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while loop with a condition update

🧠 Predict the output Topic: loops Difficulty: medium Free

What this problem practises

Loops are where off-by-one bugs live. range() excludes its stop value, the loop variable keeps its last value even after the loop ends, and break versus continue changes the flow in ways that are easy to misread when you skim.

This is an output-prediction problem: read the snippet carefully and write down exactly what it prints before anything runs. These questions are interview staples because they test whether you know the language rules, or merely recognise their syntax.

Difficulty: medium — expect to hold two interacting rules in your head at once, such as a boundary combined with a mutation.

The challenge

The code runs in PyDebug’s in-browser compiler. Open the problem, run the snippet, watch the exact error or output, and keep iterating until it is right. Signed-in solves count toward XP and your daily streak.

The snippet

The interactive editor pre-fills exactly this code — read it, then submit your prediction:

total = 0
n = 1
while n <= 4:
    total += n * 2
    n += 1
print(total)

Understand the rule

This PyDebug exercise focuses on while loop with a condition update. It is a small reproducible Python debugging task: identify the input and expected state, then predict each statement before changing code. When output differs, inspect types, boundaries, mutation, and control flow instead of guessing.

How to trace it

Read the prompt and write the expected output for while loop with a condition update.
Run the code mentally, recording values after each loop or function call.
Compare actual and expected states and locate the first divergence.
Make the smallest fix, then test a normal and a boundary case.

Common wrong fixes

Changing several lines at once and hiding the original cause.
Checking only the happy path and missing zero, empty, or boundary input.
Confusing printed representation with the stored value.

Rule to remember

Treat while loop with a condition update as a focused experiment. A reliable solution explains the output, handles its edge case, and can be verified with a small test in the browser editor.

Solve it in the interactive editor →

🔮 Learning path: Output Prediction

Problem 18 of 18 in this path. Work them in order — each one builds on the rule before it.

← sorted() with a key function 🏁 Path complete — review it →

Keep going

Output-prediction strategies — how to predict tricky snippets without running them.
Browse every loops problem in this topic.
Common Python errors and what they actually mean, plus the Python glossary for any term used on this page.