Python Programming · Week 6

Loops

Step 1 of 5

Loops are powerful constructs in programming that allow you to repeat a block of code multiple times. They are essential for automating repetitive tasks and processing collections of data efficiently.

Key Concept: Loops

Loops help you avoid writing the same code multiple times, making your programs more efficient and easier to maintain.

In Python, there are two main types of loops:

While Loops: Execute a block of code as long as a condition is true.
For Loops: Iterate over a sequence (like a list, tuple, or string) or other iterable objects.

Comparing While and For Loops

Let's look at an example that demonstrates both types of loops achieving the same result:

# For loop
print("For loop:")
for x in range(6):
    print(x)

print("\nWhile loop:")
# While loop
y = 0
while y < 6:
    print(y)
    y += 1
            

Both loops print numbers from 0 to 5, but they do so in slightly different ways:

The for loop uses the range() function to generate a sequence of numbers.
The while loop uses a counter variable that we manually increment.

Guided Exercise: Modifying Loops

Now, let's practice by modifying these loops. Follow these steps:

Change both loops to print numbers from 1 to 5 instead of 0 to 5.
Add a print statement after each loop to separate their outputs.
For the while loop, add a condition to print "Halfway there!" when the counter reaches 3.

Hint: For the for loop, use range(1, 6). For the while loop, start y at 1 and change the condition to y <= 5. Use an if statement inside the while loop to check if y == 3.

Starter code

# Modify these loops to print numbers from 1 to 5

# For loop
print("For loop:")
for x in range(1, 6):
    print(x)

print("\nWhile loop:")
# While loop
y = 1
while y <= 5:
    print(y)
    if y == 3:
        print("Halfway there!")
    y += 1

print("\nBoth loops completed!")

Write your code here

While loops in Python continue to execute a block of code as long as a given condition is true. They are particularly useful when you don't know in advance how many times you need to repeat a block of code.

Key Concept: While Loops

While loops are ideal for situations where you need to repeat an action until a specific condition is met, rather than for a predetermined number of times.

While Loop Syntax:

while condition:
    # code to be executed
    if some_condition:
        break  # optional
            

Key points about while loops:

The condition is checked at the beginning of each iteration.
If the condition is initially false, the loop body will not execute at all.
You need to make sure the loop condition eventually becomes false, or you'll create an infinite loop.
You can use 'break' to exit the loop prematurely.
You can use 'continue' to skip the rest of the current iteration and move to the next one.

Example: Using break and continue

count = 0
while count < 5:
    count += 1
    if count == 2:
        print("Skipping 2")
        continue
    if count == 4:
        print("Found 4, breaking the loop")
        break
    print(f"Current count: {count}")
print("Loop ended")
            

Guided Exercise: Countdown Timer

Let's create a countdown timer using a while loop. Follow these steps:

Start with a counter set to 10.
Use a while loop to count down from 10 to 1.
Print the current counter value in each iteration.
When the counter reaches 5, print "Halfway there!"
After the loop ends, print "Countdown finished!"

Hint: Use a while loop with the condition 'counter > 0'. Don't forget to decrement the counter in each iteration!

Starter code

# Create your countdown timer here
counter = 10

while counter > 0:
    print(counter)
    if counter == 5:
        print("Halfway there!")
    counter -= 1

print("Countdown finished!")

Write your code here

For loops in Python are used to iterate over a sequence (such as a list, tuple, string, or range) or other iterable objects. They provide a concise way to repeat a block of code a specific number of times or for each item in a collection.

Key Concept: For Loops

For loops are excellent for iterating over known sequences or when you need to perform an action a specific number of times.

For Loop Syntax:

for item in iterable:
    # code to be executed for each item
            

The range() function is often used with for loops to generate a sequence of numbers. It can take up to three arguments:

range(stop): Generates numbers from 0 to stop-1
range(start, stop): Generates numbers from start to stop-1
range(start, stop, step): Generates numbers from start to stop-1, incrementing by step

Example: Different ways to use range()

# Using range with one argument
print("Range with one argument:")
for i in range(5):
    print(i)

# Using range with two arguments
print("\nRange with two arguments:")
for i in range(2, 7):
    print(i)

# Using range with three arguments
print("\nRange with three arguments:")
for i in range(1, 10, 2):
    print(i)
            

Guided Exercise: Multiplication Table

Let's create a multiplication table using nested for loops. Follow these steps:

Create an outer loop that iterates from 1 to 5.
Create an inner loop that also iterates from 1 to 5.
Inside the inner loop, multiply the outer loop variable by the inner loop variable.
Print the result of each multiplication.
Add a blank line after each row of the table for better readability.

