting"> >>> big = 12345678901234567890 ** 3 |
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209 >>> print big |
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210 1881676372353657772490265749424677022198701224860897069000 |
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211 >>></pre> |
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212 <pre class="programlisting"> This example is to show that unlike in C or C++ there is no limit on the |
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213 value of an integer.</pre> |
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214 <p id="ch2intro_21"></a>Try this on the interactive interpreter: |
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215 <code class="literal">import this</code></p> |
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216 <p id="ch2intro_22"></a><span class="emphasis"><em>Hint: The output gives an idea of Power of Python</em></span></p> |
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217 </div> |
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218 <div class="section" title="2.2. ipython - An enhanced interactive Python interpreter"> |
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219 <div class="titlepage"><div><div><h3 class="title"> |
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220 <a name="id2757464"></a>2.2. <span class="emphasis"><em>ipython</em></span> - An enhanced interactive Python interpreter</h3></div></div></div> |
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221 <p id="ch2intro_23"></a>The power and the importance of the interactive interpreter was the highlight |
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222 of the previous section. This section provides insight into the enhanced |
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223 interpreter with more advanced set of features called <span class="strong"><strong>ipython</strong></span>. Entering |
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224 <span class="emphasis"><em>ipython</em></span> at the shell prompt fires up the interactive interpreter.</p> |
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225 <pre class="programlisting"> $ ipython |
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226 Python 2.5.2 (r252:60911, Oct 5 2008, 19:24:49) |
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227 Type "copyright", "credits" or "license" for more information. |
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228 |
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229 IPython 0.8.4 -- An enhanced Interactive Python. |
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230 ? -> Introduction and overview of IPython's features. |
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231 %quickref -> Quick reference. |
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232 help -> Python's own help system. |
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233 object? -> Details about 'object'. ?object also works, ?? prints more. |
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234 |
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235 In [1]:</pre> |
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236 <p id="ch2intro_24"></a>This is the output obtained upon firing ipython. The exact appearance may |
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237 change based on the Python version installed. The following are some of the |
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238 various features provided by <span class="strong"><strong>ipython</strong></span>:</p> |
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239 <span style="color: black"><p id="ch2intro_25"></a>Suggestions - ipython provides suggestions of the possible methods and |
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240 operations available for the given python object.</p></span><p id="ch2intro_26"></a>Eg 5:</p> |
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241 <pre class="programlisting"> In [4]: a = 6 |
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242 |
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243 In [5]: a. |
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244 a.__abs__ a.__divmod__ a.__index__ a.__neg__ a.__rand__ a.__rmod__ a.__rxor__ |
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245 a.__add__ a.__doc__ a.__init__ a.__new__ a.__rdiv__ a.__rmul__ a.__setattr__ |
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246 a.__and__ a.__float__ a.__int__ a.__nonzero__ a.__rdivmod__ a.__ror__ a.__str__ |
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247 a.__class__ a.__floordiv__ a.__invert__ a.__oct__ a.__reduce__ a.__rpow__ a.__sub__ |
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248 a.__cmp__ a.__getattribute__ a.__long__ a.__or__ a.__reduce_ex__ a.__rrshift__ a.__truediv__ |
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249 a.__coerce__ a.__getnewargs__ a.__lshift__ a.__pos__ a.__repr__ a.__rshift__ a.__xor__ |
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250 a.__delattr__ a.__hash__ a.__mod__ a.__pow__ a.__rfloordiv__ a.__rsub__ |
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251 a.__div__ a.__hex__ a.__mul__ a.__radd__ a.__rlshift__ a.__rtruediv__</pre> |
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252 <p id="ch2intro_27"></a>In this example, we initialized 'a' (a variable - a concept that will be |
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253 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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254 is pressed after typing '<span class="emphasis"><em>a.</em></span>' ipython displays the set of all possible methods |
