4.5 Chained decisions

Learning objectives

By the end of this section you should be able to

• Identify the branches taken in an if-elif and if-elif-else statement.
• Create a chained decision statement to evaluate multiple conditions.

elif

Sometimes, a complicated decision is based on more than a single condition. Ex: A travel planning site reviews the layovers on an itinerary. If a layover is greater than 24 hours, the site should suggest accommodations. Else if the layover is less than one hour, the site should alert for a possible missed connection.

Two separate if statements do not guarantee that only one branch is taken and might result in both branches being taken. Ex: The program below attempts to add a curve based on the input test score. If the input is 60, both if statements are incorrectly executed, and the resulting score is 75.

    score = int(input())
    if score < 70:
      score += 10
    # Wrong:
    if 70 <= score < 85:
      score += 5
    

Chaining decision statements with elif allows the programmer to check for multiple conditions. An elif (short for else if) statement checks a condition when the prior decision statement's condition is false. An elif statement is part of a chain and must follow an if (or elif) statement.

if-elif statement template:

    # Statements before

    if condition:
      # Body
    elif condition:
      # Body
    
    # Statements after
    

if condition_1:
  # Body 1
__ condition_2:
  # Body 2

if x > 44:
  y += 2
elif x < 50:
  y += 5

if _________:
  # Body 1
elif _________:
  # Body 2

rooms = 1
if attendees >= 100:
  rooms += 3
if attendees <= 200:
  rooms += 7
elif attendees <= 400:
  rooms += 14

if-elif-else statements

Elifs can be chained with an if-else statement to create a more complex decision statement. Ex: A program shows possible chess moves depending on the piece type. If the piece is a pawn, show moving forward one (or two) places. Else if the piece is a bishop, show diagonal moves. Else if . . . (finish for the rest of the pieces).

if hour < 8:
  print("Too early")
elif hour < 12:
  print("Good morning")
elif hour < 13:
  print("Lunchtime")
elif hour < 17:
  print("Good afternoon")
else:
  print("Too late")

_1_
if condition:
  # Body
  _2_
elif condition:
  # Body
_3_
else:
  # Body
_4_

if x < 0 and y < 0:
  y = 10
elif x < 0 and y > 0:
  y = 20
else:
  y = 30

if price < 9.99:
  order = 50
if 9.99 <= price < 19.99:
  order = 30
if price >= 19.99:
  order = 10