Counter-based while loop in Python - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of a simple counter-based while loop

What is the output of this Python code?

Python

count = 0
while count < 3:
    print(count)
    count += 1

A

1
2

B

1
2
3

C

0
1
2
3

D

0
1
2

💡 Hint

Remember, the loop runs while count is less than 3, starting from 0.

The loop starts at 0 and prints count before increasing it by 1. It stops before count reaches 3, so it prints 0, 1, and 2.

❓ Predict Output

intermediate

Value of counter after loop ends

What is the value of variable 'counter' after this code runs?

Python

counter = 5
while counter > 0:
    counter -= 2

A 0

B -1

C 1

D 2

💡 Hint

Check how counter changes each loop iteration until the condition fails.

Starting at 5, counter decreases by 2 each time: 5 -> 3 -> 1 -> -1. The loop stops when counter is no longer > 0, so final value is -1.

🔧 Debug

advanced

Identify the error in this counter-based while loop

What error does this code produce when run?

Python

i = 0
while i < 5:
    print(i)
    i += 1

A SyntaxError

B TypeError

C IndentationError

D NameError

💡 Hint

Check the syntax of the while statement line.

The while statement is missing a colon at the end, causing a SyntaxError.

❓ Predict Output

advanced

Output of a while loop with break and counter

What is the output of this code?

Python

count = 0
while True:
    if count == 3:
        break
    print(count)
    count += 1

A

1
2
3

B

0
1
2
3

C

0
1
2

D

0
1
2
3
4

💡 Hint

The loop breaks when count reaches 3, so it does not print 3.

The loop prints count starting at 0 and stops before printing 3 because of the break.

❓ Predict Output

expert

Final value of counter with complex condition

What is the final value of 'counter' after this code runs?

Python

counter = 10
while counter > 0:
    counter -= 3
    if counter == 4:
        counter += 1

A -1

B 1

C 0

D 4

💡 Hint

Track the counter value each loop iteration carefully.

The counter decreases by 3 each time: 10 -> 7 -> 4. When counter is 4, it increases by 1 to 5, then continues: 5 -> 2 -> -1. The loop stops when counter is no longer > 0, so final value is -1 after last decrement.

Practice

1.

What is the main purpose of a counter in a while loop?

easy

A. To keep track of how many times the loop has run

B. To store the final result of the loop

C. To stop the program immediately

D. To print messages inside the loop

Solution

1. Step 1: Understand the role of a counter

   A counter is a variable that counts how many times the loop runs.

2. Step 2: Identify the purpose in a while loop

   The counter helps the loop know when to stop by checking its value in the condition.

3. Final Answer:

   To keep track of how many times the loop has run -> Option A

4. Quick Check:

   Counter tracks loop runs = D [OK]

Hint: Counter counts loop repeats to control stopping [OK]

Common Mistakes:

• Thinking counter stores final result
• Confusing counter with printing inside loop
• Believing counter stops program immediately

2.

Which of the following is the correct syntax to start a counter at 0 before a while loop?

?

easy

A. counter := 0

B. while counter = 0:

C. counter == 0

D. counter = 0

Solution

1. Step 1: Identify correct variable assignment

   To start a counter, assign 0 using a single equal sign: counter = 0.

2. Step 2: Check other options for syntax errors

   while counter = 0: uses assignment in while condition (invalid), C uses comparison, D uses walrus operator incorrectly here.

3. Final Answer:

   counter = 0 -> Option D

4. Quick Check:

   Assign counter with = 0 before loop = A [OK]

Hint: Use single = to assign counter before loop [OK]

Common Mistakes:

• Using == instead of = for assignment
• Trying to assign inside while condition
• Confusing walrus operator usage

3.

What will be the output of this code?

counter = 1
while counter <= 3:
    print(counter)
    counter += 1

medium

A. 1 2 3

B. 1 2 3 4

C. 0 1 2 3

D. 3 2 1

Solution

1. Step 1: Trace the loop iterations

   Counter starts at 1, prints 1, then increments to 2, prints 2, increments to 3, prints 3, then increments to 4.

2. Step 2: Check loop condition

   Loop stops when counter is 4 because 4 <= 3 is false. So printed numbers are 1, 2, 3.

3. Final Answer:

   1 2 3 -> Option A

4. Quick Check:

   Prints 1 to 3 with counter increment = C [OK]

Hint: Loop runs while counter ≤ 3, prints 1,2,3 [OK]

Common Mistakes:

• Including 4 in output
• Starting count from 0 instead of 1
• Printing numbers in reverse

4.

Find the error in this code and choose the fix:

counter = 0
while counter < 5
    print(counter)
    counter += 1

medium

A. Change < to <= in the while condition

B. Initialize counter to 1 instead of 0

C. Add a colon ':' after the while condition

D. Remove the increment line inside the loop

Solution

1. Step 1: Identify syntax error in while loop

   The while statement is missing a colon ':' at the end of the condition line.

2. Step 2: Check other options

   Changing < to <= is not an error fix, initializing counter differently or removing increment causes logic errors.

3. Final Answer:

   Add a colon ':' after the while condition -> Option C

4. Quick Check:

   Missing colon causes syntax error = B [OK]

Hint: Always put ':' after while condition [OK]

Common Mistakes:

• Forgetting colon after while condition
• Changing loop logic instead of fixing syntax
• Removing counter increment causing infinite loop

5.

Write a counter-based while loop that prints only even numbers from 2 to 10 inclusive.

counter = 2
while counter <= 10:
    print(counter)
    ?

What should replace ? to correctly increment the counter?

hard

A. counter += 1

B. counter += 2

C. counter = counter * 2

D. counter -= 2

Solution

1. Step 1: Understand the goal

   We want to print even numbers from 2 to 10, so counter should increase by 2 each time.

2. Step 2: Choose correct increment

   Using counter += 2 moves counter from 2 to 4, 6, 8, 10 correctly.

3. Final Answer:

   counter += 2 -> Option B

4. Quick Check:

   Increment by 2 to print evens = A [OK]

Hint: Increase counter by 2 to get even numbers [OK]

Common Mistakes:

• Incrementing by 1 prints odd numbers too
• Multiplying counter causes wrong jumps
• Decrementing counter causes infinite loop