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-<head><meta http-equiv="Content-Type" content="text/html; charset=utf-8" /><title>Chapter. intro</title><link rel="stylesheet" href="/review/support/styles.css" type="text/css" /><meta name="generator" content="DocBook XSL Stylesheets V1.74.3" /><link rel="shortcut icon" type="image/png" href="/review/support/figs/favicon.png" /><script type="text/javascript" src="/review/support/jquery-min.js"></script><script type="text/javascript" src="/review/support/form.js"></script><script type="text/javascript" src="/review/support/hsbook.js"></script></head>
-<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"><div class="chapter" id="ch2intro">
-<div class="titlepage"></div>
-<div class="toc">
-<p><b>Table of Contents</b></p>
-<dl>
-<dt><span class="article"><a href="#id2915639">Basic Python</a></span></dt>
-<dd><dl>
-<dt><span class="section"><a href="#id2723402">1. Introduction</a></span></dt>
-<dt><span class="section"><a href="#id2966769">2. The Python Interpreter</a></span></dt>
-<dd><dl>
-<dt><span class="section"><a href="#id2966777">2.1. The Interactive Interpreter</a></span></dt>
-<dt><span class="section"><a href="#id2966950">2.2. <span class="emphasis"><em>ipython</em></span> - An enhanced interactive Python interpreter</a></span></dt>
-</dl></dd>
-<dt><span class="section"><a href="#id2967075">3. Editing and running a python file</a></span></dt>
-<dt><span class="section"><a href="#id2967182">4. Basic Datatypes and operators in Python</a></span></dt>
-<dd><dl>
-<dt><span class="section"><a href="#id2967216">4.1. Numbers</a></span></dt>
-<dt><span class="section"><a href="#id2967346">4.2. Variables</a></span></dt>
-<dt><span class="section"><a href="#id2967394">4.3. Strings</a></span></dt>
-<dt><span class="section"><a href="#id2967588">4.4. Boolean</a></span></dt>
-</dl></dd>
-<dt><span class="section"><a href="#id2967617">5. The <span class="strong"><strong>while</strong></span> loop</a></span></dt>
-<dt><span class="section"><a href="#id2967654">6. The <span class="strong"><strong>if</strong></span> conditional</a></span></dt>
-<dt><span class="section"><a href="#id2967723">7. <span class="strong"><strong>raw_input()</strong></span></a></span></dt>
-<dt><span class="section"><a href="#id2967877">8. <span class="strong"><strong>int()</strong></span> method</a></span></dt>
-</dl></dd>
-</dl>
-</div>
-<div class="article" title="Basic Python">
-<div class="titlepage">
-<div><div><h2 class="title">
-<a name="id2915639"></a>Basic Python</h2></div></div>
-<hr />
-</div>
-<div class="toc">
-<p><b>Table of Contents</b></p>
-<dl>
-<dt><span class="section"><a href="#id2723402">1. Introduction</a></span></dt>
-<dt><span class="section"><a href="#id2966769">2. The Python Interpreter</a></span></dt>
-<dd><dl>
-<dt><span class="section"><a href="#id2966777">2.1. The Interactive Interpreter</a></span></dt>
-<dt><span class="section"><a href="#id2966950">2.2. <span class="emphasis"><em>ipython</em></span> - An enhanced interactive Python interpreter</a></span></dt>
-</dl></dd>
-<dt><span class="section"><a href="#id2967075">3. Editing and running a python file</a></span></dt>
-<dt><span class="section"><a href="#id2967182">4. Basic Datatypes and operators in Python</a></span></dt>
-<dd><dl>
-<dt><span class="section"><a href="#id2967216">4.1. Numbers</a></span></dt>
-<dt><span class="section"><a href="#id2967346">4.2. Variables</a></span></dt>
-<dt><span class="section"><a href="#id2967394">4.3. Strings</a></span></dt>
-<dt><span class="section"><a href="#id2967588">4.4. Boolean</a></span></dt>
-</dl></dd>
-<dt><span class="section"><a href="#id2967617">5. The <span class="strong"><strong>while</strong></span> loop</a></span></dt>
-<dt><span class="section"><a href="#id2967654">6. The <span class="strong"><strong>if</strong></span> conditional</a></span></dt>
-<dt><span class="section"><a href="#id2967723">7. <span class="strong"><strong>raw_input()</strong></span></a></span></dt>
-<dt><span class="section"><a href="#id2967877">8. <span class="strong"><strong>int()</strong></span> method</a></span></dt>
-</dl>
-</div>
-<p id="ch2intro_1">This document is intended to be handed out at the end of the workshop. It has
-been designed for Engineering students who are Python beginners and have basic
-programming skills. The focus is on basic numerics and plotting using Python.</p>The system requirements:<div class="itemizedlist"><ul class="itemizedlist" type="*">
-<li class="listitem" style="list-style-type: *"><p id="ch2intro_2">Python - version 2.5.x or newer.</p></li>
-<li class="listitem" style="list-style-type: *"><p id="ch2intro_3">IPython</p></li>
-<li class="listitem" style="list-style-type: *"><p id="ch2intro_4">Text editor - scite, vim, emacs or whatever you are comfortable with.</p></li>
-</ul></div>
-<div class="section" title="1.Introduction">
-<div class="titlepage"><div><div><h2 class="title" style="clear: both">
-<a name="id2723402"></a>1.Introduction</h2></div></div></div>
-<p id="ch2intro_5">The Python programming language was created by a dutch named Guido van Rossum.
