Unit 11.1A · Term 1

While Loop & Flowcharts

The while loop repeats a block of code as long as a condition remains True. It is ideal when you don't know in advance how many times the loop should run. Flowcharts provide a visual way to design and understand loop algorithms before coding.

Learning Objectives

11.1.2.1 Write program code using a While loop
11.1.2.2 Implement a loop algorithm according to a flowchart

Lesson Presentation

11.1A-lesson-05-while-loop.pdf · Slides for classroom use

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Conceptual Anchor

The Guard at the Door

A while loop is like a guard checking a condition before letting you through the door again. Each time you finish the loop body, you return to the guard. If the condition is still True, you go through again. If it's False, the guard stops you and the program continues past the loop.

Rules & Theory

While Loop Syntax

# Basic pattern
while condition:
    # body — runs while condition is True
    # MUST modify something to eventually make condition False

# Simple counting
count = 1
while count <= 5:
    print(count)
    count += 1     # Without this → infinite loop!
# Output: 1 2 3 4 5 (each on a new line)

Flowchart Structure

Flowchart Shape	Meaning	Python Equivalent
Oval	Start / End	Beginning / End of program
Parallelogram	Input / Output	`input()` / `print()`
Rectangle	Process	Assignment, calculation
Diamond	Decision	`while condition:`
Arrow	Flow direction	Sequence of execution

While Loop Flowchart Pattern

In a flowchart, a while loop always has: (1) Initialize a variable, (2) Diamond: check condition, (3) Yes → process body → loop back to diamond, (4) No → continue to next step.

Common While Loop Patterns

# Pattern 1: Counter-controlled
count = 0
while count < 10:
    print(count)
    count += 1

# Pattern 2: Sentinel value (user says "stop")
text = input("Enter text (or 'quit'): ")
while text != "quit":
    print("You said:", text)
    text = input("Enter text (or 'quit'): ")

# Pattern 3: Accumulator
total = 0
num = int(input("Enter number (0 to stop): "))
while num != 0:
    total += num
    num = int(input("Enter number (0 to stop): "))
print("Total:", total)

# Pattern 4: Input validation
age = int(input("Enter age (1-120): "))
while age < 1 or age > 120:
    print("Invalid! Try again.")
    age = int(input("Enter age (1-120): "))

Worked Examples

1 Countdown Timer

count = 10
while count > 0:
    print(count)
    count -= 1
print("Launch!")

# Trace table:
# count | Output
# 10    | 10
# 9     | 9
# ...
# 1     | 1
# 0     | Launch!

2 Guessing Game (Flowchart Implementation)

Flowchart: Start → Set secret=7 → Input guess → Is guess == secret? → No: Too high/low → Input guess again → Yes: "Correct!" → End

secret = 7
guess = int(input("Guess the number (1-10): "))

while guess != secret:
    if guess > secret:
        print("Too high!")
    else:
        print("Too low!")
    guess = int(input("Try again: "))

print("Correct! 🎉")

3 Sum of Digits

# Extract and sum each digit of a number
num = int(input("Enter a number: "))  # e.g., 347
total = 0

while num > 0:
    digit = num % 10     # Get last digit
    total += digit       # Add to total
    num = num // 10      # Remove last digit

print("Sum of digits:", total)

# Trace for 347:
# num  | digit | total
# 347  | 7     | 7
# 34   | 4     | 11
# 3    | 3     | 14
# 0    | (stop)| 14

4 Password Checker with Attempt Limit

correct = "python123"
attempts = 3

while attempts > 0:
    pw = input("Enter password: ")
    if pw == correct:
        print("Access granted!")
        break
    else:
        attempts -= 1
        print(f"Wrong! {attempts} attempts left.")

if attempts == 0:
    print("Account locked!")

Pitfalls & Common Errors

Infinite Loop

If the condition never becomes False, the loop runs forever. Always ensure the loop variable changes inside the loop body. Press Ctrl+C to stop an infinite loop.

# BUG: count never changes!
count = 1
while count <= 5:
    print(count)
    # Missing: count += 1

Off-by-One Error

while count < 5 runs 5 times (0,1,2,3,4). while count <= 5 runs 6 times (0,1,2,3,4,5). Always trace the first and last iterations.

Sentinel Not Updated

If you use a sentinel value to exit, make sure the input is inside the loop, not just before it.

Pro-Tips for Exams

Loop Tracing Strategy

Always draw a trace table with columns for each variable + output
Check the condition BEFORE each iteration (including the first)
Count the total number of iterations carefully
For flowchart → code: map each diamond to a while condition, each rectangle to a statement

Graded Tasks

Remember

What are the three essential parts of every while loop?

Understand

Draw a flowchart for a program that sums numbers entered by the user until they enter 0.

Apply

Write a program that asks the user for a positive integer and prints its factorial using a while loop.

Apply

Implement the following flowchart: Input a number → While number > 1 → If even: divide by 2 → If odd: multiply by 3 and add 1 → Print number → Count steps.

Analyze

Trace the following code and write the output: x = 100; while x > 1: x = x // 2; print(x)

Create

Create a number guessing game where the computer picks a random number 1-100 and the user must guess it, receiving "higher" or "lower" hints. Count and display the number of attempts.

Self-Check Quiz

1. What happens if the while condition is False at the start?

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2. How many times does this loop run? i = 0; while i < 3: i += 1

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3. What causes an infinite loop?

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4. What does break do inside a while loop?

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5. What flowchart shape represents a loop condition?

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