Chapter 033 / 132 · Level 2 · Core Python

While loops and repetition

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Chapter 33 of 132 · Level 2 · Core Python · Python demonstrations with recorded outputs. Learn through reading, predicting and testing real Python code.

Code + real outputPractice + explanationsOffline Python exercises

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01 · Why this concept matters

while repeats while its condition is true. Update the state inside the loop, otherwise the condition may never become false.

Learning goal: While loops and repetition. Start by reading what the program does, then inspect its exact output. Where an example illustrates only a prerequisite, study the named advanced feature separately before using it in production.

02 · The important subtopics

Study these connected ideas as you work through the demonstration:

Loop conditions and termination
Counters sentinels and accumulators
break continue and else
Preventing infinite loops

03 · Pinpoint: read the code line by line

Line 1remaining = 3Create, update, or use a value. Notice what values enter this statement and which names it affects.
Line 2while remaining > 0:Control the next step. Notice what values enter this statement and which names it affects.
Line 3print('Try', remaining)Display a result. Notice what values enter this statement and which names it affects.
Line 4remaining -= 1Create, update, or use a value. Notice what values enter this statement and which names it affects.
Line 5print('Finished')Display a result. Notice what values enter this statement and which names it affects.

Input → operation → output

Find the literal input values and arguments, follow each operation in execution order, and verify the result below. This habit turns a code listing into a testable explanation.

04 · Runnable Python example and verified output

While loops and repetition: working demonstration

Python · runnable

PYTHON CODE

remaining = 3
while remaining > 0:
    print('Try', remaining)
    remaining -= 1
print('Finished')

Verified execution ▼

Try 3
Try 2
Try 1
Finished

Why it works: while repeats while its condition is true. Update the state inside the loop, otherwise the condition may never become false.

Worked example 2 · While Loops And Repetition in practice

Python · executed and checked

PYTHON CODE

balance = 5
while balance > 0:
    balance -= 2
    print('Left:', max(balance, 0))

Verified execution output

Left: 3
Left: 1
Left: 0

Pinpoint learning: This practical while loops and repetition demonstration uses a small input you can inspect. Run it exactly as shown, compare the displayed result, then change one value and predict how the result will change.

Understand the code line by line

1: balance = 5 — Store or update the value on the left using the expression on the right.
2: while balance > 0: — Repeat the indented block for each item or while the condition remains true.
3: balance -= 2 — Store or update the value on the left using the expression on the right.
4: print('Left:', max(balance, 0)) — Display the value so you can compare it with the verified output.

Worked example 3 · Change one input and compare

Python · executed and checked

PYTHON CODE

balance = 7
while balance > 0:
    balance -= 2
    print('Left:', max(balance, 0))

Verified execution output

Left: 5
Left: 3
Left: 1
Left: 0

Pinpoint learning: Exactly one literal input was changed from the preceding demonstration. Compare the outputs: identify the modified value, trace the operation, and explain which result changed. The displayed output was produced by executing this variant.

Understand the code line by line

1: balance = 7 — Store or update the value on the left using the expression on the right.
2: while balance > 0: — Repeat the indented block for each item or while the condition remains true.
3: balance -= 2 — Store or update the value on the left using the expression on the right.
4: print('Left:', max(balance, 0)) — Display the value so you can compare it with the verified output.

05 · Independent practice and answer guidance

Exercise A · Adapt the code

Replace the counter with five attempts; explain what would cause an infinite loop.

Exercise B · Predict

Before pressing Run, predict the first displayed line. Next, change one literal value and record the new observed output. Explain the difference in one clear sentence.

Exercise C · Debug

Make a temporary invalid change, read the exception or failed assertion if one occurs, restore the working code, and explain the repair.

Worked explanation for the original example

while repeats while its condition is true. Update the state inside the loop, otherwise the condition may never become false.

Try 3
Try 2
Try 1
Finished

06 · Build, verify and advance

Real-world task: Create a limited-attempt password practice simulator.

Mastery checklist

Run the original code; explain the important lines; change one input and verify the new outcome; use a test or recorded result to justify your conclusion.