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+<title>Chapter 2. Basic Python</title>
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+<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="#id2706129">Basic Python</a></span></dt>
+<dd><dl>
+<dt><span class="section"><a href="#id2705187">1. Introduction</a></span></dt>
+<dt><span class="section"><a href="#id2757271">2. The Python Interpreter</a></span></dt>
+<dd><dl>
+<dt><span class="section"><a href="#id2757279">2.1. The Interactive Interpreter</a></span></dt>
+<dt><span class="section"><a href="#id2757464">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="#id2757589">3. Editing and running a python file</a></span></dt>
+<dt><span class="section"><a href="#id2757696">4. Basic Datatypes and operators in Python</a></span></dt>
+<dd><dl>
+<dt><span class="section"><a href="#id2757731">4.1. Numbers</a></span></dt>
+<dt><span class="section"><a href="#id2757854">4.2. Variables</a></span></dt>
+<dt><span class="section"><a href="#id2757903">4.3. Strings</a></span></dt>
+<dt><span class="section"><a href="#id2758097">4.4. Boolean</a></span></dt>
+</dl></dd>
+<dt><span class="section"><a href="#id2758126">5. The <span class="strong"><strong>while</strong></span> loop</a></span></dt>
+<dt><span class="section"><a href="#id2758163">6. The <span class="strong"><strong>if</strong></span> conditional</a></span></dt>
+<dt><span class="section"><a href="#id2758232">7. <span class="strong"><strong>raw_input()</strong></span></a></span></dt>
+<dt><span class="section"><a href="#id2758387">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="id2706129"></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="#id2705187">1. Introduction</a></span></dt>
+<dt><span class="section"><a href="#id2757271">2. The Python Interpreter</a></span></dt>
+<dd><dl>
+<dt><span class="section"><a href="#id2757279">2.1. The Interactive Interpreter</a></span></dt>
+<dt><span class="section"><a href="#id2757464">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="#id2757589">3. Editing and running a python file</a></span></dt>
+<dt><span class="section"><a href="#id2757696">4. Basic Datatypes and operators in Python</a></span></dt>
+<dd><dl>
+<dt><span class="section"><a href="#id2757731">4.1. Numbers</a></span></dt>
+<dt><span class="section"><a href="#id2757854">4.2. Variables</a></span></dt>
+<dt><span class="section"><a href="#id2757903">4.3. Strings</a></span></dt>
+<dt><span class="section"><a href="#id2758097">4.4. Boolean</a></span></dt>
+</dl></dd>
+<dt><span class="section"><a href="#id2758126">5. The <span class="strong"><strong>while</strong></span> loop</a></span></dt>
+<dt><span class="section"><a href="#id2758163">6. The <span class="strong"><strong>if</strong></span> conditional</a></span></dt>
+<dt><span class="section"><a href="#id2758232">7. <span class="strong"><strong>raw_input()</strong></span></a></span></dt>
+<dt><span class="section"><a href="#id2758387">8. <span class="strong"><strong>int()</strong></span> method</a></span></dt>
+</dl>
+</div>
+<p id="ch2intro_1"></a>This document is intended to be handed out at the end of the workshop. It has
+been designed for Engineering students who are Python beginners and have basic
+programming skills. The focus is on basic numerics and plotting using Python.</p>
+<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">
+<div class="titlepage"><div><div><h2 class="title" style="clear: both">
+<a name="id2705187"></a>1. Introduction</h2></div></div></div>
+<p id="ch2intro_5"></a>The Python programming language was created by a dutch named Guido van Rossum.
