Branching Program: Using if, if else, nested if else, elif in Python

Branching programs

The simplest branching statement is a conditon

• A test (expression that evaluates to True or False) 
• A block of code to execute
• if the test is True!
• An optional block of code to execute if the test is False!

A simple example

x = int(raw_input('Enter an integer: '))
if x%2 == 0:  # % (modulo) returns the remainder when x/2
    print(‘’)
    print('Even')
else:
    print(‘’)
    print('Odd')
print(’Done with conditional')

Some observations

• The expression x%2 == 0 evaluates to True when the remainder of x divided by 2 is 0!
• Note that == is used for comparison, since = is reserved for assignment
• The indentation is important – each indented set of expressions denotes a block of instructions
• For example, if the last statement were indented, it would be executed as part of the else block of code
• Note how this indentation provides a visual structure that reflects the semantic structure of the program

We can have nested conditions

if x%2 == 0:
    if x%3 == 0:
        print('Divisible by 2 and 3’)
    else:
        print('Divisible by 2 and not by 3’)
elif x%3 == 0:
    print('Divisible by 3 and not by 2’)

And we can use compound Boolean

if x < y and x < z:
    print('x is least’)
elif y < z:
    print('y is least’)
else:
    print('z is least’)

• Branching programs allow us to make choices and do different things
• But still the case that at most, each statement gets executed once.
• So maximum time to run the program depends only on the length of the program
• These programs run in constant time  

Note:
This is a part of what I learn in an online Open Course Ware offered by MIT on edX
Its for my personal reference & also for those who have missed the course.
You too can enroll yourself on edX (if they are still offering the course MITx 6.00x)