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255 that are applicable on the object 'a' (an integer in this context). Ipython |
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256 provides many such datatype specific features which will be presented in the |
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257 further sections as and when the datatypes are introduced.</p> |
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258 </div> |
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259 </div> |
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260 <div class="section" title="3. Editing and running a python file"> |
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261 <div class="titlepage"><div><div><h2 class="title" style="clear: both"> |
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262 <a name="id2757589"></a>3. Editing and running a python file</h2></div></div></div> |
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263 <p id="ch2intro_28"></a>The previous sections focused on the use of the interpreter to run python code. |
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264 While the interpeter is an excellent tool to test simple solutions and |
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265 experiment with small code snippets, its main disadvantage is that everything |
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266 written in the interpreter is lost once its quit. Most of the times a program is |
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267 used by people other than the author. So the programs have to be available in |
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268 some form suitable for distribution, and hence they are written in files. This |
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269 section will focus on editing and running python files. Start by opening a text |
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270 editor ( it is recommended you choose one from the list at the top of this page ). |
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271 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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272 (python files have an extension of .py). Once done with the editing, save the |
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273 file and exit the editor.</p> |
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274 <p id="ch2intro_29"></a>Let us look at a simple example of calculating the gcd of 2 numbers using Python:</p> |
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275 <p id="ch2intro_2a"></a><span class="strong"><strong>Creating the first python script(file)</strong></span></p> |
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276 <pre class="programlisting"> $ emacs gcd.py |
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277 def gcd(x,y): |
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278 if x % y == 0: |
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279 return y |
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280 return gcd(y, x%y) |
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281 |
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282 print gcd(72, 92)</pre> |
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283 <p id="ch2intro_2b"></a>To run the script, open the shell prompt, navigate to the directory that |
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284 contains the python file and run <code class="literal">python <filename.py></code> at the prompt ( in this |
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285 case filename is gcd.py )</p> |
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286 <p id="ch2intro_2c"></a><span class="strong"><strong>Running the python script</strong></span></p> |
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287 <pre class="programlisting"> $ python gcd.py |
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288 4 |
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289 $</pre> |
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290 <p id="ch2intro_2d"></a>Another method to run a python script would be to include the line</p> |
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291 <p id="ch2intro_2e"></a><code class="literal">#! /usr/bin/python</code></p> |
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292 <p id="ch2intro_2f"></a>at the beginning of the python file and then make the file executable by</p> |
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293 <p id="ch2intro_30"></a>$ chmod a+x <span class="emphasis"><em>filename.py</em></span></p> |
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294 <p id="ch2intro_31"></a>Once this is done, the script can be run as a standalone program as follows:</p> |
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295 <p id="ch2intro_32"></a>$ ./<span class="emphasis"><em>filename.py</em></span></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="id2757696"></a>4. Basic Datatypes and operators in Python</h2></div></div></div> |
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300 <p id="ch2intro_33"></a>Python provides the following set of basic datatypes.</p> |
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301 <span style="color: black"><span style="color: black"><span style="color: black"><p id="ch2intro_34"></a>Numbers: int, float, long, complex</p></span><span style="color: black"><p id="ch2intro_35"></a>Strings</p></span><span style="color: black"><p id="ch2intro_36"></a>Boolean</p></span></span></span><div class="section" title="4.1. Numbers"> |
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302 <div class="titlepage"><div><div><h3 class="title"> |