-The idea of Python was conceived in December 1989. The name Python has nothing
-to do with the reptilian, but its been named after the 70s comedy series
-"Monty Python's Flying Circus", since it happens to be Guido's favourite
-TV series.</p>
-<p id="ch2intro_6">Current stable version of Python is 2.6.x, although Python 3.0 is also the stable
-version, it is not backwards compatible with the previous versions and is hence
-not entirely popular at the moment. This material will focus on the 2.6.x series.</p>
-<p id="ch2intro_7">Python is licensed under the Python Software Foundation License (PSF License)
-which is GPL compatible Free Software license (excepting license version 1.6 and 2.0)
-It is a no strings attached license, which means the source code is free to modify
-and redistribute.</p>
-<p id="ch2intro_8">The Python docs define Python as "Python is an interpreted, object-oriented,
-high-level programming language with dynamic semantics." A more detailed summary
-can be found at </p>
-<div class="reference">
-<div class="titlepage"><hr /></div>http://www.python.org/doc/essays/blurb.html</div>
-<p>. Python is a language that
-has been designed to help the programmer concentrate on solving the problem at hand
-and not worry about the programming language idiosyncrasies.</p>
-<p id="ch2intro_9">Python is a highly cross platform compatible language on account of it being an
-interpreted language. It is highly scalable and hence has been adapted to run on
-the Nokia 60 series phones. Python has been designed to be readable and easy to use</p>
-<p id="ch2intro_a"><span class="strong"><strong>Resources available for reference</strong></span></p>
-<div class="itemizedlist"><ul class="itemizedlist" type="*">
-<li class="listitem" style="list-style-type: *">
-<p id="ch2intro_b">Web: </p>
-<div class="reference">
-<div class="titlepage"><hr /></div>http://www.python.org</div>
-</li>
-<li class="listitem" style="list-style-type: *">
-<p id="ch2intro_c">Doc: </p>
-<div class="reference">
-<div class="titlepage"><hr /></div>http://www.python.org/doc</div>
-</li>
-<li class="listitem" style="list-style-type: *">Free Tutorials:<div class="itemizedlist"><ul class="itemizedlist" type="*">
-<li class="listitem" style="list-style-type: *">
-<p id="ch2intro_d">Official Python Tutorial: </p>
-<div class="reference">
-<div class="titlepage"><hr /></div>http://docs.python.org/tut/tut.html</div>
-</li>
-<li class="listitem" style="list-style-type: *">
-<p id="ch2intro_e">Byte of Python: </p>
-<div class="reference">
-<div class="titlepage"><hr /></div>http://www.byteofpython.info/</div>
-</li>
-<li class="listitem" style="list-style-type: *">
-<p id="ch2intro_f">Dive into Python: </p>
-<div class="reference">
-<div class="titlepage"><hr /></div>http://diveintopython.org/</div>
-</li>
-</ul></div>
-</li>
-</ul></div>
-<p id="ch2intro_10"><span class="strong"><strong>Advantages of Python - Why Python??</strong></span></p>
-<div class="itemizedlist"><ul class="itemizedlist" type="*">
-<li class="listitem" style="list-style-type: *"><p id="ch2intro_11">Python has been designed for readability and ease of use. Its been designed in
-such a fashion that it imposes readability on the programmer. Python does away
-with the braces and the semicolons and instead implements code blocks based on
-indentation, thus enhancing readability.</p></li>
-<li class="listitem" style="list-style-type: *"><p id="ch2intro_12">Python is a high level, interpreted, modular and object oriented language.
