Selection

Objectives

• To understanding selection in algorithms and code
• Define the purpose of selection (conditional statements) in programming.
• Learn the syntax of if, elif, and else statements in Python
• List and explain the relational operators used in conditions (e.g., ==, !=, >, <, >=, <=).
• Review examples of Conditional Statements
• Define the role of the elif statement in handling multiple conditions.
• Differentiate the use of elif from if and else.

Introduction

As part of our algorithm we will often encounter a moment when a decision needs to be taken, and depending on the outcome of that decision, either True or False, execute a different statement or set of statements. In Python, we use if, elif (else if), and else statements to implement selection. We also rely on the relational operators covered earlier i.e. ==, !=, > etc..

if True
    do something
else
    do this instead

Example 6

print("Welcome to the Climate Data Analyzer")

temperature = float(input("Enter the temperature in Celsius: "))

if temperature < 0:
    print("It's freezing cold!")
elif temperature >= 0 and temperature <= 20:
    print("The weather is cool.")
elif temperature > 20 and temperature <= 30:
    print("The weather is warm.")
else:
    print("It's hot outside!")

Predict what the output of the program will be for these inputs:

• "What will the program print if the temperature is -5?"
• "What will the program print if the temperature is 15?"
• "What will the program print if the temperature is 25?"
• "What will the program print if the temperature is 35?"

Run the program in a Python environment and compare the actual output with your predictions.

1. The if Statement

The if statement allows you to execute a block of code only if a specified condition is true.

if condition:
    # code to execute if condition is true

2. The elif Statement

The elif statement is used when you have multiple conditions to check. It is executed if the preceding if or elif conditions are false and its own condition is true.

if condition1:
    # code to execute if condition1 is true
elif condition2:
    # code to execute if condition2 is true

Note

It makes sense to read elif as "else if"

3. The else Statement

The else statement is used as a fallback option if none of the preceding conditions are true.

if condition:
    # code to execute if condition is true
else:
    # code to execute if condition is false

4. Conditions

The condition will always be some kind of Boolean expression that evaluates to either True or False and will use the relational operators we saw earlier:

1. Equal to (==): Checks if two values are equal.

2. Not equal to (!=): Checks if two values are not equal.

3. Greater than (>): Checks if one value is greater than another.

4. Less than (<): Checks if one value is less than another.

5. Greater than or equal to (>=): Checks if one value is greater than or equal to another.

6. Less than or equal to (<=): Checks if one value is less than or equal to another.

These conditions can be further combined using and, or and not to create complex conditional expressions.

Further Examples

Example 1: Basic if Statement

age = 16
if age >= 18:
    print("You can vote!")

Example 2: if-elif-else Statement

score = 85
if score >= 90:
    print("A")
elif score >= 80:
    print("B")
else:
    print("C")

Activity

Using the following program, make the changes suggested:

print("Welcome to the Advanced Climate Data Analyzer")

temperature = float(input("Enter the temperature in Celsius: "))
humidity = int(input("Enter the humidity percentage: "))

if temperature < 0:
    print("It's freezing cold!")
elif temperature >= 0 and temperature <= 20:
    if humidity > 80:
        print("The weather is cool and humid.")
    else:
        print("The weather is cool.")
elif temperature > 20 and temperature <= 30:
    if humidity > 60:
        print("The weather is warm and humid.")
    else:
        print("The weather is warm.")
else:
    if humidity > 40:
        print("It's hot and humid outside!")
    else:
        print("It's hot outside!")

• Add a new condition for temperatures above 35 degrees.
• Change the humidity thresholds and observe the different outputs.

Climate Quest Project

Throughout this topic we'll be working on a large scale project: Climate Quest. In this project a player embarks on a journey to combat the effects of climate change by making decisions that impact the environment. Each choice affects the outcome of the game, emphasizing the importance of individual actions in addressing climate change.

Go to task 6 - Selection

Summary

Cheat sheet for selection (and iteration)¹

Questions

1. What is the purpose of selection (conditional statements) in programming?

To execute statements in sequence

To repeat a block of code

To make decisions and execute different statements based on conditions

To define functions and classes

Conditional statements allow programs to make decisions and execute different code based on whether conditions evaluate to True or False.

