4.6: Nested Decisions

Learning Objectives

By the end of this section you should be able to

• Describe the execution paths of programs with nested if-else statements.
• Implement a program with nested if-else statements.

Nested decision statements

Suppose a programmer is writing a program that reads in a game ID and player count and prints whether the user has the right number of players for the game.

The programmer may start with:

    if game == 1 and players < 2:
      print("Not enough players")
    if game == 1 and players > 4:
      print("Too many players")
    if game == 1 and (2 <= players <= 4):
      print("Ready to start")
    if game == 2 and players < 3:
      print("Not enough players")
    if game == 2 and players > 6:
      print("Too many players")
    if game == 2 and (3 <= players <= 6):
      print("Ready to start")

The programmer realizes the code is redundant. What if the programmer could decide the game ID first and then make a decision about players? Nesting allows a decision statement to be inside another decision statement, and is indicated by an indentation level.

An improved program:

    if game == 1:
      if players < 2:
        print("Not enough players")
      elif players > 4:
        print("Too many players")
      else:
        print("Ready to start")
    if game == 2:
      if players < 3:
        print("Not enough players")
      elif players > 6:
        print("Too many players")
      else:
        print("Ready to start")
    # Test game IDs 3-end

Checkpoint: Example: Poisonous plant identification

Concepts in Practice \(\PageIndex{1}\): Using nested if-else statements

Consider the example above. Given leaf_count = 9 and leaf_shape = "teardrop", what is the output?

1. Might be poison ivy
2. Might be poison oak
3. Might be poison sumac

Answer

c. leaf_count == 3 is False , so the outer else ' s body is executed.

Concepts in Practice \(\PageIndex{2}\): Using nested if-else statements

Given num_dancers = 49, what is printed?

if num_dancers < 0:
  print("Error: num_dancers is negative")
else:
  if num_dancers % 2 == 1:
    print("Error: num_dancers is odd")
  print(num_dancers, "dancers")

1. Error: num_dancers is odd
2. 49 dancers
3. Error: num_dancers is odd
49 dancers

Answer

c. num_dancers is positive, so the else branch is taken. num_dancers is odd, so the error is printed before the program continues after the nested if and prints num_dancers.

Concepts in Practice \(\PageIndex{3}\): Using nested if-else statements

Given x = 256, y = 513, and max = 512, which of the following will execute?

if x == y:
  # Body 1
elif x < y:
  # Body 2
  if y >= max:
    # Body 3
  else:
    # Body 4
else:
  # Body 5

1. Body 2
2. Body 2, Body 3
3. Body 2, Body 5

Answer

b. 256 == 513 is False , and 256 < 513 is True , so the elif branch is taken and Body 2 executes. 513 >= 512 is True , so Body 3 executes.

Concepts in Practice \(\PageIndex{4}\): Using nested if-else statements

Given x =118, y = 300, and max = 512, which of the following will execute?

if x == y:
  # Body 1
elif x < y:
  # Body 2
  if y >= max:
    # Body 3
else:
  # Body 4
else:
  # Body 5

1. Body 2
2. Body 3
3. Error

Answer

c. The first else isn't indented and is treated as the else to the original if . Thus the second else is not connected to an if statement and produces an error.

Try It: Meal orders

Write a program that reads in a string, "lunch" or "dinner", representing the menu choice, and an integer, 1, 2, or 3, representing the user's meal choice. The program then prints the user's meal choice.

Lunch Meal Options

• 1: Caesar salad
• 2: Spicy chicken wrap
• 3: Butternut squash soup

Dinner Meal Options

• 1: Baked salmon
• 2: Turkey burger
• 3: Mushroom risotto

Ex: If the input is:
lunch
3

The output is:
Your order: Butternut squash soup

Interactive Code

Input

dinner
1

Answer

menu_choice = input()
meal_choice = int(input())
if menu_choice == "lunch":
if meal_choice == 1:
order = "Caesar salad"
elif meal_choice == 2:
order = "Spicy chicken wrap"
else:
order = "Butternut squash soup"
else:
if meal_choice == 1:
order = "Baked salmon"
elif meal_choice == 2:
order = "Turkey burger"
else:
order = "Mushroom risotto"

print("Your order:", order)