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     4 <title>Chapter 2. Basic Python</title>
       
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    15 <div class="chapter" id="ch2intro">
       
    16 <div class="titlepage"></div>
       
    17 <div class="toc">
       
    18 <p><b>Table of Contents</b></p>
       
    19 <dl>
       
    20 <dt><span class="article"><a href="#id2706129">Basic Python</a></span></dt>
       
    21 <dd><dl>
       
    22 <dt><span class="section"><a href="#id2705187">1. Introduction</a></span></dt>
       
    23 <dt><span class="section"><a href="#id2757271">2. The Python Interpreter</a></span></dt>
       
    24 <dd><dl>
       
    25 <dt><span class="section"><a href="#id2757279">2.1. The Interactive Interpreter</a></span></dt>
       
    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>
       
    27 </dl></dd>
       
    28 <dt><span class="section"><a href="#id2757589">3. Editing and running a python file</a></span></dt>
       
    29 <dt><span class="section"><a href="#id2757696">4. Basic Datatypes and operators in Python</a></span></dt>
       
    30 <dd><dl>
       
    31 <dt><span class="section"><a href="#id2757731">4.1. Numbers</a></span></dt>
       
    32 <dt><span class="section"><a href="#id2757854">4.2. Variables</a></span></dt>
       
    33 <dt><span class="section"><a href="#id2757903">4.3. Strings</a></span></dt>
       
    34 <dt><span class="section"><a href="#id2758097">4.4. Boolean</a></span></dt>
       
    35 </dl></dd>
       
    36 <dt><span class="section"><a href="#id2758126">5. The <span class="strong"><strong>while</strong></span> loop</a></span></dt>
       
    37 <dt><span class="section"><a href="#id2758163">6. The <span class="strong"><strong>if</strong></span> conditional</a></span></dt>
       
    38 <dt><span class="section"><a href="#id2758232">7. <span class="strong"><strong>raw_input()</strong></span></a></span></dt>
       
    39 <dt><span class="section"><a href="#id2758387">8. <span class="strong"><strong>int()</strong></span> method</a></span></dt>
       
    40 </dl></dd>
       
    41 </dl>
       
    42 </div>
       
    43 <div class="article" title="Basic Python">
       
    44 <div class="titlepage">
       
    45 <div><div><h2 class="title">
       
    46 <a name="id2706129"></a>Basic Python</h2></div></div>
       
    47 <hr>
       
    48 </div>
       
    49 <div class="toc">
       
    50 <p><b>Table of Contents</b></p>
       
    51 <dl>
       
    52 <dt><span class="section"><a href="#id2705187">1. Introduction</a></span></dt>
       
    53 <dt><span class="section"><a href="#id2757271">2. The Python Interpreter</a></span></dt>
       
    54 <dd><dl>
       
    55 <dt><span class="section"><a href="#id2757279">2.1. The Interactive Interpreter</a></span></dt>
       
    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>
       
    57 </dl></dd>
       
    58 <dt><span class="section"><a href="#id2757589">3. Editing and running a python file</a></span></dt>
       
    59 <dt><span class="section"><a href="#id2757696">4. Basic Datatypes and operators in Python</a></span></dt>
       
    60 <dd><dl>
       
    61 <dt><span class="section"><a href="#id2757731">4.1. Numbers</a></span></dt>
       
    62 <dt><span class="section"><a href="#id2757854">4.2. Variables</a></span></dt>
       
    63 <dt><span class="section"><a href="#id2757903">4.3. Strings</a></span></dt>
       
    64 <dt><span class="section"><a href="#id2758097">4.4. Boolean</a></span></dt>
       
    65 </dl></dd>
       
    66 <dt><span class="section"><a href="#id2758126">5. The <span class="strong"><strong>while</strong></span> loop</a></span></dt>
       
    67 <dt><span class="section"><a href="#id2758163">6. The <span class="strong"><strong>if</strong></span> conditional</a></span></dt>
       
    68 <dt><span class="section"><a href="#id2758232">7. <span class="strong"><strong>raw_input()</strong></span></a></span></dt>
       
    69 <dt><span class="section"><a href="#id2758387">8. <span class="strong"><strong>int()</strong></span> method</a></span></dt>
       
    70 </dl>
       
    71 </div>
       
    72 <p id="ch2intro_1"></a>This document is intended to be handed out at the end of the workshop. It has
       
    73 been designed for Engineering students who are Python beginners and have basic
       
    74 programming skills. The focus is on basic numerics and plotting using Python.</p>
       
    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">
       
    76 <div class="titlepage"><div><div><h2 class="title" style="clear: both">
       
    77 <a name="id2705187"></a>1. Introduction</h2></div></div></div>
       
    78 <p id="ch2intro_5"></a>The Python programming language was created by a dutch named Guido van Rossum.
       