-Python performs memory management on its own, thus the programmer need not bother
-about allocating and deallocating memory to variables. Python provides extensibility
-by providing modules which can be easily imported similar to headers in C and
-packages in Java. Python is object oriented and hence provides all the object oriented
-characteristics such as inheritance, encapsulation and polymorphism.</p></li>
-<li class="listitem" style="list-style-type: *"><p id="ch2intro_13">Python offers a highly powerful interactive programming interface in the form
-of the 'Interactive Interpreter' which will be discussed in more detail in the
-following sections.</p></li>
-<li class="listitem" style="list-style-type: *">
-<p id="ch2intro_14">Python provides a rich standard library and an extensive set of modules. The
-power of Python modules can be seen in this slightly exaggerated cartoon
-</p>
-<div class="reference">
-<div class="titlepage"><hr /></div>http://xkcd.com/353/</div>
-</li>
-<li class="listitem" style="list-style-type: *"><p id="ch2intro_15">Python interfaces well with most other programming languages such as C, C++
-and FORTRAN.</p></li>
-</ul></div>
-<p id="ch2intro_16">Although, Python has one setback. Python is not fast as some of the compiled
-languages like C or C++. Yet, the amount of flexibility and power more than make
-up for this setback.</p>
-</div>
-<div class="section" title="2.The Python Interpreter">
-<div class="titlepage"><div><div><h2 class="title" style="clear: both">
-<a name="id2966769"></a>2.The Python Interpreter</h2></div></div></div>
-<div class="section" title="2.1.The Interactive Interpreter">
-<div class="titlepage"><div><div><h3 class="title">
-<a name="id2966777"></a>2.1.The Interactive Interpreter</h3></div></div></div>
-<p id="ch2intro_17">Typing <span class="emphasis"><em>python</em></span> at the shell prompt on any standard Unix/Gnu-Linux system and
-hitting the enter key fires up the Python 'Interactive Interpreter'. The Python
-interpreter is one of the most integral features of Python. The prompt obtained
-when the interactive interpreter is similar to what is shown below. The exact
-appearance might differ based on the version of Python being used. The <code class="literal">>>></code>
-thing shown is the python prompt. When something is typed at the prompt and the
-enter key is hit, the python interpreter interprets the command entered and
-performs the appropriate action. All the examples presented in this document are
-to be tried hands on, on the interactive interpreter.</p>
-<pre class="programlisting">
-Python 2.5.2 (r252:60911, Oct 5 2008, 19:24:49)
-[GCC 4.3.2] on linux2
-Type "help", "copyright", "credits" or "license" for more information.
->>></pre>
-<p id="ch2intro_18">Lets try with an example, type <code class="literal">print 'Hello, World!'</code> at the prompt and hit
-the enter key.</p>
-<pre class="programlisting">
->>> print 'Hello, World!'
-Hello, World!</pre>
-<p id="ch2intro_19">This example was quite straight forward, and thus we have written our first
-line of Python code. Now let us try typing something arbitrary at the prompt.
-For example:</p>
-<pre class="programlisting">
->>> arbit word
- File "<stdin>", line 1
- arbit word
- ^
-SyntaxError: invalid syntax
->>></pre>
-<p id="ch2intro_1a">The interpreter gave an error message saying that 'arbit word' was invalid
-syntax which is valid. The interpreter is an amazing tool when learning to
-program in Python. The interpreter provides a help function that provides the
-necessary documentation regarding all Python syntax, constructs, modules and
-objects. Typing <span class="emphasis"><em>help()</em></span> at the prompt gives the following output:</p>
-<pre class="programlisting">
->>> help()
-
-Welcome to Python 2.5! This is the online help utility.
-
-If this is your first time using Python, you should definitely check out
-the tutorial on the Internet at http://www.python.org/doc/tut/.
-
-Enter the name of any module, keyword, or topic to get help on writing
-Python programs and using Python modules. To quit this help utility and
-return to the interpreter, just type "quit".
-
-To get a list of available modules, keywords, or topics, type "modules",
-"keywords", or "topics". Each module also comes with a one-line summary
-of what it does; to list the modules whose summaries contain a given word
-such as "spam", type "modules spam".
-
-help></pre>
-<p id="ch2intro_1b">As mentioned in the output, entering the name of any module, keyword or topic
-will provide the documentation and help regarding the same through the online
-help utility. Pressing <span class="emphasis"><em>Ctrl+d</em></span> exits the help prompt and returns to the
-python prompt.</p>
-<p id="ch2intro_1c">Let us now try a few examples at the python interpreter.</p>
-<p id="ch2intro_1d">Eg 1:</p>
-<pre class="programlisting">
->>> print 'Hello, python!'
-Hello, python!
