Nested conditional statements

Recall that an ordinary if-else statement has this syntax:

if <condition>:
    # Indented statement block
else:
    # Indented statement block

It is possible for the indented statement blocks themselves to include another if or if-else statement. This is called nesting, and it is both useful and common. Nesting might look like this (another if/else within the outer if block):

if <condition 1>:
    if <condition 2>:
        # Indented statement block
    else:
        # Indented statement block
else:
    # Indented statement block

or like this (another if/else within each of the outer if and the outer else blocks):

if <condition 1>:
    if <condition 2>:
        # Indented statement block
    else:
        # Indented statement block
else:
    if <condition 3>:
        # Indented statement block
    else:
        # Indented statement block

or even like this: (an if/else within the outer if, then another if/else within the inner if)

if <condition 1>:
    if <condition 2>:
        if <condition 3>:
            # Indented statement block
        else:
            # Indented statement block
    else:
        # Indented statement block
else:
    # Indented statement block

Notice that the last two cases each have three if conditions, but they are very different from each other. The last one has three levels of nesting, while the previous one has only two levels. In python, we always pay close attention to the indentation, because it tells us which inner block is part of which outer block. That placement controls the logical flow of the program. An inner block only gets executed if the condition for its outer block evaluates to True.

Examples of nested conditionals

CodeOutputNotes 
x = 5
y = 7
z = 10

if x < y:
    print("A")
    if z >= 20:
        print("B")
    else:
        print("C")
else:
    print("D")

A
C

The condition for the outer if is True (x < y), so the outer if block is executed and "A" is output. Next, the inner condition is checked. But z >= 20 is False, so the else block of the inner if-else is executed, and "C" is output.

 
x = 5
y = 7
z = 50

if x < y:
    print("A")
    if z >= 20:
        print("B")
    else:
        print("C")
else:
    print("D")

A
B

The condition for the outer if is True (x < y), so the outer if block is executed and "A" is output. Next, the inner condition is checked. Now z >= 20 is True, so the if block of the inner if-else is executed, and "B" is output.

 
x = 5
y = 1
z = 50

if x < y:
    print("A")
    if z >= 20:
        print("B")
    else:
        print("C")
else:
    print("D")

D

The condition for the outer if is False, so the outer else block is executed and "D" is output.

 
x = 5
y = 1
z = 50

if x < y:
    print("A")
    if z >= 20:
        print("B")
    else:
        print("C")
else:
    print("D")
    if x % 2 == 0:
        print("E")
    else:
        print("F")

D
F

The condition for the outer if is False, so the outer else block is executed and "D" is output. Next, the inner condition is checked. And x % 2 == 0 is False (since x is odd), so the else block of the inner if-else is executed, and "F" is output.

Exercise: For the last code block in the above examples, can you come up with initial values for x, y, and z so that the output is D E? Can you come up with initial values for x, y, and z so that the output is A F?

Putting it all together

The code below on the left checks if the user can vote in an upcoming election. It checks that the user is a US citizen, is 18 or over, and has registered to vote. The flowchart on the right shows the flow of control through the program, as it checks the three conditions.

Code Flowchart 
"""
A simple program that determines user eligibiity to vote
Author: COMP 1351 Instructor
Filename: voting.py
Date:
Course: COMP 1351
Assignment: Preview of nested conditional statements
Collaborators: 1351 Instructors
Internet Sources: None
"""
# Check citizenship requirement
is_citizen = input("Are you a US citizen? (Enter Y or N) ")
if is_citizen == 'Y':
    # Check age requirement
    age = int(input("How old are you? "))
    if age >= 18:
        # Check registration requirement
        is_registered = input("Are you registered to vote? (Enter Y or N) ")
        if is_registered == 'Y':
            print("You can go ahead and vote.")
        else:
            # Cannot vote because unregistered
            print("You must be registered to vote.")
    else:
        # Cannot vote due to age
        print("You must be 18 years or older to vote.")
else:
    # cannot vote due to citizenship
    print("You must be a US citizen to vote.")
Flowchart for the voting program

Key points:

  • Notice that this program has three levels of nesting.
  • Notice that if the user enters N to the first question, then no further questions are asked. Similarly, if they are too young, they are not asked the last question. This is accomplished using the structure of the if-else nesting.
  • The indentation is vital. Look at the code above and carefully notice how each if lines up directly above the else that pairs with it.
  • Copy/paste the above program into VSCode and run it. To test completely, you have to try all possible combinations of inputs, so that every possible line of code in the program gets tested. This means running the program multiple times and entering multiple possibilities at each input line. For example, what happens if the user enters P instead of Y or N? Why?