    79 The idea of Python was conceived in December 1989. The name Python has nothing
       
    80 to do with the reptilian, but its been named after the 70s comedy series
       
    81 "Monty Python's Flying Circus", since it happens to be Guido's favourite
       
    82 TV series.</p>
       
    83 <p id="ch2intro_6"></a>Current stable version of Python is 2.6.x, although Python 3.0 is also the stable
       
    84 version, it is not backwards compatible with the previous versions and is hence
       
    85 not entirely popular at the moment. This material will focus on the 2.6.x series.</p>
       
    86 <p id="ch2intro_7"></a>Python is licensed under the Python Software Foundation License (PSF License)
       
    87 which is GPL compatible Free Software license (excepting license version 1.6 and 2.0)
       
    88 It is a no strings attached license, which means the source code is free to modify
       
    89 and redistribute.</p>
       
    90 <p id="ch2intro_8"></a>The Python docs define Python as "Python is an interpreted, object-oriented,
       
    91 high-level programming language with dynamic semantics." A more detailed summary
       
    92 can be found at </p>
       
    93 <div class="reference">
       
    94 <div class="titlepage"><hr></div>http://www.python.org/doc/essays/blurb.html</div>
       
    95 <p>. Python is a language that
       
    96 has been designed to help the programmer concentrate on solving the problem at hand
       
    97 and not worry about the programming language idiosyncrasies.</p>
       
    98 <p id="ch2intro_9"></a>Python is a highly cross platform compatible language on account of it being an
       
    99 interpreted language. It is highly scalable and hence has been adapted to run on
       
   100 the Nokia 60 series phones. Python has been designed to be readable and easy to use</p>
       
   101 <p id="ch2intro_a"></a><span class="strong"><strong>Resources available for reference</strong></span></p>
       
   102 <span style="color: black"><span style="color: black"><p id="ch2intro_b"></a>Web: </p>
       
   103 <div class="reference">
       
   104 <div class="titlepage"><hr></div>http://www.python.org</div></span><span style="color: black"><p id="ch2intro_c"></a>Doc: </p>
       
   105 <div class="reference">
       
   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>
       
   107 <div class="reference">
       
   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>
       
   109 <div class="reference">
       
   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>
       
   111 <div class="reference">
       
   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>
       
   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
       
   114 such a fashion that it imposes readability on the programmer. Python does away
       
   115 with the braces and the semicolons and instead implements code blocks based on
       
   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.
       
   117 Python performs memory management on its own, thus the programmer need not bother
       
   118 about allocating and deallocating memory to variables. Python provides extensibility
       
   119 by providing modules which can be easily imported similar to headers in C and
       
   120 packages in Java. Python is object oriented and hence provides all the object oriented
       
   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
       
   122 of the 'Interactive Interpreter' which will be discussed in more detail in the
       
   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
       
   124 power of Python modules can be seen in this slightly exaggerated cartoon
       
   125 </p>
       
   126 <div class="reference">
       
   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++
       
   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
       
   129 languages like C or C++. Yet, the amount of flexibility and power more than make
       
   130 up for this setback.</p>
       
   131 </div>
       
   132 <div class="section" title="2. The Python Interpreter">
       
   133 <div class="titlepage"><div><div><h2 class="title" style="clear: both">
       
   134 <a name="id2757271"></a>2. The Python Interpreter</h2></div></div></div>
       
   135 <div class="section" title="2.1. The Interactive Interpreter">
       
   136 <div class="titlepage"><div><div><h3 class="title">
       
   137 <a name="id2757279"></a>2.1. The Interactive Interpreter</h3></div></div></div>
       
   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
       
   139 hitting the enter key fires up the Python 'Interactive Interpreter'. The Python
       
   140 interpreter is one of the most integral features of Python. The prompt obtained
       
   141 when the interactive interpreter is similar to what is shown below. The exact
       
   142 appearance might differ based on the version of Python being used. The <code class="literal">&gt;&gt;&gt;</code>
       
   143 thing shown is the python prompt. When something is typed at the prompt and the
       