->>></pre>
-<p id="ch2intro_1e">Eg 2:</p>
-<pre class="programlisting">
->>> print 4321*567890
-2453852690
->>></pre>
-<p id="ch2intro_1f">Eg 3:</p>
-<pre class="programlisting">
->>> 4321*567890
-2453852690L
->>></pre>
-<pre class="programlisting">
-Note: Notice the 'L' at the end of the output. The 'L' signifies that the
-output of the operation is of type *long*. It was absent in the previous
-example because we used the print statement. This is because *print* formats
-the output before displaying.</pre>
-<p id="ch2intro_20">Eg 4:</p>
-<pre class="programlisting">
->>> big = 12345678901234567890 ** 3
->>> print big
-1881676372353657772490265749424677022198701224860897069000
->>></pre>
-<pre class="programlisting">
-This example is to show that unlike in C or C++ there is no limit on the
-value of an integer.</pre>
-<p id="ch2intro_21">Try this on the interactive interpreter:
-<code class="literal">import this</code></p>
-<p id="ch2intro_22"><span class="emphasis"><em>Hint: The output gives an idea of Power of Python</em></span></p>
-</div>
-<div class="section" title="2.2.ipython - An enhanced interactive Python interpreter">
-<div class="titlepage"><div><div><h3 class="title">
-<a name="id2966950"></a>2.2.<span class="emphasis"><em>ipython</em></span> - An enhanced interactive Python interpreter</h3></div></div></div>
-<p id="ch2intro_23">The power and the importance of the interactive interpreter was the highlight
-of the previous section. This section provides insight into the enhanced
-interpreter with more advanced set of features called <span class="strong"><strong>ipython</strong></span>. Entering
-<span class="emphasis"><em>ipython</em></span> at the shell prompt fires up the interactive interpreter.</p>
-<pre class="programlisting">
-$ ipython
-Python 2.5.2 (r252:60911, Oct 5 2008, 19:24:49)
-Type "copyright", "credits" or "license" for more information.
-
-IPython 0.8.4 -- An enhanced Interactive Python.
-? -> Introduction and overview of IPython's features.
-%quickref -> Quick reference.
-help -> Python's own help system.
-object? -> Details about 'object'. ?object also works, ?? prints more.
-
-In [1]:</pre>
-<p id="ch2intro_24">This is the output obtained upon firing ipython. The exact appearance may
-change based on the Python version installed. The following are some of the
-various features provided by <span class="strong"><strong>ipython</strong></span>:</p>
-<p id="ch2intro_25">Suggestions - ipython provides suggestions of the possible methods and
-operations available for the given python object.</p>
-<p id="ch2intro_26">Eg 5:</p>
-<pre class="programlisting">
-In [4]: a = 6
-
-In [5]: a.
-a.__abs__ a.__divmod__ a.__index__ a.__neg__ a.__rand__ a.__rmod__ a.__rxor__
-a.__add__ a.__doc__ a.__init__ a.__new__ a.__rdiv__ a.__rmul__ a.__setattr__
-a.__and__ a.__float__ a.__int__ a.__nonzero__ a.__rdivmod__ a.__ror__ a.__str__
-a.__class__ a.__floordiv__ a.__invert__ a.__oct__ a.__reduce__ a.__rpow__ a.__sub__
-a.__cmp__ a.__getattribute__ a.__long__ a.__or__ a.__reduce_ex__ a.__rrshift__ a.__truediv__
-a.__coerce__ a.__getnewargs__ a.__lshift__ a.__pos__ a.__repr__ a.__rshift__ a.__xor__
-a.__delattr__ a.__hash__ a.__mod__ a.__pow__ a.__rfloordiv__ a.__rsub__
-a.__div__ a.__hex__ a.__mul__ a.__radd__ a.__rlshift__ a.__rtruediv__</pre>
-<p id="ch2intro_27">In this example, we initialized 'a' (a variable - a concept that will be
-discussed in the subsequent sections.) to 6. In the next line when the <span class="emphasis"><em>tab</em></span> key
-is pressed after typing '<span class="emphasis"><em>a.</em></span>' ipython displays the set of all possible methods
-that are applicable on the object 'a' (an integer in this context). Ipython
-provides many such datatype specific features which will be presented in the
-further sections as and when the datatypes are introduced.</p>
-</div>
-</div>
-<div class="section" title="3.Editing and running a python file">
-<div class="titlepage"><div><div><h2 class="title" style="clear: both">
-<a name="id2967075"></a>3.Editing and running a python file</h2></div></div></div>
-<p id="ch2intro_28">The previous sections focused on the use of the interpreter to run python code.
-While the interpeter is an excellent tool to test simple solutions and
-experiment with small code snippets, its main disadvantage is that everything
-written in the interpreter is lost once its quit. Most of the times a program is
-used by people other than the author. So the programs have to be available in
-some form suitable for distribution, and hence they are written in files. This
-section will focus on editing and running python files. Start by opening a text
-editor ( it is recommended you choose one from the list at the top of this page ).
