Unit 11.1A · Term 1

While Loop & Flowcharts

A while loop repeats a block of code while a condition remains True. It is especially useful when the number of repetitions is not known in advance. Flowcharts help us design the loop before translating the algorithm into Python.

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

While / Iteration

Craig'n'Dave — iteration in the three basic programming constructs. The iteration section begins at about 03:39.

Conceptual Anchor

The Guard at the Door

A while loop is like a guard checking a condition before each entry. If the condition is True, you enter the loop body. When the body finishes, you return to the guard. If the condition becomes False, you leave the loop.

1. Check condition

2. True → enter body

3. State changes

4. Return and check again

5. False → leave loop

Initialise count = 1

→

Check count <= 5?

→

Body print(count)

→

Update count += 1

After the update, execution returns to the condition. If the condition is False, the loop ends.

Anatomy of a While Loop

count = 1

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

1. Initialise count = 1

Create the starting state before the first condition check.

2. Condition count <= 5

The body runs only when this expression is True.

3. Body print(count)

The statements repeated during each iteration.

4. Update count += 1

Changes state so the condition can eventually become False.

More precise rule

Something relevant to the condition must eventually change, otherwise the loop may never terminate.

A while loop can run zero times

Initial value x = 10

The variable starts at 10.

First check x < 5

10 < 5 is False.

Body print(x)

Skipped because the first check is already False.

Output nothing

The body executes zero times.

x = 10

while x < 5:
    print(x)

# No output

While Loop Tracer

Notice that the condition is checked one more time after the final successful iteration.

Check	count before	count <= 5	Output	count after update

Change the Loop

Start value Limit Step

Generated loop

Output

Summary

While Loop Flowcharts

START

Oval Start / End

INPUT

Parallelogram Input / Output

PROCESS

Rectangle Assignment / calculation

Diamond Condition / decision

Arrow Flow direction

Flowchart Shape	Meaning	Python Equivalent
Oval	Start / End	Beginning / end of algorithm
Parallelogram	Input / Output	`input()` / `print()`
Rectangle	Process	Assignment / calculation
Diamond	Decision	`while condition:`
Arrow	Flow direction	Sequence / loop-back path

count = 1

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

# Output:
# 1
# 2
# 3
# 4
# 5

The loop-back arrow is the key

The Yes/True path enters the body and eventually returns to the same decision diamond. The No/False path leaves the loop.

Flowchart Path Tracer

Common While-Loop Patterns

Choose a while-loop pattern:

Loop Simulators

Sentinel Accumulator Simulator

Values separated by commas — use 0 to stop

Input Validation Simulator

Enter age

When Should I Use while?

Unknown repetitions

You do not know in advance how many iterations will be needed.

Condition-controlled

Repetition continues while a Boolean condition remains True.

User-controlled stop

Input determines when the loop finishes.

Validation

Repeat until acceptable input is received.

Preview of the next lesson

When the number of repetitions is known in advance, a for loop is often more suitable.

Design a while loop with three questions

1. Starting state?

What values must exist before the first condition check?

2. Continue while what?

Write the Boolean condition controlling repetition.

3. What changes?

Which state changes after each iteration?

4. How does it stop?

Identify the state that makes the condition False.

Debugging While Loops

Infinite Loop Detector

Follow how the condition changes after each iteration.

Off-by-One Lab

count = 0

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

Output: 0, 1, 2, 3, 4 → 5 iterations.

count = 0

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

Output: 0, 1, 2, 3, 4, 5 → 6 iterations.

count = 0; while count <= 5: count += 1

Predict before choosing.

Trace Table Builder

x = 20

while x > 2:
    x = x // 2
    print(x)

Iteration	x before	x after `// 2`	Output

Fill in every cell, then check your trace.

Worked Examples

Choose an example:

Extension: break

break immediately exits the nearest loop, even when the loop condition is still True.

while condition:
    # repeated statements

    if something:
        break

# execution continues here

Flowchart → Code Challenge

Choose the Python code that matches the algorithm.

While Error Lab

Identify the most important issue.

Pitfalls & Common Errors

Infinite Loop

If the condition never becomes False, the loop continues indefinitely. Changing a variable is not enough — it must change in a direction that can eventually make the condition False.

Off-by-One Error

< and <= often produce different iteration counts. Trace the first and last successful checks.

Sentinel Not Updated

When a sentinel controls the loop, a new value must be obtained inside the loop body. Otherwise the condition keeps testing the same old value.

Update Moves the Wrong Way

x += 1 is wrong if the loop needs x to decrease toward zero. Always compare the update with the stopping condition.

Invalid Starting State

The condition is checked before the first iteration. A starting value may cause the body to run zero times.

Numeric Input Still Needs Conversion

input() returns str. Convert with int() or float() when the loop compares numeric values.

Pro-Tips for Exams

Loop Tracing Strategy

Write a trace table with a column for every changing variable and the output.
Check the condition before every iteration, including the first.
Remember that one final False check happens after the last successful iteration.
For flowchart → code, map the decision diamond to the while condition.
Follow the loop-back arrow carefully.
Ask: what changes, and how will that eventually make the condition False?

Graded Tasks

Remember

State the four parts of a common counter-controlled while loop: initialisation, condition, body and update.

Understand

Explain how a while loop can execute zero times.

Understand

Identify why the following is infinite: x = 10; while x > 0: x += 1.

Apply

Write a countdown using while that prints 10 down to 1 and then prints "Launch!".

Apply

Read integers until the user enters 0. Accumulate and output the total of all non-zero values entered.

Apply

Convert a flowchart with the structure Initialise → condition diamond → process → update → loop back into valid Python code.

Analyze

Trace: x = 100; while x > 1: x = x // 2; print(x). Produce a complete trace table and exact output.

Create

Create a number guessing or input-validation program in which while repeats until a correct/valid value is entered.

Challenge: Collatz sequence

Start with a positive integer. While it is greater than 1: if even divide by 2; if odd multiply by 3 and add 1. Print every value and count the steps.

Exam Practice

Q1: Write the exact output. [3]

x = 10

while x > 1:
    x = x // 2
    print(x)

Reveal marking points

5 2 1

Q2: Explain why this loop never terminates. [2]

x = 1

while x <= 5:
    print(x)

Reveal marking points

x never changes inside the loop. Therefore x remains 1 and the condition x <= 5 remains True.

Q3: Write Python code for this algorithm. [4]
Set total to 0 → input number → while number is not 0 → add number to total → input next number → output total.

Reveal marking points

total = 0 number = int(input("Number: ")) while number != 0: total += number number = int(input("Number: ")) print(total)

Q4: How many times does the body execute? [1]
i = 0; while i <= 5: i += 1

Reveal marking point

6 times.

Self-Check Quiz

Eight quick checks Click each answer to reveal

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

Click to reveal answer

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

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3. What commonly causes an infinite loop?

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4. What does break do?

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5. Which flowchart shape represents the loop condition?

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6. With count = 0, what is the difference between count < 5 and count <= 5?

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7. Why must a sentinel value usually be updated inside the loop?

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8. Where does execution go after the loop body finishes?

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