conditionals.rst
author amit
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Hello friends. Welcome to this spoken tutorial on Getting started with
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strings.
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{{{ Show the slide containing the title }}}
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{{{ Show the slide containing the outline }}}
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In this tutorial, we will learn the basic conditional constructs
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available in Python. We learn the if/else, if/elif/else and ternary
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conditional constructs available in Python. 
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{{{ Shift to terminal and start ipython }}}
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To begin with let us start ipython, by typing::
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  ipython
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on the terminal
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Whenever we have two possible states that can occur depending on a
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whether a certain condition we can use if/else construct in
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Python. Say for example we have a variable "a" which stores integers
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and we are required to find out whether the value of the variable "a"
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is an even number or an odd number. To test out conditional statements
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as an example, let us say the value of the variable "a" is 5::
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  a = 5
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In such a case we can write the if/else block as::
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  if a % 2 == 0:
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      print "Even"
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  else:
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      print "Odd"
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When the value of the variable "a" is divided by 2 and the remainder
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is 0 i.e. the result of the operation "a modulo 2" is 0 the condition
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"a % 2 == 0" evaluates to True, so the code within the if block gets
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executed. This means that the value of "a" is Even. 
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If the operation "a modulo 2" is not 0 the condition "a % 2 == 0"
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evaluates to False and hence the code block within else gets executed
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which means that the value of "a" is Odd. 
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Note in such a case only one of the two blocks get executed depending
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on whether the condition is True or False.
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There is a very important sytactic element to understand here. All the
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statements which are inside a certain code block are indented by 4
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spaces. The statement which starts a new code block after it, i.e. the
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if statement in this example ends with a colon (:). So the next
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immediate line will be inside the if block and hence indented by 4
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spaces. To come out of the code block we have to come back to the
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previous indentation level as shown in the else line here. Again the
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line following else will be in a new block so else line ends with a
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colon and the following block of code is indented by 4.
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As we use if/else statement when we have a condition which can take
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one of the two states, we may have conditions which can take more than
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two states. In such a scenario Python provides if/elif/else
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statements. Let us take an example. We have a variable "a" which holds
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integer values. We need to print "positive" if the value of a is
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positive, "negative" if it is negative and "zero" if the value of the
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variable "a" is 0. Let us use if/elif/else ladder for it. For the
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purposes of testing our code let us assume that the value of a is -3::
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  a = -3
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  if a > 0:
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      print "positive"
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  elif a < 0:
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      print "negative"
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  else:
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      print "zero"
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This if/elif/else ladder is self explanatory. All the syntax and rules
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as said for if/else statements hold. The only addition here is the
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elif statement which can have another condition of its own.
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Here, exactly one block of code is executed and that block of code
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corresponds to the condition which first evaluates to True. Even if
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there is a situation where multiple conditions evaluate to True all
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the subsequent conditions other than the first one which evaluates to
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True are neglected. Consequently, the else block gets executed if and
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only if all the conditions evaluate to False.
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Also, the else block in both if/else statement and if/elif/else is
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optional. We can have a single if statement or just if/elif statements
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without having else block at all. Also, there can be any number of
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elif's within an if/elif/else ladder. For example
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{{{ Show slide for this }}}
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  if user == 'admin':
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      # Do admin operations
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  elif user == 'moderator':
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      # Do moderator operations
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  elif user == 'client':
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      # Do customer operations
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{{{ end of slide switch to ipython }}}
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is completely valid. Note that there are multiple elif blocks and there
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is no else block.
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In addition to these conditional statements, Python provides a very
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convenient ternary conditional operator. Let us take the following
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example where we read the marks data from a data file which is
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obtained as a string as we read a file. The marks can be in the range
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of 0 to 100 or 'AA' if the student is absent. In such a case to obtain
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the marks as an integer we can use the ternary conditional
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operator. Let us say the string score is stored in score_str
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variable::
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  score_str = 'AA'
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Now let us use the ternary conditional operator::
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  score = int(score_str) if score_str != 'AA' else 0
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This is just the if/else statement block which written in a more
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convenient form and is very helpful when we have only one statement
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for each block. This conditional statement effectively means as we
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would have exactly specified in the English language which will be
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like score is integer of score_str is score_str is not 'AA' otherwise
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it is 0. This means that we make the scores of the students who were
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absent for the exam 0.
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Moving on, there are certain situations where we will have to no
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operations or statements within the block of code. For example, we
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have a code where we are waiting for the keyboard input. If the user
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enters "s" as the input we would perform some operation nothing
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otherwise. In such cases "pass" statement comes very handy::
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  a = raw_input("Enter 'c' to calculate and exit, 'd' to display the existing
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  results exit and 'x' to exit and any other key to continue: ")
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  if a == 'c':
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     # Calculate the marks and exit
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  elif a == 'd':
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     # Display the results and exit
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  elif a == 'x':
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     # Exit the program
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  else:
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     pass
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In this case "pass" statement acts as a place holder for the block of
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code. It is equivalent to a null operation. It literally does
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nothing. So "pass" statement can be used as a null operation
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statement, or it can used as a place holder when the actual code
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implementation for a particular block of code is not known yet but has
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to be filled up later.
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{{{ Show summary slide }}}
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This brings us to the end of the tutorial session on conditional
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statements in Python. In this tutorial session we learnt
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  * What are conditional statements
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  * if/else statement
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  * if/elif/else statement
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  * Ternary conditional statement - C if X else Y
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  * and the "pass" statement
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{{{ Show the "sponsored by FOSSEE" slide }}}
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This tutorial was created as a part of FOSSEE project, NME ICT, MHRD India
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Hope you have enjoyed and found it useful.
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Thankyou
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.. Author              : Madhu
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   Internal Reviewer 1 :         [potential reviewer: Puneeth]
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   Internal Reviewer 2 :         [potential reviewer: Anoop]
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   External Reviewer   :
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