-In the editor type down python code and save the file with an extension <span class="strong"><strong>.py</strong></span>
-(python files have an extension of .py). Once done with the editing, save the
-file and exit the editor.</p>
-<p id="ch2intro_29">Let us look at a simple example of calculating the gcd of 2 numbers using Python:</p>
-<p id="ch2intro_2a"><span class="strong"><strong>Creating the first python script(file)</strong></span></p>
-<pre class="programlisting">
-$ emacs gcd.py
- def gcd(x,y):
- if x % y == 0:
- return y
- return gcd(y, x%y)
-
- print gcd(72, 92)</pre>
-<p id="ch2intro_2b">To run the script, open the shell prompt, navigate to the directory that
-contains the python file and run <code class="literal">python <filename.py></code> at the prompt ( in this
-case filename is gcd.py )</p>
-<p id="ch2intro_2c"><span class="strong"><strong>Running the python script</strong></span></p>
-<pre class="programlisting">
-$ python gcd.py
-4
-$</pre>
-<p id="ch2intro_2d">Another method to run a python script would be to include the line</p>
-<p id="ch2intro_2e"><code class="literal">#! /usr/bin/python</code></p>
-<p id="ch2intro_2f">at the beginning of the python file and then make the file executable by</p>
-<p id="ch2intro_30">$ chmod a+x <span class="emphasis"><em>filename.py</em></span></p>
-<p id="ch2intro_31">Once this is done, the script can be run as a standalone program as follows:</p>
-<p id="ch2intro_32">$ ./<span class="emphasis"><em>filename.py</em></span></p>
-</div>
-<div class="section" title="4.Basic Datatypes and operators in Python">
-<div class="titlepage"><div><div><h2 class="title" style="clear: both">
-<a name="id2967182"></a>4.Basic Datatypes and operators in Python</h2></div></div></div>
-<p id="ch2intro_33">Python provides the following set of basic datatypes.</p>
-<div class="itemizedlist"><ul class="itemizedlist" type="*">
-<li class="listitem" style="list-style-type: *"><p id="ch2intro_34">Numbers: int, float, long, complex</p></li>
-<li class="listitem" style="list-style-type: *"><p id="ch2intro_35">Strings</p></li>
-<li class="listitem" style="list-style-type: *"><p id="ch2intro_36">Boolean</p></li>
-</ul></div>
-<div class="section" title="4.1.Numbers">
-<div class="titlepage"><div><div><h3 class="title">
-<a name="id2967216"></a>4.1.Numbers</h3></div></div></div>
-<p id="ch2intro_37">Numbers were introduced in the examples presented in the interactive interpreter
-section. Numbers include types as mentioned earlier viz., int (integers), float
-(floating point numbers), long (large integers), complex (complex numbers with
-real and imaginary parts). Python is not a strongly typed language, which means
-the type of a variable need not mentioned during its initialization. Let us look
-at a few examples.</p>
-<p id="ch2intro_38">Eg 6:</p>
-<pre class="programlisting">
->>> a = 1 #here a is an integer variable</pre>
-<p id="ch2intro_39">Eg 7:</p>
-<pre class="programlisting">
->>> lng = 122333444455555666666777777788888888999999999 #here lng is a variable of type long
->>> lng
-122333444455555666666777777788888888999999999L #notice the trailing 'L'
->>> print lng
-122333444455555666666777777788888888999999999 #notice the absence of the trailing 'L'
->>> lng+1
-122333444455555666666777777788888889000000000L</pre>
-<p id="ch2intro_3a">Long numbers are the same as integers in almost all aspects. They can be used in
-operations just like integers and along with integers without any distinction.
-The only distinction comes during type checking (which is not a healthy practice).
-Long numbers are tucked with a trailing 'L' just to signify that they are long.
-Notice that in the example just lng at the prompt displays the value of the variable
-with the 'L' whereas <code class="literal">print lng</code> displays without the 'L'. This is because print
-formats the output before printing. Also in the example, notice that adding an
-integer to a long does not give any errors and the result is as expected. So for
-all practical purposes longs can be treated as ints.</p>
-<p id="ch2intro_3b">Eg 8:</p>
-<pre class="programlisting">
->>> fl = 3.14159 #fl is a float variable
->>> e = 1.234e-4 #e is also a float variable, specified in the exponential form
->>> a = 1
->>> b = 2
->>> a/b #integer division
-0
->>> a/fl #floating point division
-0.31831015504887655
->>> e/fl
-3.9279473133031364e-05</pre>
-<p id="ch2intro_3c">Floating point numbers, simply called floats are real numbers with a decimal point.
-The example above shows the initialization of a float variable. Shown also in this
-example is the difference between integer division and floating point division.
