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4.5: Chained Decisions

  • Page ID
    117544
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    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: Using elif
    1.
    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
  • elif
  • else
  • 2.
    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
    3.
    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]
    4.
    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
      
    5.
    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

    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: Using elif within if-elif-else statements
    6.
    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
  • Lunchtime
  • Good afternoon
  • Too late
  • 7.
    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
    8.
    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
    9.
    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
      
    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
    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

    This page titled 4.5: Chained Decisions is shared under a CC BY 4.0 license and was authored, remixed, and/or curated by OpenStax via source content that was edited to the style and standards of the LibreTexts platform.