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

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")

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

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