   144 enter key is hit, the python interpreter interprets the command entered and
       
   145 performs the appropriate action. All the examples presented in this document are
       
   146 to be tried hands on, on the interactive interpreter.</p>
       
   147 <pre class="programlisting"> Python 2.5.2 (r252:60911, Oct  5 2008, 19:24:49)
       
   148 [GCC 4.3.2] on linux2
       
   149 Type "help", "copyright", "credits" or "license" for more information.
       
   150 &gt;&gt;&gt;</pre>
       
   151 <p id="ch2intro_18"></a>Lets try with an example, type <code class="literal">print 'Hello, World!'</code> at the prompt and hit
       
   152 the enter key.</p>
       
   153 <pre class="programlisting"> &gt;&gt;&gt; print 'Hello, World!'
       
   154 Hello, World!</pre>
       
   155 <p id="ch2intro_19"></a>This example was quite straight forward, and thus we have written our first
       
   156 line of Python code. Now let us try typing something arbitrary at the prompt.
       
   157 For example:</p>
       
   158 <pre class="programlisting"> &gt;&gt;&gt; arbit word
       
   159   File "&lt;stdin&gt;", line 1
       
   160     arbit word
       
   161             ^
       
   162 SyntaxError: invalid syntax
       
   163 &gt;&gt;&gt;</pre>
       
   164 <p id="ch2intro_1a"></a>The interpreter gave an error message saying that 'arbit word' was invalid
       
   165 syntax which is valid. The interpreter is an amazing tool when learning to
       
   166 program in Python. The interpreter provides a help function that provides the
       
   167 necessary documentation regarding all Python syntax, constructs, modules and
       
   168 objects. Typing <span class="emphasis"><em>help()</em></span> at the prompt gives the following output:</p>
       
   169 <pre class="programlisting"> &gt;&gt;&gt; help()
       
   170 
       
   171 Welcome to Python 2.5!  This is the online help utility.
       
   172 
       
   173 If this is your first time using Python, you should definitely check out
       
   174 the tutorial on the Internet at http://www.python.org/doc/tut/.
       
   175 
       
   176 Enter the name of any module, keyword, or topic to get help on writing
       
   177 Python programs and using Python modules.  To quit this help utility and
       
   178 return to the interpreter, just type "quit".
       
   179 
       
   180 To get a list of available modules, keywords, or topics, type "modules",
       
   181 "keywords", or "topics".  Each module also comes with a one-line summary
       
   182 of what it does; to list the modules whose summaries contain a given word
       
   183 such as "spam", type "modules spam".
       
   184 
       
   185 help&gt;</pre>
       
   186 <p id="ch2intro_1b"></a>As mentioned in the output, entering the name of any module, keyword or topic
       
   187 will provide the documentation and help regarding the same through the online
       
   188 help utility. Pressing <span class="emphasis"><em>Ctrl+d</em></span> exits the help prompt and returns to the
       
   189 python prompt.</p>
       
   190 <p id="ch2intro_1c"></a>Let us now try a few examples at the python interpreter.</p>
       
   191 <p id="ch2intro_1d"></a>Eg 1:</p>
       
   192 <pre class="programlisting"> &gt;&gt;&gt; print 'Hello, python!'
       
   193 Hello, python!
       
   194 &gt;&gt;&gt;</pre>
       
   195 <p id="ch2intro_1e"></a>Eg 2:</p>
       
   196 <pre class="programlisting"> &gt;&gt;&gt; print 4321*567890
       
   197 2453852690
       
   198 &gt;&gt;&gt;</pre>
       
   199 <p id="ch2intro_1f"></a>Eg 3:</p>
       
   200 <pre class="programlisting"> &gt;&gt;&gt; 4321*567890
       
   201 2453852690L
       
   202 &gt;&gt;&gt;</pre>
       
   203 <pre class="programlisting"> Note: Notice the 'L' at the end of the output. The 'L' signifies that the
       
   204 output of the operation is of type *long*. It was absent in the previous
       
   205 example because we used the print statement. This is because *print* formats
       
   206 the output before displaying.</pre>
       
   207 <p id="ch2intro_20"></a>Eg 4:</p>
       
   208 <pre class="programlisting"> &gt;&gt;&gt; big = 12345678901234567890 ** 3
       
   209 &gt;&gt;&gt; print big
       
   210 1881676372353657772490265749424677022198701224860897069000
       
   211 &gt;&gt;&gt;</pre>
       
   212 <pre class="programlisting"> This example is to show that unlike in C or C++ there is no limit on the
       