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303 <a name="id2757731"></a>4.1. Numbers</h3></div></div></div> |
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304 <p id="ch2intro_37"></a>Numbers were introduced in the examples presented in the interactive interpreter |
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305 section. Numbers include types as mentioned earlier viz., int (integers), float |
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306 (floating point numbers), long (large integers), complex (complex numbers with |
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307 real and imaginary parts). Python is not a strongly typed language, which means |
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308 the type of a variable need not mentioned during its initialization. Let us look |
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309 at a few examples.</p> |
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310 <p id="ch2intro_38"></a>Eg 6:</p> |
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311 <pre class="programlisting"> >>> a = 1 #here a is an integer variable</pre> |
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312 <p id="ch2intro_39"></a>Eg 7:</p> |
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313 <pre class="programlisting"> >>> lng = 122333444455555666666777777788888888999999999 #here lng is a variable of type long |
|
314 >>> lng |
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315 122333444455555666666777777788888888999999999L #notice the trailing 'L' |
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316 >>> print lng |
|
317 122333444455555666666777777788888888999999999 #notice the absence of the trailing 'L' |
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318 >>> lng+1 |
|
319 122333444455555666666777777788888889000000000L</pre> |
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320 <p id="ch2intro_3a"></a>Long numbers are the same as integers in almost all aspects. They can be used in |
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321 operations just like integers and along with integers without any distinction. |
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322 The only distinction comes during type checking (which is not a healthy practice). |
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323 Long numbers are tucked with a trailing 'L' just to signify that they are long. |
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324 Notice that in the example just lng at the prompt displays the value of the variable |
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325 with the 'L' whereas <code class="literal">print lng</code> displays without the 'L'. This is because print |
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326 formats the output before printing. Also in the example, notice that adding an |
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327 integer to a long does not give any errors and the result is as expected. So for |
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328 all practical purposes longs can be treated as ints.</p> |
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329 <p id="ch2intro_3b"></a>Eg 8:</p> |
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330 <pre class="programlisting"> >>> fl = 3.14159 #fl is a float variable |
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331 >>> e = 1.234e-4 #e is also a float variable, specified in the exponential form |
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332 >>> a = 1 |
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333 >>> b = 2 |
|
334 >>> a/b #integer division |
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335 0 |
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336 >>> a/fl #floating point division |
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337 0.31831015504887655 |
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338 >>> e/fl |
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339 3.9279473133031364e-05</pre> |
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340 <p id="ch2intro_3c"></a>Floating point numbers, simply called floats are real numbers with a decimal point. |
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341 The example above shows the initialization of a float variable. Shown also in this |
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342 example is the difference between integer division and floating point division. |
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343 'a' and 'b' here are integer variables and hence the division gives 0 as the quotient. |
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344 When either of the operands is a float, the operation is a floating point division, |
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345 and the result is also a float as illustrated.</p> |
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346 <p id="ch2intro_3d"></a>Eg 9:</p> |
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347 <pre class="programlisting"> >>> cplx = 3 + 4j #cplx is a complex variable |
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348 >>> cplx |
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349 (3+4j) |
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350 >>> print cplx.real #prints the real part of the complex number |
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351 3.0 |
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352 >>> print cplx.imag #prints the imaginary part of the complex number |
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353 4.0 |
