While loop execution flow in Python - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the basic structure of a while loop in Python?

A while loop starts with the keyword while, followed by a condition and a colon. The code inside the loop is indented and runs repeatedly as long as the condition is true.

beginner

What happens if the condition in a while loop is initially false?

The code inside the while loop does not run at all, and the program continues with the code after the loop.

beginner

How does the while loop decide when to stop?

The while loop checks the condition before each iteration. If the condition becomes false, the loop stops running.

intermediate

What can cause an infinite loop in a while loop?

If the condition never becomes false because the code inside the loop does not change the variables involved, the loop will run forever, causing an infinite loop.

intermediate

How can you safely exit a while loop before the condition becomes false?

You can use the break statement inside the loop to stop it immediately, regardless of the condition.

What does the while loop check before running its code block?

A The loop condition

B The number of times it has run

C The last value printed

D The function return value

The while loop checks the condition before each iteration to decide if it should run the code inside.

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

A The loop runs once

B The loop does not run at all

C The loop runs infinitely

D The loop runs twice

If the condition is false at the start, the while loop skips the code inside and continues after the loop.

Which statement can stop a while loop immediately?

A continue

B pass

C return

D break

The break statement stops the loop immediately, exiting it regardless of the condition.

What is a common cause of an infinite while loop?

A Using a break statement

B Changing the loop variable inside the loop

C Not changing the loop condition inside the loop

D Using an else clause

If the loop condition never changes to false, the loop will run forever, causing an infinite loop.

When does the while loop check its condition?

A Before running the loop code

B After running the loop code

C Only once at the start

D Only if a break is used

The while loop checks the condition before each iteration to decide whether to run the loop code.

Explain how a while loop executes step-by-step.

Think about what happens before and after the loop code runs.

Describe what can cause an infinite loop and how to prevent it.

Focus on the condition and how it changes.

Practice

1.

What does a while loop do in Python?

easy

A. Stops the program immediately

B. Runs code only once

C. Repeats code as long as a condition is true

D. Runs code a fixed number of times

Solution

1. Step 1: Understand the purpose of a while loop

   A while loop runs repeatedly while its condition is true.

2. Step 2: Compare options with this behavior

   Only Repeats code as long as a condition is true describes repeating code while a condition is true.

3. Final Answer:

   Repeats code as long as a condition is true -> Option C

4. Quick Check:

   While loop = repeat while true [OK]

Hint: While loops run repeatedly while condition is true [OK]

Common Mistakes:

• Thinking while loops run a fixed number of times
• Confusing while with if statement
• Believing while loops run only once

2.

Which of the following is the correct syntax to start a while loop in Python?

?

easy

A. while x > 0:

B. while (x > 0)

C. while x > 0 {}

D. while x > 0 then

Solution

1. Step 1: Recall Python while loop syntax

   Python uses a colon after the condition and no parentheses or braces.

2. Step 2: Match options with correct syntax

   while x > 0: uses 'while x > 0:' which is correct syntax.

3. Final Answer:

   while x > 0: -> Option A

4. Quick Check:

   Colon ends while condition line [OK]

Hint: Use colon after condition, no braces or 'then' [OK]

Common Mistakes:

• Adding braces {} like other languages
• Using 'then' keyword
• Putting condition in parentheses unnecessarily

3.

What will be the output of this code?

count = 3
while count > 0:
    print(count)
    count -= 1
print("Done")

medium

A. Done

B. 3 2 1 Done

C. 3 2 1 0 Done

D. 3 2 1 2 Done

Solution

1. Step 1: Trace the while loop iterations

   count starts at 3, prints 3, then decreases to 2, prints 2, then 1, then stops when count is 0.

2. Step 2: Note the final print after loop

   After loop ends, "Done" is printed.

3. Final Answer:

   3 2 1 Done -> Option B

4. Quick Check:

   Prints 3,2,1 then Done [OK]

Hint: Count decreases before loop ends, stops at zero [OK]

Common Mistakes:

• Expecting 0 to print inside loop
• Forgetting to decrease count
• Thinking loop runs forever

4.

Find the error in this code snippet:

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

medium

A. Missing colon after while condition

B. Variable i is not initialized

C. Indentation error in print statement

D. Infinite loop because i never changes

Solution

1. Step 1: Check while loop syntax

   The while line must end with a colon (:).

2. Step 2: Identify the missing colon

   The code misses ':' after 'while i < 5', causing syntax error.

3. Final Answer:

   Missing colon after while condition -> Option A

4. Quick Check:

   While line needs colon [OK]

Hint: Always put colon after while condition [OK]

Common Mistakes:

• Forgetting colon after while
• Misindenting inside loop
• Not updating loop variable

5.

Consider this code:

n = 5
result = 1
while n > 1:
    result *= n
    n -= 1
print(result)

What does this code calculate?

hard

A. Sum of numbers from 1 to 5

B. Infinite loop with no output

C. Product of numbers from 1 to 4

D. Factorial of 5 (5!)

Solution

1. Step 1: Understand the loop's multiplication

   result multiplies by n each time, starting at 5 down to 2.

2. Step 2: Recognize factorial pattern

   Multiplying 5*4*3*2*1 equals 5 factorial (5!).

3. Final Answer:

   Factorial of 5 (5!) -> Option D

4. Quick Check:

   Multiplying down to 1 = factorial [OK]

Hint: Multiplying down from n to 1 calculates factorial [OK]

Common Mistakes:

• Thinking it sums numbers instead of multiplies
• Stopping multiplication at 2 instead of 1
• Assuming infinite loop