Lesson – While Loops & Loop Control in Python

Objective

By the end of this lesson, students will be able to:

• Understand how while loops work.

• Use break and continue to control loops.

• Write programs that loop until a condition is met.

• Avoid infinite loops.

1. What is a While Loop?

A while loop repeats a block of code as long as a condition is True.
This is different from a for loop (which loops over a sequence or a set number of times).

Syntax:

while <condition>:
    <code to run>

• The <condition> is checked before each iteration.

• If the condition is True, the loop runs.

• If the condition is False, the loop stops.

Example – Counting Down

count = 5
while count > 0:
    print("Counting down:", count)
    count -= 1  # decreases count
print("Blast off!")

Output:

Counting down: 5
Counting down: 4
Counting down: 3
Counting down: 2
Counting down: 1
Blast off!

Key points:

• The loop stops when count > 0 becomes False.

• If we forget count -= 1, the loop never ends (infinite loop!).

2. Loop Control Statements

Python gives us two important tools to control loops:

break – Exit the loop completely

while True:
    text = input("Type 'quit' to exit: ")
    if text == "quit":
        print("Goodbye!")
        break
    print("You typed:", text)

• break jumps out of the loop, no more iterations.

continue – Skip the rest of the current loop iteration

n = 0
while n < 10:
    n += 1
    if n % 2 == 0:   # if even, skip
        continue
    print(n)

Output:

1
3
5
7
9

• Even numbers are skipped.

• The loop still continues for other numbers.

3. Combining while, break, and continue

Example – Stop when total > 100

total = 0
num = 1
while True:
    total += num
    if total > 100:
        print("Reached above 100. Stopping.")
        break
    num += 1
print("Final total:", total)

Example – Input validation

while True:
    val = int(input("Enter an odd number: "))
    if val % 2 == 0:
        print("That's even! Try again.")
        continue
    break
print("You entered an odd number:", val)

• Keeps asking until a valid odd number is entered.

4. Common Mistakes to Avoid

❌ Forgetting to update loop variables → infinite loop.
❌ Using while True without a break.
❌ Writing conditions that can never be False.

5. Mini-Exercises

1- Basic While Loop
Write a loop to print numbers 1 to 5. (Hint: You can do this similarly to the for loop examples, just make sure to increment your counter or you’ll loop infinitely.)

2- Break Example
Modify it to stop if the number is 3. (so it would only print 1, 2 and then stop).

3-Number Guessing Game

• Create a number-guessing mini-game: the program has a secret number (say 7). Use a while loop to repeatedly ask the user to guess the number. If they guess correctly, break and congratulate them. If they guess too high or too low, print a hint and continue. Use input() to get user guesses inside the loop. (Be sure to convert input to int since it input() returns a string.)

6. How to Run the Code

1. Open your Python file (example: lesson_while.py) in your IDE.

2. Copy and paste one of the examples.

3. Save the file.

4. Run:

   python lesson_while.py
   

5. Watch the output and modify the code to test different conditions.