   213 value of an integer.</pre>
       
   214 <p id="ch2intro_21"></a>Try this on the interactive interpreter:
       
   215 <code class="literal">import this</code></p>
       
   216 <p id="ch2intro_22"></a><span class="emphasis"><em>Hint: The output gives an idea of Power of Python</em></span></p>
       
   217 </div>
       
   218 <div class="section" title="2.2. ipython - An enhanced interactive Python interpreter">
       
   219 <div class="titlepage"><div><div><h3 class="title">
       
   220 <a name="id2757464"></a>2.2. <span class="emphasis"><em>ipython</em></span> - An enhanced interactive Python interpreter</h3></div></div></div>
       
   221 <p id="ch2intro_23"></a>The power and the importance of the interactive interpreter was the highlight
       
   222 of the previous section. This section provides insight into the enhanced
       
   223 interpreter with more advanced set of features called <span class="strong"><strong>ipython</strong></span>. Entering
       
   224 <span class="emphasis"><em>ipython</em></span> at the shell prompt fires up the interactive interpreter.</p>
       
   225 <pre class="programlisting"> $ ipython
       
   226 Python 2.5.2 (r252:60911, Oct  5 2008, 19:24:49)
       
   227 Type "copyright", "credits" or "license" for more information.
       
   228 
       
   229 IPython 0.8.4 -- An enhanced Interactive Python.
       
   230 ?         -&gt; Introduction and overview of IPython's features.
       
   231 %quickref -&gt; Quick reference.
       
   232 help      -&gt; Python's own help system.
       
   233 object?   -&gt; Details about 'object'. ?object also works, ?? prints more.
       
   234 
       
   235 In [1]:</pre>
       
   236 <p id="ch2intro_24"></a>This is the output obtained upon firing ipython. The exact appearance may
       
   237 change based on the Python version installed. The following are some of the
       
   238 various features provided by <span class="strong"><strong>ipython</strong></span>:</p>
       
   239 <span style="color: black"><p id="ch2intro_25"></a>Suggestions - ipython provides suggestions of the possible methods and
       
   240 operations available for the given python object.</p></span><p id="ch2intro_26"></a>Eg 5:</p>
       
   241 <pre class="programlisting"> In [4]: a = 6
       
   242 
       
   243 In [5]: a.
       
   244 a.__abs__           a.__divmod__        a.__index__         a.__neg__          a.__rand__          a.__rmod__          a.__rxor__
       
   245 a.__add__           a.__doc__           a.__init__          a.__new__          a.__rdiv__          a.__rmul__          a.__setattr__
       
   246 a.__and__           a.__float__         a.__int__           a.__nonzero__      a.__rdivmod__       a.__ror__           a.__str__
       
   247 a.__class__         a.__floordiv__      a.__invert__        a.__oct__          a.__reduce__        a.__rpow__          a.__sub__
       
   248 a.__cmp__           a.__getattribute__  a.__long__          a.__or__           a.__reduce_ex__     a.__rrshift__       a.__truediv__
       
   249 a.__coerce__        a.__getnewargs__    a.__lshift__        a.__pos__          a.__repr__          a.__rshift__        a.__xor__
       
   250 a.__delattr__       a.__hash__          a.__mod__           a.__pow__          a.__rfloordiv__     a.__rsub__
       
   251 a.__div__           a.__hex__           a.__mul__           a.__radd__         a.__rlshift__       a.__rtruediv__</pre>
       
   252 <p id="ch2intro_27"></a>In this example, we initialized 'a' (a variable - a concept that will be
       
   253 discussed in the subsequent sections.) to 6. In the next line when the <span class="emphasis"><em>tab</em></span> key
       
   254 is pressed after typing '<span class="emphasis"><em>a.</em></span>' ipython displays the set of all possible methods
       
   255 that are applicable on the object 'a' (an integer in this context). Ipython
       
   256 provides many such datatype specific features which will be presented in the
       
   257 further sections as and when the datatypes are introduced.</p>
       
   258 </div>
       
   259 </div>
       
   260 <div class="section" title="3. Editing and running a python file">
       
   261 <div class="titlepage"><div><div><h2 class="title" style="clear: both">
       
   262 <a name="id2757589"></a>3. Editing and running a python file</h2></div></div></div>
       
   263 <p id="ch2intro_28"></a>The previous sections focused on the use of the interpreter to run python code.
       
