Python Looping Constructs: For, While, and Loop Control Statements

Python Looping Constructs: For, While, and Loop Control Statements

Loops are essential for automating repetitive tasks in Python. In this guide, you’ll learn how to use for and while loops, control flow with break, continue, and pass, and apply these concepts to real-world scenarios.

For Loops with range()

The range() function generates sequences for iterating with for loops. It has three forms:

1. range(stop)

for i in range(5):
    print(i)  # Output: 0, 1, 2, 3, 4

2. range(start, stop)

for i in range(2, 6):
    print(i)  # Output: 2, 3, 4, 5

3. range(start, stop, step)

for i in range(0, 10, 2):
    print(i)  # Output: 0, 2, 4, 6, 8

While Loops

while loops repeat code as long as a condition is True:

count = 0
while count < 3:
    print("Count is", count)
    count += 1  # Output: 0, 1, 2

Loop Control Statements

1. break: Exit the Loop Immediately

for num in [1, 2, 3, 4, 5]:
    if num == 3:
        break  # Stops the loop
    print(num)  # Output: 1, 2

2. continue: Skip to the Next Iteration

for num in range(5):
    if num % 2 == 0:
        continue  # Skip even numbers
    print(num)  # Output: 1, 3

3. pass: Placeholder for Future Code

for num in range(3):
    pass  # Do nothing (used as a placeholder)

When to Use Loop Control

• break: Stop early when a condition is met (e.g., finding the first match).
• continue: Skip unwanted iterations (e.g., invalid inputs).
• pass: Temporarily fill empty code blocks during development.

Practice Work

Exercise 1: Fix the Infinite Loop

Correct this while loop to avoid running infinitely:

count = 0
while count < 5:
    print(count)

count = 0
while count < 5:
    print(count)
    count += 1

Exercise 2: Use continue

Modify this loop to skip numbers divisible by 3:

for num in range(10):
    print(num)

for num in range(10):
    if num % 3 == 0:
        continue
    print(num)

Exercise 3: Use break

Stop the loop when the number 5 is found:

numbers = [1, 3, 5, 7, 9]
for num in numbers:
    print(num)

numbers = [1, 3, 5, 7, 9]
for num in numbers:
    if num == 5:
        break
    print(num)

Workshop: Real-World Applications

Workshop 1: Number Guessing Game

Use a while loop to let the user guess a secret number (e.g., 7).

secret = 7
while True:
    guess = int(input("Guess a number: "))
    if guess == secret:
        print("Correct!")
        break
    print("Try again.")

Workshop 2: Filter Invalid Data

Use continue to skip negative numbers in a list:

data = [5, -2, 10, -8, 3]
for num in data:
    # Your code here
    print(num)

data = [5, -2, 10, -8, 3]
for num in data:
    if num < 0:
        continue
    print(num)

Workshop 3: Password Attempt Limiter

Allow 3 login attempts using a while loop and break:

attempts = 0
correct_password = "admin123"

while attempts < 3:
    password = input("Enter password: ")
    if password == correct_password:
        print("Access granted!")
        break
    attempts += 1
else:
    print("Too many failed attempts.")

Best Practices

• Avoid Infinite Loops: Ensure while loops have an exit condition.
• Prefer for over while: Use for when iterating over known sequences.
• Use Descriptive Variable Names: e.g., attempts instead of n.

Conclusion

Mastering loops and control statements unlocks the power of automation in Python. Practice with the exercises and workshops to build confidence in using for, while, break, and continue effectively.

Next Steps: Explore Python List Comprehensions or Python Functions.