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354 >>> print cplx*fl #multiplies the real and imag parts of the complex number with the multiplier |
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355 (9.42477+12.56636j) |
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356 >>> abs(cplx) #returns the absolute value of the complex number |
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357 5.0</pre> |
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358 <p id="ch2intro_3e"></a>Python provides a datatype for complex numbers. Complex numbers are initialized |
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359 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 |
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360 imaginary parts of the complex number as shown. The <span class="emphasis"><em>abs()</em></span> returns the absolute |
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361 value of the complex number.</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="id2757854"></a>4.2. Variables</h3></div></div></div> |
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366 <p id="ch2intro_3f"></a>Variables are just names that represent a value. Variables have already been |
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367 introduced in the various examples from the previous sections. Certain rules about |
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368 using variables:</p> |
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369 <span style="color: black"><span style="color: black"><span style="color: black"><p id="ch2intro_40"></a>Variables have to be initialized or assigned a value before being used.</p></span><span style="color: black"><p id="ch2intro_41"></a>Variable names can consist of letters, digits and underscores(_).</p></span><span style="color: black"><p id="ch2intro_42"></a>Variable names cannot begin with digits, but can contain digits in them.</p></span></span></span><p id="ch2intro_43"></a>In reference to the previous section examples, 'a', 'b', 'lng', 'fl', 'e' and 'cplx' |
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370 are all variables of various datatypes.</p> |
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371 <pre class="programlisting"> Note: Python is not a strongly typed language and hence an integer variable can at a |
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372 later stage be used as a float variable as well.</pre> |
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373 </div> |
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374 <div class="section" title="4.3. Strings"> |
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375 <div class="titlepage"><div><div><h3 class="title"> |
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376 <a name="id2757903"></a>4.3. Strings</h3></div></div></div> |
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377 <p id="ch2intro_44"></a>Strings are one of the essential data structures of any programming language. |
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378 The <code class="literal">print "Hello, World!"</code> program was introduced in the earlier section, and |
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379 the <span class="emphasis"><em>"Hello, World!"</em></span> in the print statement is a string. A string is basically |
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380 a set of characters. Strings can be represented in various ways shown below:</p> |
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381 <pre class="programlisting"> s = 'this is a string' # a string variable can be represented using single quotes |
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382 s = 'This one has "quotes" inside!' # The string can have quotes inside it as shown |
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383 s = "I have 'single-quotes' inside!" |
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384 l = "A string spanning many lines\ |
|
385 one more line\ |
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386 yet another" # a string can span more than a single line. |
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387 t = """A triple quoted string does # another way of representing multiline strings. |
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388 not need to be escaped at the end and |
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389 "can have nested quotes" etc."""</pre> |
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390 <p id="ch2intro_45"></a>Try the following on the interpreter: |
|
391 <code class="literal">s = 'this is a string with 'quotes' of similar kind'</code></p> |
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392 <p id="ch2intro_46"></a><span class="strong"><strong>Exercise: How to use single quotes within single quotes in a string as shown |
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393 in the above example without getting an error?</strong></span></p> |
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394 <div class="section" title="4.3.1. String operations"> |
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395 <div class="titlepage"><div><div><h4 class="title"> |
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396 <a name="id2757955"></a>4.3.1. String operations</h4></div></div></div> |
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397 <p id="ch2intro_47"></a>A few basic string operations are presented here.</p> |
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398 <p id="ch2intro_48"></a><span class="strong"><strong>String concatenation</strong></span> |
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399 String concatenation is done by simple addition of two strings.</p> |
