.. Objectives

.. ----------

.. Clearly state the objectives of the LO (along with RBT level)

.. Prerequisites

.. -------------

..   1. Name of LO-1

..   2. Name of LO-2

..   3. Name of LO-3

.. Author              : Madhu

   Internal Reviewer   : 

   External Reviewer   :

   Checklist OK?       : <put date stamp here, if OK> [2010-10-05]

Script

------

{{{ Show the slide containing the title }}}

Hello friends. Welcome to this spoken tutorial on Getting started with

strings.

{{{ Show the slide containing the outline }}}

In this tutorial, we will learn the basic conditional constructs

available in Python. We learn the if/else, if/elif/else and ternary

conditional constructs available in Python. 

{{{ Shift to terminal and start ipython }}}

To begin with let us start ipython, by typing::

  ipython

on the terminal

Whenever we have two possible states that can occur depending on a

whether a certain condition we can use if/else construct in

Python. Say for example we have a variable "a" which stores integers

and we are required to find out whether the value of the variable "a"

is an even number or an odd number. To test out conditional statements

as an example, let us say the value of the variable "a" is 5::

  a = 5

In such a case we can write the if/else block as::

  if a % 2 == 0:

      print "Even"

  else:

      print "Odd"

When the value of the variable "a" is divided by 2 and the remainder

is 0 i.e. the result of the operation "a modulo 2" is 0 the condition

"a % 2 == 0" evaluates to True, so the code within the if block gets

executed. This means that the value of "a" is Even. 

If the operation "a modulo 2" is not 0 the condition "a % 2 == 0"

evaluates to False and hence the code block within else gets executed

which means that the value of "a" is Odd. 

Note in such a case only one of the two blocks get executed depending

on whether the condition is True or False.

There is a very important sytactic element to understand here. All the

statements which are inside a certain code block are indented by 4

spaces. The statement which starts a new code block after it, i.e. the

if statement in this example ends with a colon (:). So the next

immediate line will be inside the if block and hence indented by 4

spaces. To come out of the code block we have to come back to the

previous indentation level as shown in the else line here. Again the

line following else will be in a new block so else line ends with a

colon and the following block of code is indented by 4.

As we use if/else statement when we have a condition which can take

one of the two states, we may have conditions which can take more than

two states. In such a scenario Python provides if/elif/else

statements. Let us take an example. We have a variable "a" which holds

integer values. We need to print "positive" if the value of a is

positive, "negative" if it is negative and "zero" if the value of the

variable "a" is 0. Let us use if/elif/else ladder for it. For the

purposes of testing our code let us assume that the value of a is -3::

  a = -3

  if a > 0:

      print "positive"

  elif a < 0:

      print "negative"

  else:

      print "zero"

This if/elif/else ladder is self explanatory. All the syntax and rules

as said for if/else statements hold. The only addition here is the

elif statement which can have another condition of its own.

Here, exactly one block of code is executed and that block of code

corresponds to the condition which first evaluates to True. Even if

there is a situation where multiple conditions evaluate to True all

the subsequent conditions other than the first one which evaluates to

True are neglected. Consequently, the else block gets executed if and

only if all the conditions evaluate to False.

Also, the else block in both if/else statement and if/elif/else is

optional. We can have a single if statement or just if/elif statements

without having else block at all. Also, there can be any number of

elif's within an if/elif/else ladder. For example

{{{ Show slide for this }}}

  if user == 'admin':

      # Do admin operations

  elif user == 'moderator':

      # Do moderator operations

  elif user == 'client':

      # Do customer operations

{{{ end of slide switch to ipython }}}

is completely valid. Note that there are multiple elif blocks and there

is no else block.

In addition to these conditional statements, Python provides a very

convenient ternary conditional operator. Let us take the following

example where we read the marks data from a data file which is

obtained as a string as we read a file. The marks can be in the range

of 0 to 100 or 'AA' if the student is absent. In such a case to obtain

the marks as an integer we can use the ternary conditional

operator. Let us say the string score is stored in score_str

variable::

  score_str = 'AA'

Now let us use the ternary conditional operator::

  score = int(score_str) if score_str != 'AA' else 0

This is just the if/else statement block which written in a more

convenient form and is very helpful when we have only one statement

for each block. This conditional statement effectively means as we

would have exactly specified in the English language which will be

like score is integer of score_str is score_str is not 'AA' otherwise

it is 0. This means that we make the scores of the students who were

absent for the exam 0.

Moving on, there are certain situations where we will have to no

operations or statements within the block of code. For example, we

have a code where we are waiting for the keyboard input. If the user

enters "s" as the input we would perform some operation nothing

otherwise. In such cases "pass" statement comes very handy::

  a = raw_input("Enter 'c' to calculate and exit, 'd' to display the existing

  results exit and 'x' to exit and any other key to continue: ")

  if a == 'c':

     # Calculate the marks and exit

  elif a == 'd':

     # Display the results and exit

  elif a == 'x':

     # Exit the program

  else:

     pass

In this case "pass" statement acts as a place holder for the block of

code. It is equivalent to a null operation. It literally does

nothing. So "pass" statement can be used as a null operation

statement, or it can used as a place holder when the actual code

implementation for a particular block of code is not known yet but has

to be filled up later.

{{{ Show summary slide }}}

This brings us to the end of the tutorial session on conditional

statements in Python. In this tutorial session we learnt

  * What are conditional statements

  * if/else statement

  * if/elif/else statement

  * Ternary conditional statement - C if X else Y

  * and the "pass" statement

{{{ Show the "sponsored by FOSSEE" slide }}}

This tutorial was created as a part of FOSSEE project, NME ICT, MHRD India

Hope you have enjoyed and found it useful.

Thank you!