   264 While the interpeter is an excellent tool to test simple solutions and
       
   265 experiment with small code snippets, its main disadvantage is that everything
       
   266 written in the interpreter is lost once its quit. Most of the times a program is
       
   267 used by people other than the author. So the programs have to be available in
       
   268 some form suitable for distribution, and hence they are written in files. This
       
   269 section will focus on editing and running python files. Start by opening a text
       
   270 editor ( it is recommended you choose one from the list at the top of this page ).
       
   271 In the editor type down python code and save the file with an extension <span class="strong"><strong>.py</strong></span>
       
   272 (python files have an extension of .py). Once done with the editing, save the
       
   273 file and exit the editor.</p>
       
   274 <p id="ch2intro_29"></a>Let us look at a simple example of calculating the gcd of 2 numbers using Python:</p>
       
   275 <p id="ch2intro_2a"></a><span class="strong"><strong>Creating the first python script(file)</strong></span></p>
       
   276 <pre class="programlisting"> $ emacs gcd.py
       
   277   def gcd(x,y):
       
   278     if x % y == 0:
       
   279       return y
       
   280     return gcd(y, x%y)
       
   281 
       
   282   print gcd(72, 92)</pre>
       
   283 <p id="ch2intro_2b"></a>To run the script, open the shell prompt, navigate to the directory that
       
   284 contains the python file and run <code class="literal">python &lt;filename.py&gt;</code> at the prompt ( in this
       
   285 case filename is gcd.py )</p>
       
   286 <p id="ch2intro_2c"></a><span class="strong"><strong>Running the python script</strong></span></p>
       
   287 <pre class="programlisting"> $ python gcd.py
       
   288 4
       
   289 $</pre>
       
   290 <p id="ch2intro_2d"></a>Another method to run a python script would be to include the line</p>
       
   291 <p id="ch2intro_2e"></a><code class="literal">#! /usr/bin/python</code></p>
       
   292 <p id="ch2intro_2f"></a>at the beginning of the python file and then make the file executable by</p>
       
   293 <p id="ch2intro_30"></a>$ chmod a+x <span class="emphasis"><em>filename.py</em></span></p>
       
   294 <p id="ch2intro_31"></a>Once this is done, the script can be run as a standalone program as follows:</p>
       
   295 <p id="ch2intro_32"></a>$ ./<span class="emphasis"><em>filename.py</em></span></p>
       
   296 </div>
       
   297 <div class="section" title="4. Basic Datatypes and operators in Python">
       
   298 <div class="titlepage"><div><div><h2 class="title" style="clear: both">
       
   299 <a name="id2757696"></a>4. Basic Datatypes and operators in Python</h2></div></div></div>
       
   300 <p id="ch2intro_33"></a>Python provides the following set of basic datatypes.</p>
       
   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">
       
   302 <div class="titlepage"><div><div><h3 class="title">
       
   303 <a name="id2757731"></a>4.1. Numbers</h3></div></div></div>
       
   304 <p id="ch2intro_37"></a>Numbers were introduced in the examples presented in the interactive interpreter
       
   305 section. Numbers include types as mentioned earlier viz., int (integers), float
       
   306 (floating point numbers), long (large integers), complex (complex numbers with
       
   307 real and imaginary parts). Python is not a strongly typed language, which means
       
   308 the type of a variable need not mentioned during its initialization. Let us look
       
   309 at a few examples.</p>
       
   310 <p id="ch2intro_38"></a>Eg 6:</p>
       
   311 <pre class="programlisting"> &gt;&gt;&gt; a = 1 #here a is an integer variable</pre>
       
   312 <p id="ch2intro_39"></a>Eg 7:</p>
       
   313 <pre class="programlisting"> &gt;&gt;&gt; lng = 122333444455555666666777777788888888999999999 #here lng is a variable of type long
       
   314 &gt;&gt;&gt; lng
       
   315 122333444455555666666777777788888888999999999L #notice the trailing 'L'
       
   316 &gt;&gt;&gt; print lng
       
   317 122333444455555666666777777788888888999999999 #notice the absence of the trailing 'L'
       
   318 &gt;&gt;&gt; lng+1
       
   319 122333444455555666666777777788888889000000000L</pre>
       
   320 <p id="ch2intro_3a"></a>Long numbers are the same as integers in almost all aspects. They can be used in
       
   321 operations just like integers and along with integers without any distinction.
       
