Python: Branching code flow using if - else - elif

Branching programs allow us to make choices and do different things. Let’s have look at the following flow diagram.

Observing the flow diagram:

• 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

Table of Contents

• A simple example of if and else in Python:
• Nested if, else and elif in Python:
• Compound conditions inside if

Such a desire can be achieved using if else clauses in Python.

A simple example of if and else in Python:

x = int(raw_input('Enter an integer: ')) # Will wait for you to enter an int
if x%2 == 0:  # % (modulo) returns the remainder when x/2
    print('Even')
else:
    print('Odd')
print('Done with condition')

Analysis of the above code:

• 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

Nested if, else and elif in Python:

An example program to check if a number if divisible by 2 and/or 3.

x = int(raw_input('Enter an integer: ')) # Will wait for you to enter an int
if x%2 == 0:
    if x%3 == 0:
        print('Divisible by 2 and 3')
    else:
        print('Divisible by 2 but not by 3')
elif x%3 == 0:
    print('Divisible by 3 but not by 2')
else:
    print('Not divisible by either 2 or 3')

Analysis of the above code:

• Notice the if else block inside the first if block. This is called nesting.
• We have an elif block, this is executed when the code does not flow through if. elif is, as you might have guessed, just a short form for ‘else if’.
• The last else block is executed with the code does not flow through either of if or elif

Compound conditions inside if

Let’s consider a program to find the minimum of three integers.

x, y, z = map(int, raw_input('Enter three space separated integer: ').split())
if x < y and x < z:
    print('Minimum among ' + str(x) + ', ' + str(y) + ', '+ str(z) + ' is ' + str(x))
elif y < z:
    print('Minimum among ' + str(x) + ', ' + str(y) + ', '+ str(z) + ' is ' + str(y))
else:
    print('Minimum among ' + str(x) + ', ' + str(y) + ', '+ str(z) + ' is ' + str(z))

Analysis of the above code:

• The first statement itself is a mess here, what is does is, reads a string from input and tries to split them by space and then convert each of the parts into an integer type. If the conversion of all of them is successful, it assigns each of them to x, y, and z respectively.
• We check for two conditions in the if clause, and then print a human readable string by joining several strings and numbers. Since x, y, and z were integers, we first need to convert them into str objects to be able to join them
• elif and else are quite understandable

Next, we will discuss how to perform repetitive tasks by creating a loop in Python.

Note: This is a part of what I learned in an online Open Course Ware offered by MIT on edX. Its for my personal reference & also for those who would like to revisit the course.