|
400 <pre class="programlisting"> >>> x = 'Hello' |
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401 >>> y = ' Python' |
|
402 >>> print x+y |
|
403 Hello Python</pre> |
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404 <p id="ch2intro_49"></a><span class="emphasis"><em>Try this yourself:</em></span></p> |
|
405 <pre class="programlisting"> >>> somenum = 13 |
|
406 >>> print x+somenum</pre> |
|
407 <p id="ch2intro_4a"></a>The problem with the above example is that here a string variable and an integer |
|
408 variable are trying to be concantenated. To obtain the desired result from the |
|
409 above example the str(), repr() and the `` can be used.</p> |
|
410 <p id="ch2intro_4b"></a><span class="strong"><strong>str()</strong></span> simply converts a value to a string in a reasonable form. |
|
411 <span class="strong"><strong>repr()</strong></span> creates a string that is a representation of the value.</p> |
|
412 <p id="ch2intro_4c"></a>The difference can be seen in the example shown below:</p> |
|
413 <pre class="programlisting"> >>> str(1000000000000000000000000000000000000000000000000L) |
|
414 '1000000000000000000000000000000000000000000000000' |
|
415 >>> repr(1000000000000000000000000000000000000000000000000L) |
|
416 '1000000000000000000000000000000000000000000000000L'</pre> |
|
417 <p id="ch2intro_4d"></a>It can be observed that the 'L' in the long value shown was omitted by str(), |
|
418 whereas repr() converted that into a string too. An alternative way of using |
|
419 repr(value) is <code class="literal">`value`</code>.</p> |
|
420 <p id="ch2intro_4e"></a>A few more examples:</p> |
|
421 <pre class="programlisting"> >>> x = "Let's go \nto Pycon" |
|
422 >>> print x |
|
423 Let's go |
|
424 to Pycon</pre> |
|
425 <p id="ch2intro_4f"></a>In the above example, notice that the 'n'(newline) character is formatted and |
|
426 the string is printed on two lines. The strings discussed until now were normal |
|
427 strings. Other than these there are two other types of strings namely, raw strings |
|
428 and unicode strings.</p> |
|
429 <p id="ch2intro_50"></a><span class="strong"><strong>Raw strings</strong></span> are strings which are unformatted, that is the backslashes() are |
|
430 not parsed and are left as it is in the string. Raw strings are represented with |
|
431 an 'r' at the start of a string. |
|
432 Let us look at an example</p> |
|
433 <pre class="programlisting"> >>> x = r"Let's go \nto Pycon" |
|
434 >>> print x |
|
435 Let's go \nto Pycon</pre> |
|
436 <p id="ch2intro_51"></a>Note: The 'n' is not being parsed into a new line and is left as it is.</p> |
|
437 <p id="ch2intro_52"></a><span class="emphasis"><em>Try this yourself:</em></span></p> |
|
438 <pre class="programlisting"> >>> x = r"Let's go to Pycon\"</pre> |
|
439 <p id="ch2intro_53"></a><span class="strong"><strong>Unicode strings</strong></span> are strings where the characters are Unicode characters as |
|
440 opposed to ASCII characters. Unicode strings are represented with a 'u' at the |
|
441 start of the string. |
|
442 Let us look at an example:</p> |
|
443 <pre class="programlisting"> >>> x = u"Let's go to Pycon!" |
|
444 >>> print x |
|
445 Let's go to Pycon!</pre> |
|
446 </div> |
|
447 </div> |
|
448 <div class="section" title="4.4. Boolean"> |
|
449 <div class="titlepage"><div><div><h3 class="title"> |
|
450 <a name="id2758097"></a>4.4. Boolean</h3></div></div></div> |
|
451 <p id="ch2intro_54"></a>Python also provides special Boolean datatype. A boolean variable can assume a |
|
452 value of either <span class="emphasis"><em>True</em></span> or <span class="emphasis"><em>False</em></span> (Note the capitalizations).</p> |
|
453 <p id="ch2intro_55"></a>Let us look at examples:</p> |
|
454 <pre class="programlisting"> >>> t = True |
|
455 >>> f = not t |
|
456 >>> print f |
|
457 False |
|
458 >>> f or t |
|
459 True |
|
460 >>> f and t |
|
461 False</pre> |
|
462 </div> |
|
463 </div> |
|
464 <div class="section" title="5. The while loop"> |
|
465 <div class="titlepage"><div><div><h2 class="title" style="clear: both"> |
|
466 <a name="id2758126"></a>5. The <span class="strong"><strong>while</strong></span> loop</h2></div></div></div> |
|
467 <p id="ch2intro_56"></a>The Python <span class="strong"><strong>while</strong></span> loop is similar to the C/C++ while loop. The syntax is as |
|
468 follows:</p> |
|
469 <pre class="programlisting"> statement 0 |
|
470 while condition: |
|
471 statement 1 #while block |
|
472 statement 2 #while block |
|
473 statement 3 #outside the while block.</pre> |
|
474 <p id="ch2intro_57"></a>Let us look at an example:</p> |
|
475 <pre class="programlisting"> >>> x = 1 |
|
476 >>> while x <= 5: |
|
477 ... print x |
|
478 ... x += 1 |
|
479 ... |
|
480 1 |
|
481 2 |
|
482 3 |
|
483 4 |
|
484 5</pre> |
|
485 </div> |
|
486 <div class="section" title="6. The if conditional"> |
|
487 <div class="titlepage"><div><div><h2 class="title" style="clear: both"> |
|
488 <a name="id2758163"></a>6. The <span class="strong"><strong>if</strong></span> conditional</h2></div></div></div> |
|
489 <p id="ch2intro_58"></a>The Python <span class="strong"><strong>if</strong></span> block provides the conditional execution of statements. |
|
490 If the condition evaluates as true the block of statements defined under the if |
|
491 block are executed.</p> |
|
492 <p id="ch2intro_59"></a>If the first block is not executed on account of the condition not being satisfied, |
|
493 the set of statements in the <span class="strong"><strong>else</strong></span> block are executed.</p> |
|
494 <p id="ch2intro_5a"></a>The <span class="strong"><strong>elif</strong></span> block provides the functionality of evaluation of multiple conditions |
