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

Checkpoint: Example: Livestream features

Concepts in Practice \(\PageIndex{1}\): Using elif

Fill in the blank to execute Body 2 when condition_1 is false and condition_2 is true.

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

1. if
2. elif
3. else

Answer

b. elif will evaluate condition_2 if condition_1 is False and execute Body 2 if condition_2 is True .

Concepts in Practice \(\PageIndex{2}\): Using elif

Given x = 42 and y = 0, what is the final value of y?

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

1. 2
2. 5
3. 7

Answer

b. x > 44, 42 > 44, is False , so x < 50 is evaluated. 42 < 50 is True , so y = 0 + 5. Only one of the if and elif branches is taken.

Concepts in Practice \(\PageIndex{3}\): Using elif

Which conditions complete the code such that if x is less than 0, Body 1 executes, else if x equals 0, Body 2 executes.

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

1. x < 0
x == 0
2. x == 0
x < 0
3. x <= 0
   [no condition]

Answer

a. If x < 0, Body 1 executes. Else if x == 0, Body 2 executes.

Concepts in Practice \(\PageIndex{4}\): Using elif

Which of the following is a valid chained decision statement?

1. if condition_1:
     # Body 1
   elif condition_2:
     # Body 2
   

2. if condition_1:
     # Body 1
     elif condition_2:
       # Body 2
   

3. elif condition_1:
     # Body 1
   if condition_2:
     # Body 2

Answer

a. The elif must be chained to a preceding statement and at the same indentation level.

Concepts in Practice \(\PageIndex{5}\): Using elif

Given attendees = 350, what is the final value of rooms?

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

1. 4
2. 15
3. 18

Answer

c. The if and if-elif statements are not chained. The first if evaluates to True and executes. Then the if-elif is evaluated and the elif branch executes.

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).

Checkpoint: Example: Possible chess moves

Concepts in Practice \(\PageIndex{6}\): Using elif within if-elif-else statements

Given hour = 12, what is printed?

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. Good morning
2. Lunchtime
3. Good afternoon
4. Too late

Answer

b. The third branch executes if hour < 8 and hour < 12 is False and hour < 13 is True .

Concepts in Practice \(\PageIndex{7}\): Using elif within if-elif-else statements

Where can an elif statement be added?

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

1. 1
2. 2
3. 3
4. 4

Answer

c. An elif can be chained to another elif to form a longer chained decision.

Concepts in Practice \(\PageIndex{8}\): Using elif within if-elif-else statements

Given x = -1 and y = -2, what is the final value of y?

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

1. 10
2. 20
3. 30

Answer

a. Only one branch is executed in a chained decision statement. -1 < 0 and -2 < 0 is True , so y = 10.

Concepts in Practice \(\PageIndex{9}\): Using elif within if-elif-else statements

How could the following statements be rewritten as a chained statement?

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

1. if price < 9.99:
     order = 50
   else:
     order = 30
   order = 10
   

2. if price < 9.99:
     order = 50
   elif price < 19.99:
     order = 30
   elif price == 19.99:
     order = 10
   

3. if price < 9.99:
     order = 50
   elif price < 19.99:
     order = 30
   else:
     order = 10

Answer

c. The first branch executes if price < 9.99. The second branch executes if price < 19.99 and the first condition fails: that is, price >= 9.99. The third branch executes if the first and second conditions fail: that is, price >= 19.99. Chaining can simplify decision statements.

Try It: Crochet hook size conversion

Write a program that reads in a crochet hook's US size and computes the metric diameter in millimeters. (A subset of sizes is used.) If the input does not match B-G, the diameter should be assigned with -1.0. Ex: If the input is D, the output is "3.25 mm".

Size conversions for US size: mm

• B : 2.25
• C : 2.75
• D : 3.25
• E : 3.5
• F : 3.75
• G : 4.0

Interactive Code

# Read in hook and compute diameter in mm 

print(diameter, "mm")

Input

F

Answer

hook = input()
diameter = 0

if hook == "B":
diameter = 2.25
elif hook == "C":
diameter = 2.75
elif hook == "D":
diameter = 3.25
elif hook == "E":
diameter = 3.5
elif hook == "F":
diameter = 3.75
elif hook == "G":
diameter = 4.0
else:
diameter = -1.0

print(diameter, "mm")

Try It: Color wavelengths

Write a program that reads in an integer representing a visible light wavelength in nanometers. Print the corresponding color using the following inclusive ranges:

• Violet: 380–449
• Blue: 450–484
• Cyan: 485–499
• Green: 500–564
• Yellow: 565–589
• Orange: 590–624
• Red: 625–750

Assume the input is within the visible light spectrum, 380-750 inclusive.

Given input:

    550

The output is:

    Green

Interactive Code

Input

550

Answer

color = int(input())

if color < 450:
print('Violet')
elif color < 485:
print('Blue')
elif color < 500:
print('Cyan')
elif color < 565:
print('Green')
elif color < 590:
print('Yellow')
elif color < 625:
print('Orange')
else:
print('Red')