+The idea of Python was conceived in December 1989. The name Python has nothing
+to do with the reptilian, but its been named after the 70s comedy series
+"Monty Python's Flying Circus", since it happens to be Guido's favourite
+TV series.</p>
+<p id="ch2intro_6"></a>Current stable version of Python is 2.6.x, although Python 3.0 is also the stable
+version, it is not backwards compatible with the previous versions and is hence
+not entirely popular at the moment. This material will focus on the 2.6.x series.</p>
+<p id="ch2intro_7"></a>Python is licensed under the Python Software Foundation License (PSF License)
+which is GPL compatible Free Software license (excepting license version 1.6 and 2.0)
+It is a no strings attached license, which means the source code is free to modify
+and redistribute.</p>
+<p id="ch2intro_8"></a>The Python docs define Python as "Python is an interpreted, object-oriented,
+high-level programming language with dynamic semantics." A more detailed summary
+can be found at </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"></a>Python is a highly cross platform compatible language on account of it being an
+interpreted language. It is highly scalable and hence has been adapted to run on
+the Nokia 60 series phones. Python has been designed to be readable and easy to use</p>
+<p id="ch2intro_a"></a><span class="strong"><strong>Resources available for reference</strong></span></p>
+<span style="color: black"><span style="color: black"><p id="ch2intro_b"></a>Web: </p>
+<div class="reference">
+<div class="titlepage"><hr></div>http://www.python.org</div></span><span style="color: black"><p id="ch2intro_c"></a>Doc: </p>
+<div class="reference">
+<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>
+<div class="reference">
+<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>
+<div class="reference">
+<div class="titlepage"><hr></div>http://www.byteofpython.info/</div></span><span style="color: black"><p id="ch2intro_f"></a>Dive into Python: </p>
+<div class="reference">
+<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>
+<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
+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></span><span style="color: black"><p id="ch2intro_12"></a>Python is a high level, interpreted, modular and object oriented language.
+Python performs memory management on its own, thus the programmer need not bother
+about allocating and deallocating memory to variables. Python provides extensibility
+by providing modules which can be easily imported similar to headers in C and
+packages in Java. Python is object oriented and hence provides all the object oriented
+characteristics such as inheritance, encapsulation and polymorphism.</p></span><span style="color: black"><p id="ch2intro_13"></a>Python offers a highly powerful interactive programming interface in the form
+of the 'Interactive Interpreter' which will be discussed in more detail in the
+following sections.</p></span><span style="color: black"><p id="ch2intro_14"></a>Python provides a rich standard library and an extensive set of modules. The
+power of Python modules can be seen in this slightly exaggerated cartoon
+</p>
+<div class="reference">
+<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++
+and FORTRAN.</p></span></span><p id="ch2intro_16"></a>Although, Python has one setback. Python is not fast as some of the compiled
+languages like C or C++. Yet, the amount of flexibility and power more than make
+up for this setback.</p>
+</div>
+<div class="section" title="2. The Python Interpreter">
+<div class="titlepage"><div><div><h2 class="title" style="clear: both">
+<a name="id2757271"></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="id2757279"></a>2.1. The Interactive Interpreter</h3></div></div></div>
+<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
+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"></a>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"></a>This example was quite straight forward, and thus we have written our first
+line of Python code. Now let us try typing something arbitrary at the prompt.
+For example:</p>
+<pre class="programlisting"> >>> arbit word
+ File "<stdin>", line 1
+ arbit word
+ ^
+SyntaxError: invalid syntax
+>>></pre>
+<p id="ch2intro_1a"></a>The interpreter gave an error message saying that 'arbit word' was invalid
+syntax which is valid. The interpreter is an amazing tool when learning to
+program in Python. The interpreter provides a help function that provides the
+necessary documentation regarding all Python syntax, constructs, modules and
+objects. Typing <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"></a>As mentioned in the output, entering the name of any module, keyword or topic
+will provide the documentation and help regarding the same through the online
+help utility. Pressing <span class="emphasis"><em>Ctrl+d</em></span> exits the help prompt and returns to the
+python prompt.</p>
+<p id="ch2intro_1c"></a>Let us now try a few examples at the python interpreter.</p>
+<p id="ch2intro_1d"></a>Eg 1:</p>
+<pre class="programlisting"> >>> print 'Hello, python!'
