while loops, sentinels and avoiding infinite loops. Learn through short examples, practical tasks and targeted questions.
A while loop checks its condition before every repetition. It works well for menus and input that continues until a sentinel value is entered. Identify which variable changes inside the loop and explain how the program eventually stops.
Watch out: If the loop condition never becomes false, the program can repeat indefinitely. Check both the starting state and the update.
while loops, sentinels and avoiding infinite loops. Study the code and predict what it does before running it. Change a value or string and observe how the output changes.
counter = 3
while counter > 0:
print(counter)
counter -= 1
print("Go!")Build a simple menu that repeats until the user enters Q.
Plan your inputs, processing and outputs. Write the code in Mu, IDLE or PyCharm; test at least three cases.
choice = ""
while choice != "Q":
choice = input("A to continue; Q to quit: ").upper()
print("Goodbye")Identify the problem and explain how to fix it before revealing the answer.
count = 0
while count < 3:
print(count)count = 0
while count < 3:
print(count)
count += 1Compare the two snippets. Identify the changed line, run the corrected version and explain the effect.
Extension: Improve your solution to deal with unexpected input, add a second feature or display results more clearly.
Reflect: What was the hardest part? What would you test if someone else used your program?
Want extra practice? Try interactive Python tracing and debugging questions →
A while loop only terminates if the condition eventually becomes False or the loop exits another way. Check that the loop variable changes and practise a dry run with the smallest valid input.
Check: If total never changes enough to make the condition False, the loop cannot terminate normally.