-'a' and 'b' here are integer variables and hence the division gives 0 as the quotient.
-When either of the operands is a float, the operation is a floating point division,
-and the result is also a float as illustrated.</p>
-<p id="ch2intro_3d">Eg 9:</p>
-<pre class="programlisting">
->>> cplx = 3 + 4j #cplx is a complex variable
->>> cplx
-(3+4j)
->>> print cplx.real #prints the real part of the complex number
-3.0
->>> print cplx.imag #prints the imaginary part of the complex number
-4.0
->>> print cplx*fl #multiplies the real and imag parts of the complex number with the multiplier
-(9.42477+12.56636j)
->>> abs(cplx) #returns the absolute value of the complex number
-5.0</pre>
-<p id="ch2intro_3e">Python provides a datatype for complex numbers. Complex numbers are initialized
-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
-imaginary parts of the complex number as shown. The <span class="emphasis"><em>abs()</em></span> returns the absolute
-value of the complex number.</p>
-</div>
-<div class="section" title="4.2.Variables">
-<div class="titlepage"><div><div><h3 class="title">
-<a name="id2967346"></a>4.2.Variables</h3></div></div></div>
-<p id="ch2intro_3f">Variables are just names that represent a value. Variables have already been
-introduced in the various examples from the previous sections. Certain rules about
-using variables:</p>
-<div class="itemizedlist"><ul class="itemizedlist" type="*">
-<li class="listitem" style="list-style-type: *"><p id="ch2intro_40">Variables have to be initialized or assigned a value before being used.</p></li>
-<li class="listitem" style="list-style-type: *"><p id="ch2intro_41">Variable names can consist of letters, digits and underscores(_).</p></li>
-<li class="listitem" style="list-style-type: *"><p id="ch2intro_42">Variable names cannot begin with digits, but can contain digits in them.</p></li>
-</ul></div>
-<p id="ch2intro_43">In reference to the previous section examples, 'a', 'b', 'lng', 'fl', 'e' and 'cplx'
-are all variables of various datatypes.</p>
-<pre class="programlisting">
-Note: Python is not a strongly typed language and hence an integer variable can at a
-later stage be used as a float variable as well.</pre>
-</div>
-<div class="section" title="4.3.Strings">
-<div class="titlepage"><div><div><h3 class="title">
-<a name="id2967394"></a>4.3.Strings</h3></div></div></div>
-<p id="ch2intro_44">Strings are one of the essential data structures of any programming language.
-The <code class="literal">print "Hello, World!"</code> program was introduced in the earlier section, and
-the <span class="emphasis"><em>"Hello, World!"</em></span> in the print statement is a string. A string is basically
-a set of characters. Strings can be represented in various ways shown below:</p>
-<pre class="programlisting">
-s = 'this is a string' # a string variable can be represented using single quotes
-s = 'This one has "quotes" inside!' # The string can have quotes inside it as shown
-s = "I have 'single-quotes' inside!"
-l = "A string spanning many lines
-one more line
-yet another" # a string can span more than a single line.
-t = """A triple quoted string does # another way of representing multiline strings.
-not need to be escaped at the end and
-"can have nested quotes" etc."""</pre>
-<p id="ch2intro_45">Try the following on the interpreter:
-<code class="literal">s = 'this is a string with 'quotes' of similar kind'</code></p>
-<p id="ch2intro_46"><span class="strong"><strong>Exercise: How to use single quotes within single quotes in a string as shown
-in the above example without getting an error?</strong></span></p>
-<div class="section" title="4.3.1.String operations">
-<div class="titlepage"><div><div><h4 class="title">
-<a name="id2967445"></a>4.3.1.String operations</h4></div></div></div>
-<p id="ch2intro_47">A few basic string operations are presented here.</p>
-<p id="ch2intro_48"><span class="strong"><strong>String concatenation</strong></span>
-String concatenation is done by simple addition of two strings.</p>
-<pre class="programlisting">
->>> x = 'Hello'
->>> y = ' Python'
->>> print x+y
-Hello Python</pre>
-<p id="ch2intro_49"><span class="emphasis"><em>Try this yourself:</em></span></p>
-<pre class="programlisting">
->>> somenum = 13
->>> print x+somenum</pre>
-<p id="ch2intro_4a">The problem with the above example is that here a string variable and an integer
-variable are trying to be concantenated. To obtain the desired result from the
-above example the str(), repr() and the `` can be used.</p>
-<p id="ch2intro_4b"><span class="strong"><strong>str()</strong></span> simply converts a value to a string in a reasonable form.