+Hello, python!
+>>></pre>
+<p id="ch2intro_1e"></a>Eg 2:</p>
+<pre class="programlisting"> >>> print 4321*567890
+2453852690
+>>></pre>
+<p id="ch2intro_1f"></a>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"></a>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"></a>Try this on the interactive interpreter:
+<code class="literal">import this</code></p>
+<p id="ch2intro_22"></a><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="id2757464"></a>2.2. <span class="emphasis"><em>ipython</em></span> - An enhanced interactive Python interpreter</h3></div></div></div>
+<p id="ch2intro_23"></a>The power and the importance of the interactive interpreter was the highlight
+of the previous section. This section provides insight into the enhanced
+interpreter with more advanced set of features called <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"></a>This is the output obtained upon firing ipython. The exact appearance may
+change based on the Python version installed. The following are some of the
+various features provided by <span class="strong"><strong>ipython</strong></span>:</p>
+<span style="color: black"><p id="ch2intro_25"></a>Suggestions - ipython provides suggestions of the possible methods and
+operations available for the given python object.</p></span><p id="ch2intro_26"></a>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"></a>In this example, we initialized 'a' (a variable - a concept that will be
+discussed in the subsequent sections.) to 6. In the next line when the <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="id2757589"></a>3. Editing and running a python file</h2></div></div></div>
+<p id="ch2intro_28"></a>The previous sections focused on the use of the interpreter to run python code.
+While the interpeter is an excellent tool to test simple solutions and
+experiment with small code snippets, its main disadvantage is that everything
+written in the interpreter is lost once its quit. Most of the times a program is
+used by people other than the author. So the programs have to be available in
+some form suitable for distribution, and hence they are written in files. This
+section will focus on editing and running python files. Start by opening a text
+editor ( it is recommended you choose one from the list at the top of this page ).
+In the editor type down python code and save the file with an extension <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"></a>Let us look at a simple example of calculating the gcd of 2 numbers using Python:</p>
+<p id="ch2intro_2a"></a><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"></a>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"></a><span class="strong"><strong>Running the python script</strong></span></p>
+<pre class="programlisting"> $ python gcd.py
+4
+$</pre>
+<p id="ch2intro_2d"></a>Another method to run a python script would be to include the line</p>
+<p id="ch2intro_2e"></a><code class="literal">#! /usr/bin/python</code></p>
+<p id="ch2intro_2f"></a>at the beginning of the python file and then make the file executable by</p>
+<p id="ch2intro_30"></a>$ chmod a+x <span class="emphasis"><em>filename.py</em></span></p>
+<p id="ch2intro_31"></a>Once this is done, the script can be run as a standalone program as follows:</p>
+<p id="ch2intro_32"></a>$ ./<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="id2757696"></a>4. Basic Datatypes and operators in Python</h2></div></div></div>
+<p id="ch2intro_33"></a>Python provides the following set of basic datatypes.</p>
+<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">
+<div class="titlepage"><div><div><h3 class="title">
+<a name="id2757731"></a>4.1. Numbers</h3></div></div></div>
+<p id="ch2intro_37"></a>Numbers were introduced in the examples presented in the interactive interpreter
+section. Numbers include types as mentioned earlier viz., int (integers), float
+(floating point numbers), long (large integers), complex (complex numbers with
+real and imaginary parts). Python is not a strongly typed language, which means
+the type of a variable need not mentioned during its initialization. Let us look
+at a few examples.</p>
+<p id="ch2intro_38"></a>Eg 6:</p>
+<pre class="programlisting"> >>> a = 1 #here a is an integer variable</pre>
+<p id="ch2intro_39"></a>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"></a>Long numbers are the same as integers in almost all aspects. They can be used in
+operations just like integers and along with integers without any distinction.
+The only distinction comes during type checking (which is not a healthy practice).