   322 The only distinction comes during type checking (which is not a healthy practice).
       
   323 Long numbers are tucked with a trailing 'L' just to signify that they are long.
       
   324 Notice that in the example just lng at the prompt displays the value of the variable
       
   325 with the 'L' whereas <code class="literal">print lng</code> displays without the 'L'. This is because print
       
   326 formats the output before printing. Also in the example, notice that adding an
       
   327 integer to a long does not give any errors and the result is as expected. So for
       
   328 all practical purposes longs can be treated as ints.</p>
       
   329 <p id="ch2intro_3b"></a>Eg 8:</p>
       
   330 <pre class="programlisting"> &gt;&gt;&gt; fl = 3.14159 #fl is a float variable
       
   331 &gt;&gt;&gt; e = 1.234e-4 #e is also a float variable, specified in the exponential form
       
   332 &gt;&gt;&gt; a = 1
       
   333 &gt;&gt;&gt; b = 2
       
   334 &gt;&gt;&gt; a/b #integer division
       
   335 0
       
   336 &gt;&gt;&gt; a/fl #floating point division
       
   337 0.31831015504887655
       
   338 &gt;&gt;&gt; e/fl
       
   339 3.9279473133031364e-05</pre>
       
   340 <p id="ch2intro_3c"></a>Floating point numbers, simply called floats are real numbers with a decimal point.
       
   341 The example above shows the initialization of a float variable. Shown also in this
       
   342 example is the difference between integer division and floating point division.
       
   343 'a' and 'b' here are integer variables and hence the division gives 0 as the quotient.
       
   344 When either of the operands is a float, the operation is a floating point division,
       
   345 and the result is also a float as illustrated.</p>
       
   346 <p id="ch2intro_3d"></a>Eg 9:</p>
       
   347 <pre class="programlisting"> &gt;&gt;&gt; cplx = 3 + 4j #cplx is a complex variable
       
   348 &gt;&gt;&gt; cplx
       
   349 (3+4j)
       
   350 &gt;&gt;&gt; print cplx.real #prints the real part of the complex number
       
   351 3.0
       
   352 &gt;&gt;&gt; print cplx.imag #prints the imaginary part of the complex number
       
   353 4.0
       
   354 &gt;&gt;&gt; print cplx*fl  #multiplies the real and imag parts of the complex number with the multiplier
       
   355 (9.42477+12.56636j)
       
   356 &gt;&gt;&gt; abs(cplx) #returns the absolute value of the complex number
       
   357 5.0</pre>
       
   358 <p id="ch2intro_3e"></a>Python provides a datatype for complex numbers. Complex numbers are initialized
       
   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
       
   360 imaginary parts of the complex number as shown. The <span class="emphasis"><em>abs()</em></span> returns the absolute
       
   361 value of the complex number.</p>
       
   362 </div>
       
   363 <div class="section" title="4.2. Variables">
       
   364 <div class="titlepage"><div><div><h3 class="title">
       
   365 <a name="id2757854"></a>4.2. Variables</h3></div></div></div>
       
   366 <p id="ch2intro_3f"></a>Variables are just names that represent a value. Variables have already been
       
   367 introduced in the various examples from the previous sections. Certain rules about
       
   368 using variables:</p>
       
   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'
       
   370 are all variables of various datatypes.</p>
       
   371 <pre class="programlisting"> Note: Python is not a strongly typed language and hence an integer variable can at a
       
   372 later stage be used as a float variable as well.</pre>
       
   373 </div>
       
   374 <div class="section" title="4.3. Strings">
       
   375 <div class="titlepage"><div><div><h3 class="title">
       
   376 <a name="id2757903"></a>4.3. Strings</h3></div></div></div>
       
   377 <p id="ch2intro_44"></a>Strings are one of the essential data structures of any programming language.
       
   378 The <code class="literal">print "Hello, World!"</code> program was introduced in the earlier section, and
       
   379 the <span class="emphasis"><em>"Hello, World!"</em></span> in the print statement is a string. A string is basically
       
   380 a set of characters. Strings can be represented in various ways shown below:</p>
       
   381 <pre class="programlisting"> s = 'this is a string'              # a string variable can be represented using single quotes
       
   382 s = 'This one has "quotes" inside!' # The string can have quotes inside it as shown
       
   383 s = "I have 'single-quotes' inside!"
       