-<span class="strong"><strong>repr()</strong></span> creates a string that is a representation of the value.</p>
-<p id="ch2intro_4c">The difference can be seen in the example shown below:</p>
-<pre class="programlisting">
->>> str(1000000000000000000000000000000000000000000000000L)
-'1000000000000000000000000000000000000000000000000'
->>> repr(1000000000000000000000000000000000000000000000000L)
-'1000000000000000000000000000000000000000000000000L'</pre>
-<p id="ch2intro_4d">It can be observed that the 'L' in the long value shown was omitted by str(),
-whereas repr() converted that into a string too. An alternative way of using
-repr(value) is <code class="literal">`value`</code>.</p>
-<p id="ch2intro_4e">A few more examples:</p>
-<pre class="programlisting">
->>> x = "Let's go nto Pycon"
->>> print x
-Let's go
-to Pycon</pre>
-<p id="ch2intro_4f">In the above example, notice that the 'n'(newline) character is formatted and
-the string is printed on two lines. The strings discussed until now were normal
-strings. Other than these there are two other types of strings namely, raw strings
-and unicode strings.</p>
-<p id="ch2intro_50"><span class="strong"><strong>Raw strings</strong></span> are strings which are unformatted, that is the backslashes() are
-not parsed and are left as it is in the string. Raw strings are represented with
-an 'r' at the start of a string.
-Let us look at an example</p>
-<pre class="programlisting">
->>> x = r"Let's go nto Pycon"
->>> print x
-Let's go nto Pycon</pre>
-<p id="ch2intro_51">Note: The 'n' is not being parsed into a new line and is left as it is.</p>
-<p id="ch2intro_52"><span class="emphasis"><em>Try this yourself:</em></span></p>
-<pre class="programlisting">
->>> x = r"Let's go to Pycon "</pre>
-<p id="ch2intro_53"><span class="strong"><strong>Unicode strings</strong></span> are strings where the characters are Unicode characters as
-opposed to ASCII characters. Unicode strings are represented with a 'u' at the
-start of the string.
-Let us look at an example:</p>
-<pre class="programlisting">
->>> x = u"Let's go to Pycon!"
->>> print x
-Let's go to Pycon!</pre>
-</div>
-</div>
-<div class="section" title="4.4.Boolean">
-<div class="titlepage"><div><div><h3 class="title">
-<a name="id2967588"></a>4.4.Boolean</h3></div></div></div>
-<p id="ch2intro_54">Python also provides special Boolean datatype. A boolean variable can assume a
-value of either <span class="emphasis"><em>True</em></span> or <span class="emphasis"><em>False</em></span> (Note the capitalizations).</p>
-<p id="ch2intro_55">Let us look at examples:</p>
-<pre class="programlisting">
->>> t = True
->>> f = not t
->>> print f
-False
->>> f or t
-True
->>> f and t
-False</pre>
-</div>
-</div>
-<div class="section" title="5.The while loop">
-<div class="titlepage"><div><div><h2 class="title" style="clear: both">
-<a name="id2967617"></a>5.The <span class="strong"><strong>while</strong></span> loop</h2></div></div></div>
-<p id="ch2intro_56">The Python <span class="strong"><strong>while</strong></span> loop is similar to the C/C++ while loop. The syntax is as
-follows:</p>
-<pre class="programlisting">
-statement 0
-while condition:
- statement 1 #while block
- statement 2 #while block
-statement 3 #outside the while block.</pre>
-<p id="ch2intro_57">Let us look at an example:</p>
-<pre class="programlisting">
->>> x = 1
->>> while x <= 5:
-... print x
-... x += 1
-...
-1
-2
-3
-4
-5</pre>
-</div>
-<div class="section" title="6.The if conditional">
-<div class="titlepage"><div><div><h2 class="title" style="clear: both">
-<a name="id2967654"></a>6.The <span class="strong"><strong>if</strong></span> conditional</h2></div></div></div>
-<p id="ch2intro_58">The Python <span class="strong"><strong>if</strong></span> block provides the conditional execution of statements.
-If the condition evaluates as true the block of statements defined under the if
-block are executed.</p>
-<p id="ch2intro_59">If the first block is not executed on account of the condition not being satisfied,
-the set of statements in the <span class="strong"><strong>else</strong></span> block are executed.</p>
-<p id="ch2intro_5a">The <span class="strong"><strong>elif</strong></span> block provides the functionality of evaluation of multiple conditions
-as shown in the example.</p>
-<p id="ch2intro_5b">The syntax is as follows:</p>
-<pre class="programlisting">
-if condition :
- statement_1
- statement_2
-
-elif condition:
- statement_3
- statement_4
-else:
- statement_5
- statement_6</pre>
-<p id="ch2intro_5c">Let us look at an example:</p>
-<pre class="programlisting">
->>> n = raw_input("Input a number:")
->>> if n < 0:
- print n," is negative"
- elif n > 0:
- print n," is positive"
- else:
- print n, " is 0"</pre>
-</div>
-<div class="section" title="7.raw_input()">
-<div class="titlepage"><div><div><h2 class="title" style="clear: both">
-<a name="id2967723"></a>7.<span class="strong"><strong>raw_input()</strong></span>
-</h2></div></div></div>
-<p id="ch2intro_5d">In the previous example we saw the call to the raw_input() subroutine.