Submit

2. What is the correct syntax for an if statement in Python?

if (condition) {}

if condition:

if condition then:

if: condition

The correct syntax for an if statement in Python is to use if condition: followed by an indented block of code.

Submit

3. Which relational operator checks if two values are not equal in Python?

==

!=

>

<

The != operator checks if two values are not equal.

Submit

4. What will the program output if the temperature is 25?

temperature = 25
if temperature < 0:
    print("It's freezing cold!")
elif temperature >= 0 and temperature <= 20:
    print("The weather is cool.")
elif temperature > 20 and temperature <= 30:
    print("The weather is warm.")
else:
    print("It's hot outside!")

"It's freezing cold!"

"The weather is cool."

"The weather is warm."

"It's hot outside!"

The temperature 25 falls within the range of 20 < temperature <= 30, so the program prints 'The weather is warm.'

Submit

5. What is the role of the elif statement in Python?

To execute a block of code if an initial if condition is false and its own condition is true

To execute a block of code regardless of any conditions

To terminate the program

To loop through a block of code multiple times

The elif statement is used to check additional conditions if the previous if or elif conditions were false.

Submit

6. Which of the following conditions will check if a value is both greater than 10 and less than 20?

value > 10 or value < 20

value > 10 and value < 20

value > 10 and value > 20

value < 10 and value < 20

The condition value > 10 and value < 20 correctly checks if a value is greater than 10 and less than 20.

Submit

7. What will the following code output if score = 85?

score = 85
if score >= 90:
    print("A")
elif score >= 80:
    print("B")
else:
    print("C")

"A"

"B"

"C"

"None"

The score 85 falls within the range of 80 <= score < 90, so the program will print 'B'.

Submit

8. Which of the following conditions will always evaluate to True?

5 > 10

10 == 10

20 < 15

8 != 8

10 == 10 is always true because the two values are equal.

Submit

9. What does the else statement do in Python?

It checks if a condition is true

It provides an alternative block of code to execute if the preceding if and elif conditions are false

It repeats a block of code multiple times

It defines a new function

The else statement in Python provides a fallback block of code that is executed if none of the preceding if or elif conditions are true.

Submit

10. What is the output of the following code if temperature = -5?

if temperature < 0:
    print("It's freezing cold!")
elif temperature >= 0 and temperature <= 20:
    print("The weather is cool.")
else:
    print("It's hot outside!")

"It's freezing cold!"

"The weather is cool."

"It's hot outside!"

"None"

Since the temperature is -5, which is less than 0, the program will print 'It's freezing cold!'.

Submit

Programming Tasks

For each of the following tasks write a program using Python.

Grade Calculator

Write a Python program that takes a test score as input and prints the corresponding letter grade ('A', 'B', 'C', 'D', or 'F') based on the following grading scale A: 90 or above, B: 80-89, C: 70-79, D: 60-69, F: Below 60

Read more ...

Leap Year Checker

Write a Python program that checks if a given year is a leap year. A leap year is divisible by 4 but not divisible by 100 unless it is divisible by 400. Print 'Leap year' or 'Not a leap year' accordingly. Use the following pseudocode to help with the logic:

Read more ...

Rock, Paper, Scissors

Write a Python program that lets a user play the game of Rock, Paper, Scissors against the computer. Implement the game logic and provide instructions for the user to input their choice.

Read more ...

BMI Calculator

Write a Python program to calculate the Body Mass Index BMI or a user given a height and a weight. The formula to use: bmi = weight / (height ** 2). Output the result 'Underweight' if the BMI < 18.5; 'Obese' if the BMI is > 30.5; 'Normal Weight' if the BMI is between 18.5 and < 25; 'Overweight' if the BMI is between 25 and 30.

Read more ...

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1. [https://ehmatthes.github.io/pcc_3e/cheat_sheets/(https://ehmatthes.github.io/pcc_3e/cheat_sheets/)] ↩