Counter-based while loop in Python - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a counter-based while loop?

A counter-based while loop is a loop that repeats a set of instructions while a counter variable meets a condition. The counter changes each time the loop runs, usually increasing or decreasing by 1.

beginner

How do you initialize a counter for a while loop in Python?

You create a variable before the loop starts and set it to a starting number, often 0 or 1. For example:

counter = 0

beginner

Why must you update the counter inside the while loop?

If you don't change the counter inside the loop, the condition may never become false, causing an infinite loop that never stops.

beginner

Write a simple counter-based while loop that prints numbers 1 to 5.

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

beginner

What happens if the counter condition is false at the start of the while loop?

The loop body will not run even once because the condition is checked before the loop starts.

What is the first step when creating a counter-based while loop?

A Write the print statement

B Initialize the counter variable

C Update the counter inside the loop

D Check the condition after the loop

You must first create and set the counter variable before the loop starts.

What happens if you forget to update the counter inside the while loop?

A The loop will run only once

B The counter resets automatically

C The program will crash immediately

D The loop will run forever (infinite loop)

Without updating, the condition never changes, so the loop never ends.

Which of these is a correct condition for a counter starting at 0 to run 5 times?

A counter < 5

B counter > 5

C counter == 5

D counter != 0

Using 'counter < 5' runs the loop while counter is 0,1,2,3,4 which is 5 times.

Where do you put the counter update in a while loop?

A After the loop ends

B Before the loop starts

C Inside the loop body

D Outside the program

The counter must be updated inside the loop to change the condition each time.

What will this code print?

counter = 3
while counter > 0:
    print(counter)
    counter -= 1

A 3 2 1

B 1 2 3

C 0 1 2

D Nothing

The loop prints 3, then 2, then 1, then stops when counter is 0.

Explain how a counter-based while loop works and why updating the counter is important.

Think about how the loop knows when to stop.

Write a simple Python while loop that counts down from 5 to 1 and prints each number.

Start with counter = 5 and reduce it by 1 each time.

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