-The <span class="strong"><strong>raw_input()</strong></span> method is used to take user inputs through the console.
-Unlike <span class="strong"><strong>input()</strong></span> which assumes the data entered by the user as a standard python
-expression, <span class="strong"><strong>raw_input()</strong></span> treats all the input data as raw data and converts
-everything into a string. To illustrate this let us look at an example.</p>
-<pre class="programlisting">
->>> input("Enter a number thats a palindrome:")
-Enter a number thats a palindrome:121
-121
-
->>> input("Enter your name:")
-Enter your name:PythonFreak
-Traceback (most recent call last):
- File "<stdin>", line 1, in <module>
- File "<string>", line 1, in <module>
-NameError: name 'PythonFreak' is not defined</pre>
-<p id="ch2intro_5e">As shown above the <span class="strong"><strong>input()</strong></span> assumes that the data entered is a valid Python
-expression. In the first call it prompts for an integer input and when entered
-it accepts the integer as an integer, whereas in the second call, when the string
-is entered without the quotes, <span class="strong"><strong>input()</strong></span> assumes that the entered data is a valid
-Python expression and hence it raises and exception saying PythonFreak is not
-defined.</p>
-<pre class="programlisting">
->>> input("Enter your name:")
-Enter your name:'PythonFreak'
-'PythonFreak'
->>></pre>
-<p id="ch2intro_5f">Here the name is accepted because its entered as a string (within quotes). But
-its unreasonable to go on using quotes each time a string is entered. Hence the
-alternative is to use <span class="strong"><strong>raw_input()</strong></span>.</p>
-<p id="ch2intro_60">Let us now look at how <span class="strong"><strong>raw_input()</strong></span> operates with an example.</p>
-<pre class="programlisting">
->>> raw_input("Enter your name:")
-Enter your name:PythonFreak
-'PythonFreak'</pre>
-<p id="ch2intro_61">Observe that the <span class="strong"><strong>raw_input()</strong></span> is converting it into a string all by itself.</p>
-<pre class="programlisting">
->>> pal = raw_input("Enter a number thats a palindrome:")
-Enter a number thats a palindrome:121
-'121'</pre>
-<p id="ch2intro_62">Observe that <span class="strong"><strong>raw_input()</strong></span> is converting the integer 121 also to a string as
-'121'. Let us look at another example:</p>
-<pre class="programlisting">
->>> pal = raw_input("Enter a number thats a palindrome:")
-Enter a number thats a palindrome:121
->>> pal + 2
-Traceback (most recent call last):
- File "<stdin>", line 1, in <module>
-TypeError: cannot concatenate 'str' and 'int' objects
->>> pal
-'121'</pre>
-<p id="ch2intro_63">Observe here that the variable <span class="emphasis"><em>pal</em></span> is a string and hence integer operations
-cannot be performed on it. Hence the exception is raised.</p>
-</div>
-<div class="section" title="8.int() method">
-<div class="titlepage"><div><div><h2 class="title" style="clear: both">
-<a name="id2967877"></a>8.<span class="strong"><strong>int()</strong></span> method</h2></div></div></div>
-<p id="ch2intro_64">Generally for computing purposes, the data used is not strings or raw data but
-on integers, floats and similar mathematical data structures. The data obtained
-from <span class="strong"><strong>raw_input()</strong></span> is raw data in the form of strings. In order to obtain integers
-from strings we use the method <span class="strong"><strong>int()</strong></span>.</p>
-<p id="ch2intro_65">Let us look at an example.</p>
-<pre class="programlisting">
->>> intpal = int(pal)
->>> intpal
-121</pre>
-<p id="ch2intro_66">In the previous example it was observed that <span class="emphasis"><em>pal</em></span> was a string variable. Here
-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>
-<p id="ch2intro_67"><span class="emphasis"><em>Try This Yourself:</em></span></p>
-<pre class="programlisting">
->>> stringvar = raw_input("Enter a name:")
-Enter a name:Guido Van Rossum
->>> stringvar
-'Guido Van Rossum'
->>> numvar = int(stringvar)</pre>
-</div>
-</div>
-</div></body>
-</html>
-