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+++ b/conditionals.rst	Sat Sep 18 01:25:21 2010 +0530
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+Hello friends. Welcome to this spoken tutorial on Getting started with
+strings.
+
+{{{ Show the slide containing the title }}}
+
+{{{ 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.
+Thankyou
+ 
+.. Author              : Madhu
+   Internal Reviewer 1 :         [potential reviewer: Puneeth]
+   Internal Reviewer 2 :         [potential reviewer: Anoop]
+   External Reviewer   :
+