Hint: Use two for loops with range(1, 6). Use print() with end=" " to print on the same line, and an empty print() to move to the next line.

Starter code

# Create your multiplication table here

for i in range(1, 6):
    for j in range(1, 6):
        print(f"{i * j:2d}", end=" ")
    print()  # Move to the next line after each row

Write your code here

Understanding how for loops work internally can help you use them more effectively. In Python, a for loop works by iterating over an iterable object (like a list, tuple, or range).

Key Concept: Iteration

Iteration is the process of going through each item in a sequence, one at a time. For loops automate this process, making it easy to perform actions on each item.

Here's what happens in each iteration of a for loop:

The next item from the iterable is assigned to the loop variable.
The code block inside the loop is executed.
Steps 1 and 2 are repeated until there are no more items in the iterable.

Example: Iterating over different data types

# Iterating over a list
fruits = ["apple", "banana", "cherry"]
print("Iterating over a list:")
for fruit in fruits:
    print(f"Current fruit: {fruit}")

# Iterating over a string
print("\nIterating over a string:")
for char in "Python":
    print(f"Current character: {char}")

# Iterating over a dictionary
print("\nIterating over a dictionary:")
person = {"name": "Alice", "age": 30, "city": "New York"}
for key, value in person.items():
    print(f"{key}: {value}")
            

Guided Exercise: Analyzing Student Scores

Let's create a program that analyzes student scores. Follow these steps:

Create a list of dictionaries, where each dictionary represents a student with 'name' and 'score' keys.
Use a for loop to iterate over this list.
Calculate and print the average score.
Find and print the name of the student with the highest score.
Count and print the number of students who scored above 80.

Hint: Use a list comprehension to extract all scores. Use the sum() and len() functions to calculate the average. Use max() with a key function to find the highest score.

Starter code

# Analyze student scores here
students = [
    {"name": "Alice", "score": 85},
    {"name": "Bob", "score": 92},
    {"name": "Charlie", "score": 78},
    {"name": "David", "score": 95},
    {"name": "Eve", "score": 88}
]

# Calculate average score
scores = [student["score"] for student in students]
average_score = sum(scores) / len(scores)
print(f"Average score: {average_score:.2f}")

# Find student with highest score
top_student = max(students, key=lambda x: x["score"])
print(f"Top student: {top_student['name']} with score {top_student['score']}")

# Count students scoring above 80
high_scorers = sum(1 for student in students if student["score"] > 80)
print(f"Number of students scoring above 80: {high_scorers}")

Write your code here

Now that we've learned about loops, let's put our knowledge into practice by creating a simple number guessing game. This project will help reinforce the concepts we've covered and show how loops can be used in a real-world scenario.

Project Goal

Create a game where the computer generates a random number, and the player tries to guess it. The game will provide hints and keep track of the number of attempts.

Project Requirements:

Your Number Guessing Game should:

Generate a random number between 1 and 100.
Ask the player to guess the number.
Provide feedback on whether the guess is too high, too low, or correct.
Keep track of the number of attempts.
Allow the player to play again if they want to.

Implementation Steps:

Import the random module to generate random numbers.
Create a function to generate a random number.
Use a while loop to keep the game running until the player guesses correctly.
Use conditional statements to provide feedback on the player's guess.
Implement a counter to keep track of the number of attempts.
Use another while loop to allow multiple games if the player wants to continue.

Hint: Use random.randint(1, 100) to generate a random number. Use input() to get the player's guess, and don't forget to convert it to an integer!

The template below provides a starting point for your Number Guessing Game. Fill in the functions and add any additional functionality you think would make the game more engaging!

Starter code

import random

def generate_number():
    return random.randint(1, 100)

def play_game():
    number_to_guess = generate_number()
    attempts = 0

    print("Welcome to the Number Guessing Game!")
    print("I'm thinking of a number between 1 and 100.")

    while True:
        guess = int(input("Enter your guess: "))
        attempts += 1

        if guess < number_to_guess:
            print("Too low! Try again.")
        elif guess > number_to_guess:
            print("Too high! Try again.")
        else:
            print(f"Congratulations! You guessed the number in {attempts} attempts!")
            return attempts

def main():
    while True:
        play_game()
        play_again = input("Do you want to play again? (yes/no): ").lower()
        if play_again != 'yes':
            print("Thanks for playing! Goodbye!")
            break

main()

Write your code here

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