|
495 as shown in the example.</p> |
|
496 <p id="ch2intro_5b"></a>The syntax is as follows:</p> |
|
497 <pre class="programlisting"> if condition : |
|
498 statement_1 |
|
499 statement_2 |
|
500 |
|
501 elif condition: |
|
502 statement_3 |
|
503 statement_4 |
|
504 else: |
|
505 statement_5 |
|
506 statement_6</pre> |
|
507 <p id="ch2intro_5c"></a>Let us look at an example:</p> |
|
508 <pre class="programlisting"> >>> n = raw_input("Input a number:") |
|
509 >>> if n < 0: |
|
510 print n," is negative" |
|
511 elif n > 0: |
|
512 print n," is positive" |
|
513 else: |
|
514 print n, " is 0"</pre> |
|
515 </div> |
|
516 <div class="section" title="7. raw_input()"> |
|
517 <div class="titlepage"><div><div><h2 class="title" style="clear: both"> |
|
518 <a name="id2758232"></a>7. <span class="strong"><strong>raw_input()</strong></span> |
|
519 </h2></div></div></div> |
|
520 <p id="ch2intro_5d"></a>In the previous example we saw the call to the raw_input() subroutine. |
|
521 The <span class="strong"><strong>raw_input()</strong></span> method is used to take user inputs through the console. |
|
522 Unlike <span class="strong"><strong>input()</strong></span> which assumes the data entered by the user as a standard python |
|
523 expression, <span class="strong"><strong>raw_input()</strong></span> treats all the input data as raw data and converts |
|
524 everything into a string. To illustrate this let us look at an example.</p> |
|
525 <pre class="programlisting"> >>> input("Enter a number thats a palindrome:") |
|
526 Enter a number thats a palindrome:121 |
|
527 121 |
|
528 |
|
529 >>> input("Enter your name:") |
|
530 Enter your name:PythonFreak |
|
531 Traceback (most recent call last): |
|
532 File "<stdin>", line 1, in <module> |
|
533 File "<string>", line 1, in <module> |
|
534 NameError: name 'PythonFreak' is not defined</pre> |
|
535 <p id="ch2intro_5e"></a>As shown above the <span class="strong"><strong>input()</strong></span> assumes that the data entered is a valid Python |
|
536 expression. In the first call it prompts for an integer input and when entered |
|
537 it accepts the integer as an integer, whereas in the second call, when the string |
|
538 is entered without the quotes, <span class="strong"><strong>input()</strong></span> assumes that the entered data is a valid |
|
539 Python expression and hence it raises and exception saying PythonFreak is not |
|
540 defined.</p> |
|
541 <pre class="programlisting"> >>> input("Enter your name:") |
|
542 Enter your name:'PythonFreak' |
|
543 'PythonFreak' |
|
544 >>></pre> |
|
545 <p id="ch2intro_5f"></a>Here the name is accepted because its entered as a string (within quotes). But |
|
546 its unreasonable to go on using quotes each time a string is entered. Hence the |
|
547 alternative is to use <span class="strong"><strong>raw_input()</strong></span>.</p> |
|
548 <p id="ch2intro_60"></a>Let us now look at how <span class="strong"><strong>raw_input()</strong></span> operates with an example.</p> |
|
549 <pre class="programlisting"> >>> raw_input("Enter your name:") |
|
550 Enter your name:PythonFreak |
|
551 'PythonFreak'</pre> |
|
552 <p id="ch2intro_61"></a>Observe that the <span class="strong"><strong>raw_input()</strong></span> is converting it into a string all by itself.</p> |
|
553 <pre class="programlisting"> >>> pal = raw_input("Enter a number thats a palindrome:") |
|
554 Enter a number thats a palindrome:121 |
|
555 '121'</pre> |
|
556 <p id="ch2intro_62"></a>Observe that <span class="strong"><strong>raw_input()</strong></span> is converting the integer 121 also to a string as |
|
557 '121'. Let us look at another example:</p> |
|
558 <pre class="programlisting"> >>> pal = raw_input("Enter a number thats a palindrome:") |
|
559 Enter a number thats a palindrome:121 |
|
560 >>> pal + 2 |
|
561 Traceback (most recent call last): |
|
562 File "<stdin>", line 1, in <module> |
|
563 TypeError: cannot concatenate 'str' and 'int' objects |
|
564 >>> pal |
|
565 '121'</pre> |
|
566 <p id="ch2intro_63"></a>Observe here that the variable <span class="emphasis"><em>pal</em></span> is a string and hence integer operations |
|
567 cannot be performed on it. Hence the exception is raised.</p> |
|
568 </div> |
|
569 <div class="section" title="8. int() method"> |
|
570 <div class="titlepage"><div><div><h2 class="title" style="clear: both"> |
|
571 <a name="id2758387"></a>8. <span class="strong"><strong>int()</strong></span> method</h2></div></div></div> |
|
572 <p id="ch2intro_64"></a>Generally for computing purposes, the data used is not strings or raw data but |
|
573 on integers, floats and similar mathematical data structures. The data obtained |
|
574 from <span class="strong"><strong>raw_input()</strong></span> is raw data in the form of strings. In order to obtain integers |
|
575 from strings we use the method <span class="strong"><strong>int()</strong></span>.</p> |
|
576 <p id="ch2intro_65"></a>Let us look at an example.</p> |
|
577 <pre class="programlisting"> >>> intpal = int(pal) |
|
578 >>> intpal |
|
579 121</pre> |
|
580 <p id="ch2intro_66"></a>In the previous example it was observed that <span class="emphasis"><em>pal</em></span> was a string variable. Here |
|
581 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> |
|
582 <p id="ch2intro_67"></a><span class="emphasis"><em>Try This Yourself:</em></span></p> |
|
583 <pre class="programlisting"> >>> stringvar = raw_input("Enter a name:") |
|
584 Enter a name:Guido Van Rossum |
|
585 >>> stringvar |
|
586 'Guido Van Rossum' |
|
587 >>> numvar = int(stringvar)</pre> |
|
588 </div> |
|
589 </div> |
|
590 </div></body> |
|
591 </html> |
|