Infinite loop prevention in Python - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is an infinite loop?

An infinite loop is a loop that never stops running because its exit condition is never met.

beginner

How can you prevent an infinite loop in Python?

Make sure the loop's condition will eventually become false by updating variables inside the loop.

beginner

What role does a loop counter play in preventing infinite loops?

A loop counter tracks how many times the loop runs and can be used to stop the loop after a set number of iterations.

intermediate

Why is it important to test loops with different inputs?

Testing with different inputs helps ensure the loop stops correctly and does not run forever.

intermediate

What is a common debugging method to find infinite loops?

Adding print statements inside the loop to see how variables change and if the exit condition is reached.

What happens if a loop's exit condition is never met?

A The loop skips all iterations.

B The loop runs once.

C The program crashes immediately.

D The loop runs forever (infinite loop).

If the exit condition is never met, the loop keeps running endlessly, causing an infinite loop.

Which of these helps prevent infinite loops?

A Using only while True without breaks.

B Leaving the loop condition unchanged.

C Updating loop variables inside the loop.

D Ignoring loop counters.

Updating loop variables ensures the exit condition can eventually be met, stopping the loop.

What is a loop counter used for?

A To count how many times the loop runs.

B To make the loop run infinitely.

C To skip loop iterations.

D To crash the program.

A loop counter tracks iterations and can help stop the loop after a set number.

Which debugging method helps find infinite loops?

A Adding print statements inside the loop.

B Removing all loop conditions.

C Ignoring variable changes.

D Running the program without testing.

Print statements show variable changes and help check if the loop will stop.

What is a safe way to write a while loop to avoid infinite loops?

A Use while True without any break.

B Ensure the condition will become false by changing variables inside the loop.

C Never update variables inside the loop.

D Write loops without conditions.

Changing variables inside the loop helps the condition become false and stops the loop.

Explain what causes an infinite loop and how you can prevent it in Python.

Think about what makes a loop stop running.

Describe how you would debug a program that seems stuck in an infinite loop.

How can you see what the program is doing inside the loop?

Practice

1. What is the main reason an infinite loop happens in Python?

easy

A. The loop runs only once

B. The loop uses a break statement

C. The loop condition never becomes false

D. The loop has no variables

Solution

1. Step 1: Understand loop condition

   An infinite loop occurs when the condition controlling the loop never changes to false.

2. Step 2: Identify cause of infinite loop

   If the loop condition never becomes false, the loop keeps running forever.

3. Final Answer:

   The loop condition never becomes false -> Option C

4. Quick Check:

   Infinite loop = condition never false [OK]

Hint: Check if loop condition can ever become false [OK]

Common Mistakes:

• Thinking break causes infinite loops
• Assuming loops run only once cause infinite loops
• Ignoring the loop condition

2. Which of these loop structures correctly prevents an infinite loop?

easy

A. while True:
print('Hello')

B. while x < 5:
x += 1

C. while x == 0:
pass

D. while x > 0:
print(x)

Solution

1. Step 1: Check each loop's condition and body

   while True: print('Hello') loops forever because condition is always True with no break. while x < 5: x += 1 increases x, so condition will become false. while x == 0: pass loops forever because x never changes. while x > 0: print(x) loops forever because x is never changed.

2. Step 2: Identify loop that ends

   Only while x < 5: x += 1 changes x to eventually stop the loop.

3. Final Answer:

   while x < 5: x += 1 -> Option B

4. Quick Check:

   Change loop variable to end loop [OK]

Hint: Look for loops that update the condition variable [OK]

Common Mistakes:

• Choosing loops with constant True condition
• Ignoring variable updates inside loop
• Assuming print stops loops

3. What will be the output of this code?

i = 0
while i < 3:
    print(i)
    i -= 1

medium

A. SyntaxError

B. 0 1 2

C. 0

D. 0 -1 -2 ... (infinite loop)

Solution

1. Step 1: Analyze loop condition and variable change

   i starts at 0 and loop runs while i < 3. Inside loop, i decreases by 1 each time.

2. Step 2: Determine if loop ends

   Since i decreases, it will go 0, -1, -2, ... always less than 3, so loop never ends.

3. Final Answer:

   0 -1 -2 ... (infinite loop) -> Option D

4. Quick Check:

   Variable moves away from exit condition = infinite loop [OK]

Hint: Check if loop variable moves toward or away from condition [OK]

Common Mistakes:

• Assuming i increases automatically
• Thinking loop stops after first print
• Confusing syntax errors with logic errors

4. Find the error that causes an infinite loop in this code:

count = 5
while count > 0:
    print(count)

medium

A. Missing decrement of count

B. Missing increment of count

C. Wrong comparison operator

D. Syntax error in while statement

Solution

1. Step 1: Check loop condition and body

   The loop runs while count > 0, but count is never changed inside the loop.

2. Step 2: Identify missing update

   Without decreasing count, the condition stays true forever, causing infinite loop.

3. Final Answer:

   Missing decrement of count -> Option A

4. Quick Check:

   Loop variable must change to end loop [OK]

Hint: Look for missing variable update inside loop [OK]

Common Mistakes:

• Thinking increment needed instead of decrement
• Assuming loop ends automatically
• Confusing syntax errors with logic errors

5. You want to print numbers from 1 to 5 using a while loop without causing an infinite loop. Which code correctly does this?

hard

A. num = 1
while num <= 5:
print(num)
num += 1

B. num = 5
while num > 0:
print(num)
num += 1

C. num = 1
while num < 5:
print(num)
num -= 1

D. num = 1
while num <= 5:
print(num)

Solution

1. Step 1: Check loop conditions and variable updates

   num = 1 while num <= 5: print(num) never changes num, so infinite loop. num = 1 while num <= 5: print(num) num += 1 increases num, stopping at 6. num = 1 while num < 5: print(num) num -= 1 decreases num, moving away from stop condition. num = 5 while num > 0: print(num) num += 1 increases num starting at 5, so loop never ends.

2. Step 2: Confirm correct loop behavior

   Only num = 1 while num <= 5: print(num) num += 1 prints 1 to 5 and stops correctly.

3. Final Answer:

   num = 1 while num <= 5: print(num) num += 1 -> Option A

4. Quick Check:

   Update loop variable toward exit condition [OK]

Hint: Ensure loop variable moves toward stopping condition [OK]

Common Mistakes:

• Forgetting to update loop variable
• Updating variable in wrong direction
• Using wrong loop condition