+Long numbers are tucked with a trailing 'L' just to signify that they are long.
+Notice that in the example just lng at the prompt displays the value of the variable
+with the 'L' whereas <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"></a>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"></a>Floating point numbers, simply called floats are real numbers with a decimal point.
+The example above shows the initialization of a float variable. Shown also in this
+example is the difference between integer division and floating point division.
+'a' and 'b' here are integer variables and hence the division gives 0 as the quotient.
+When either of the operands is a float, the operation is a floating point division,
+and the result is also a float as illustrated.</p>
+<p id="ch2intro_3d"></a>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"></a>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="id2757854"></a>4.2. Variables</h3></div></div></div>
+<p id="ch2intro_3f"></a>Variables are just names that represent a value. Variables have already been
+introduced in the various examples from the previous sections. Certain rules about
+using variables:</p>
+<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'
+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="id2757903"></a>4.3. Strings</h3></div></div></div>
+<p id="ch2intro_44"></a>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"></a>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"></a><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="id2757955"></a>4.3.1. String operations</h4></div></div></div>
+<p id="ch2intro_47"></a>A few basic string operations are presented here.</p>
+<p id="ch2intro_48"></a><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"></a><span class="emphasis"><em>Try this yourself:</em></span></p>
+<pre class="programlisting"> >>> somenum = 13
+>>> print x+somenum</pre>
+<p id="ch2intro_4a"></a>The problem with the above example is that here a string variable and an integer
+variable are trying to be concantenated. To obtain the desired result from the
+above example the str(), repr() and the `` can be used.</p>
+<p id="ch2intro_4b"></a><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"></a>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"></a>It can be observed that the 'L' in the long value shown was omitted by str(),
+whereas repr() converted that into a string too. An alternative way of using
+repr(value) is <code class="literal">`value`</code>.</p>
+<p id="ch2intro_4e"></a>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"></a>In the above example, notice that the 'n'(newline) character is formatted and
+the string is printed on two lines. The strings discussed until now were normal
+strings. Other than these there are two other types of strings namely, raw strings
+and unicode strings.</p>
+<p id="ch2intro_50"></a><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"></a>Note: The 'n' is not being parsed into a new line and is left as it is.</p>
+<p id="ch2intro_52"></a><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"></a><span class="strong"><strong>Unicode strings</strong></span> are strings where the characters are Unicode characters as
+opposed to ASCII characters. Unicode strings are represented with a 'u' at the
+start of the string.
+Let us look at an example:</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="id2758097"></a>4.4. Boolean</h3></div></div></div>
+<p id="ch2intro_54"></a>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"></a>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="id2758126"></a>5. The <span class="strong"><strong>while</strong></span> loop</h2></div></div></div>
+<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
+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"></a>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="id2758163"></a>6. The <span class="strong"><strong>if</strong></span> conditional</h2></div></div></div>
+<p id="ch2intro_58"></a>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"></a>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"></a>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"></a>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"></a>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="id2758232"></a>7. <span class="strong"><strong>raw_input()</strong></span>
+</h2></div></div></div>
+<p id="ch2intro_5d"></a>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"></a>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"></a>Here the name is accepted because its entered as a string (within quotes). But
+its unreasonable to go on using quotes each time a string is entered. Hence the
+alternative is to use <span class="strong"><strong>raw_input()</strong></span>.</p>
+<p id="ch2intro_60"></a>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"></a>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"></a>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"></a>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="id2758387"></a>8. <span class="strong"><strong>int()</strong></span> method</h2></div></div></div>
+<p id="ch2intro_64"></a>Generally for computing purposes, the data used is not strings or raw data but
+on integers, floats and similar mathematical data structures. The data obtained
+from <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"></a>Let us look at an example.</p>
+<pre class="programlisting"> >>> intpal = int(pal)
+>>> intpal
+121</pre>
+<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
+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"></a><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>