   384 l = "A string spanning many lines\
       
   385 one more line\
       
   386 yet another"                        # a string can span more than a single line.
       
   387 t = """A triple quoted string does  # another way of representing multiline strings.
       
   388 not need to be escaped at the end and
       
   389 "can have nested quotes" etc."""</pre>
       
   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>
       
   392 <p id="ch2intro_46"></a><span class="strong"><strong>Exercise: How to use single quotes within single quotes in a string as shown
       
   393 in the above example without getting an error?</strong></span></p>
       
   394 <div class="section" title="4.3.1. String operations">
       
   395 <div class="titlepage"><div><div><h4 class="title">
       
   396 <a name="id2757955"></a>4.3.1. String operations</h4></div></div></div>
       
   397 <p id="ch2intro_47"></a>A few basic string operations are presented here.</p>
       
   398 <p id="ch2intro_48"></a><span class="strong"><strong>String concatenation</strong></span>
       
   399 String concatenation is done by simple addition of two strings.</p>
       
   400 <pre class="programlisting"> &gt;&gt;&gt; x = 'Hello'
       
   401 &gt;&gt;&gt; y = ' Python'
       
   402 &gt;&gt;&gt; print x+y
       
   403 Hello Python</pre>
       
   404 <p id="ch2intro_49"></a><span class="emphasis"><em>Try this yourself:</em></span></p>
       
   405 <pre class="programlisting"> &gt;&gt;&gt; somenum = 13
       
   406 &gt;&gt;&gt; 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"> &gt;&gt;&gt; str(1000000000000000000000000000000000000000000000000L)
       
   414 '1000000000000000000000000000000000000000000000000'
       
   415 &gt;&gt;&gt; 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"> &gt;&gt;&gt; x = "Let's go \nto Pycon"
       
   422 &gt;&gt;&gt; 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"> &gt;&gt;&gt; x = r"Let's go \nto Pycon"
       
   434 &gt;&gt;&gt; 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"> &gt;&gt;&gt; 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"> &gt;&gt;&gt; x = u"Let's go to Pycon!"
       
   444 &gt;&gt;&gt; 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"> &gt;&gt;&gt; t = True
       
   455 &gt;&gt;&gt; f = not t
       
   456 &gt;&gt;&gt; print f
       
   457 False
       
   458 &gt;&gt;&gt; f or t
       
   459 True
       
   460 &gt;&gt;&gt; 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"> &gt;&gt;&gt; x = 1
       
   476 &gt;&gt;&gt; while x &lt;= 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"> &gt;&gt;&gt; n = raw_input("Input a number:")
       
   509 &gt;&gt;&gt; if n &lt; 0:
       
   510       print n," is negative"
       
   511       elif n &gt; 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"> &gt;&gt;&gt; input("Enter a number thats a palindrome:")
       
   526 Enter a number thats a palindrome:121
       
   527 121
       
   528 
       
   529 &gt;&gt;&gt; input("Enter your name:")
       
   530 Enter your name:PythonFreak
       
   531 Traceback (most recent call last):
       
   532   File "&lt;stdin&gt;", line 1, in &lt;module&gt;
       
   533   File "&lt;string&gt;", line 1, in &lt;module&gt;
       
   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"> &gt;&gt;&gt; input("Enter your name:")
       
   542 Enter your name:'PythonFreak'
       
   543 'PythonFreak'
       
   544 &gt;&gt;&gt;</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"> &gt;&gt;&gt; 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"> &gt;&gt;&gt; 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"> &gt;&gt;&gt; pal = raw_input("Enter a number thats a palindrome:")
       
   559 Enter a number thats a palindrome:121
       
   560 &gt;&gt;&gt; pal + 2
       
   561 Traceback (most recent call last):
       
   562   File "&lt;stdin&gt;", line 1, in &lt;module&gt;
       
   563 TypeError: cannot concatenate 'str' and 'int' objects
       
   564 &gt;&gt;&gt; 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"> &gt;&gt;&gt; intpal = int(pal)
       
   578 &gt;&gt;&gt; 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"> &gt;&gt;&gt; stringvar = raw_input("Enter a name:")
       
   584 Enter a name:Guido Van Rossum
       
   585 &gt;&gt;&gt; stringvar
       
   586 'Guido Van Rossum'
       
   587 &gt;&gt;&gt; numvar = int(stringvar)</pre>
       
   588 </div>
       
   589 </div>
       
   590 </div></body>
       
   591 </html>