While Loops

Why read this? The real power of computers is repetition — doing something millions of times without getting bored. While loops let you repeat code as long as a condition is true. They're the foundation of everything from calculating compound interest to processing every line of a file.

🎯 Learning Objectives

1. Write while loops that repeat code based on a condition
2. Avoid and fix infinite loops
3. Use accumulator patterns (sum, product, count)
4. Implement sentinel-controlled loops (repeat until a signal value)
5. Choose between while and for loops

📋 Prerequisites

• Conditionals — if/elif/else — Loop conditions use boolean expressions.
• Variables & Data Types — Updating variables inside loops.

📖 Core Content

9.1 What Problem Do While Loops Solve?

9.2 The while Loop — Basic Structure

python
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# runnable
count = 1
while count <= 5:
    print("Count:", count)
    count += 1  # CRITICAL: update the variable!
print("Loop ended!")

pseudo
Count: 1
Count: 2
Count: 3
Count: 4
Count: 5
Loop ended!

1. Check count <= 5 → True, enter loop
2. Print count (1), then count += 1 (count becomes 2)
3. Back to top, check again: 2 <= 5 → True
4. ... repeat until count becomes 6
5. 6 <= 5 → False, exit loop Diagram Rendering diagram

9.3 The Infinite Loop — and How to Avoid It

python
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# runnable - CAUTION: This would run forever!
# count = 1
# while count <= 5:
#     print("Hello")   # count NEVER changes!
# Run with caution - press Ctrl+C to stop
# Instead, always ensure the condition can become False:
count = 1
while count <= 5:
    print("Hello", count)
    count += 1  # This line prevents infinity

• Ctrl+C (or ⌘+C on Mac) interrupts the program
• Close the terminal window
• In Repl.it, click the Stop button

9.4 Sentinel-Controlled Loops

python
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# runnable
total = 0
count = 0
print("Enter numbers to sum. Enter -1 to stop.")
num = float(input("Enter a number: "))
while num != -1:          # -1 is the sentinel
    total += num
    count += 1
    num = float(input("Enter a number: "))
if count > 0:
    print(f"Total: {total}")
    print(f"Average: {total/count:.2f}")
else:
    print("No numbers entered.")

9.5 Worked Example 1: Factorial

python
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# runnable
n = int(input("Enter n: "))
factorial = 1
i = 1
while i <= n:
    factorial *= i  # same as: factorial = factorial * i
    i += 1
print(f"{n}! = {factorial}")

pseudo
Enter n: 5
5! = 120

9.6 Worked Example 2: Sum of Digits

python
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# runnable
num = int(input("Enter a number: "))
original = num
sum_digits = 0
while num > 0:
    digit = num % 10      # get last digit
    sum_digits += digit
    num //= 10            # remove last digit
print(f"Sum of digits of {original} = {sum_digits}")

pseudo
Enter a number: 1234
Sum of digits of 1234 = 10

9.7 Worked Example 3: Reverse a Number

python
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# runnable
num = int(input("Enter a number: "))
original = num
reversed_num = 0
while num > 0:
    digit = num % 10
    reversed_num = reversed_num * 10 + digit
    num //= 10
print(f"Reverse of {original} is {reversed_num}")

pseudo
Enter a number: 1234
Reverse of 1234 is 4321

9.8 Worked Example 4: Fibonacci Sequence

python
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# runnable
n = int(input("How many Fibonacci terms? "))
a, b = 0, 1
count = 0
while count < n:
    print(a, end=" ")
    a, b = b, a + b
    count += 1
print()

pseudo
Enter how many Fibonacci terms? 10
0 1 1 2 3 5 8 13 21 34

9.9 Worked Example 5: Collatz Conjecture

python
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# runnable
n = int(input("Enter a positive integer: "))
steps = 0
while n != 1:
    print(n, end=" → ")
    if n % 2 == 0:
        n //= 2
    else:
        n = n * 3 + 1
    steps += 1
print("1")
print(f"Reached 1 in {steps} steps.")

pseudo
Enter a positive integer: 6
6 → 3 → 10 → 5 → 16 → 8 → 4 → 2 → 1
Reached 1 in 8 steps.

📐 Key Concepts Reference

⚠️ Common Pitfalls

Pitfall 1: Infinite Loop (Forgot to Update)

python
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i = 1
while i <= 10:
    print(i)
    # forgot: i += 1

Pitfall 2: Off-by-One Errors

Pitfall 3: Sentinel Loop — Forgetting to Re-Read

python
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num = float(input("Enter number: "))
while num != -1:
    total += num
    # forgot to read next num — infinite loop!

Pitfall 4: Using = Instead of == in Condition

📝 Practice Questions

Q1: How many times does this loop print?

python
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i = 0
while i < 5:
    print(i)
    i += 1

Answer: 5 times (i = 0, 1, 2, 3, 4). When i becomes 5, 5 < 5 is False, so loop exits. Q2: What's the output?

python
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x = 10
while x > 0:
    x -= 3
print(x)

Answer:

pseudo
-2

Trace: x=10 → 7 → 4 → 1 → -2. When x = -2, -2 > 0 is False, exit. Print -2. Q3: Write a loop that prints all even numbers from 2 to 20.

Answer:

python
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# runnable
i = 2
while i <= 20:
    print(i, end=" ")
    i += 2

Output: 2 4 6 8 10 12 14 16 18 20 Q4: What does this program do?

python
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n = 12345
count = 0
while n > 0:
    n //= 10
    count += 1
print(count)

Answer: It counts the number of digits. Output: 5 Q5: Write a loop that computes the sum: 1² + 2² + 3² + ... + n².

Answer:

python
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# runnable
n = int(input("Enter n: "))
i = 1
total = 0
while i <= n:
    total += i ** 2
    i += 1
print(f"Sum of squares: {total}")

Q6: What's wrong with this code?

python
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x = 5
while x > 0:
    print(x)

Answer: Infinite loop! x never changes. It will print 5 forever. Fix: add x -= 1 inside the loop. Q7: Write a program that keeps asking for a password until the user enters "secret".

Answer:

python
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# runnable
password = input("Enter password: ")
while password != "secret":
    print("Wrong! Try again.")
    password = input("Enter password: ")
print("Access granted!")

Q8: What's the output of this trace?

python
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a, b = 0, 1
count = 0
while count < 5:
    print(a, end=" ")
    a, b = b, a + b
    count += 1

Answer:

pseudo
0 1 1 2 3

Fibonacci sequence: F₀=0, F₁=1, F₂=1, F₃=2, F₄=3 Q9: Write code to find the GCD of two numbers using Euclid's algorithm.

Answer:

python
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# runnable
a = int(input("Enter a: "))
b = int(input("Enter b: "))
x, y = a, b
while y != 0:
    x, y = y, x % y
print(f"GCD({a}, {b}) = {x}")

Q10: Write a loop that prints the multiplication table for a given number.

Answer:

python
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# runnable
n = int(input("Enter number: "))
i = 1
while i <= 10:
    print(f"{n} × {i} = {n * i}")
    i += 1

🔗 Cross-References

• Next Topic: For Loops & range()
• Previous Topic: Modules & Import
• BSCS1001 Computational Thinking: Loops are the "iteration" construct in algorithmic thinking.
• Reference: Python for Everybody, Chapter 5 — "Iteration"
• Video: L26: Introduction to while loop, L31: Tutorial on while loop Join Discord Previous8. Modules & ImportNext10. For Loops & range()