While–else behavior in Python - Mini Project: Build & Apply

Understanding While-else Behavior in Python

📖 Scenario: Imagine you are checking a list of numbers to find if any number is divisible by 7. You want to know if you found such a number or if none exists.

🎯 Goal: You will write a Python program that uses a while loop with an else clause to check numbers and print a message depending on whether a divisible number is found.

📋 What You'll Learn

Create a list called numbers with the exact values: [10, 13, 22, 35, 40]

Create a variable called index and set it to 0

Use a while loop with the condition index < len(numbers)

Inside the loop, check if numbers[index] is divisible by 7 using %

If divisible, print Found a number divisible by 7: <number> and break

Use an else clause on the while loop to print No number divisible by 7 found if loop completes without break

Increment index by 1 inside the loop

💡 Why This Matters

🌍 Real World

Checking lists or data for specific conditions and knowing if none matched is common in data processing and validation.

💼 Career

Understanding loop control and the <code>else</code> clause helps in writing clear and efficient code for tasks like searching, filtering, and error checking.

1

Create the list of numbers

Create a list called numbers with these exact values: [10, 13, 22, 35, 40]

Python

# Create the list called numbers with the values [10, 13, 22, 35, 40]
# Your code here

Hint

Use square brackets [] to create a list and separate values with commas.

2

Set up the index variable

Create a variable called index and set it to 0

Python

numbers = [10, 13, 22, 35, 40]
# Create a variable called index and set it to 0
# Your code here

Hint

Use = to assign the value 0 to the variable index.

3

Write the while loop with else

Write a while loop with the condition index < len(numbers). Inside the loop, check if numbers[index] % 7 == 0. If true, print Found a number divisible by 7: {numbers[index]} and use break. Increment index by 1 inside the loop. Add an else clause on the while loop that prints No number divisible by 7 found.

Python

numbers = [10, 13, 22, 35, 40]
index = 0
# Write the while loop with else as described
# Your code here

Hint

Remember the else after a while runs only if the loop did not break.

4

Run the program to see the output

Run the program and print the output. The program should print Found a number divisible by 7: 35.

Python

numbers = [10, 13, 22, 35, 40]
index = 0
while index < len(numbers):
    if numbers[index] % 7 == 0:
        print(f"Found a number divisible by 7: {numbers[index]}")
        break
    index += 1
else:
    print("No number divisible by 7 found")
# Run the program to see the output

Hint

If you see this output, your program works correctly!

Practice

1.

What happens to the else block in a while loop if the loop ends normally (without a break)?

easy

A. The else block runs before the loop starts.

B. The else block never runs.

C. The else block runs after the loop finishes all iterations.

D. The else block runs only if the loop has a break.

Solution

1. Step 1: Understand the while-else structure

   The else block after a while loop runs only if the loop finishes all iterations without encountering a break.

2. Step 2: Analyze loop ending conditions

   If the loop ends normally (condition becomes false), the else block executes. If a break occurs, it skips the else.

3. Final Answer:

   The else block runs after the loop finishes all iterations. -> Option C

4. Quick Check:

   while-else else runs if no break [OK]

Hint: Else runs only if while loop ends without break [OK]

Common Mistakes:

• Thinking else runs always after while
• Believing else runs only if break occurs
• Confusing else with finally block

2.

Which of the following is the correct syntax for a while loop with an else block in Python?

?

easy

A. while condition:
# code
else
# code

B. while condition:
else:
# code
# code

C. while condition:
# code
else:
# code

D. while condition:
# code
else:
# code

Solution

1. Step 1: Recall Python while-else syntax

   The else block must be aligned with the while, not indented inside it.

2. Step 2: Check each option's indentation and keywords

   while condition: # code else: # code correctly places else: aligned with while and indents code blocks properly.

3. Final Answer:

   while condition: # code else: # code -> Option D

4. Quick Check:

   Else aligned with while, colon included [OK]

Hint: Else must align with while, not inside loop body [OK]

Common Mistakes:

• Indenting else inside while block
• Missing colon after else
• Placing else before while

3.

What is the output of this code?

i = 0
while i < 3:
    print(i)
    i += 1
else:
    print('Done')

medium

A. 0
1
2

B. 0
1
2
Done

C. Done

D. 0
1
2
3
Done

Solution

1. Step 1: Trace the while loop iterations

   Variable i starts at 0 and increments by 1 each loop until i < 3 is false. It prints 0, 1, 2.

2. Step 2: Check else block execution

   Since the loop ends normally (i becomes 3, condition false), the else block runs and prints 'Done'.

3. Final Answer:

   0 1 2 Done -> Option B

4. Quick Check:

   Loop prints 0-2, else prints Done [OK]

Hint: Else runs after loop if no break, prints 'Done' [OK]

Common Mistakes:

• Ignoring else block output
• Expecting 3 to print inside loop
• Thinking else runs only on break

4.

Find the error in this code snippet:

i = 0
while i < 5:
    if i == 3:
        break
    print(i)
    i += 1
else:
print('Finished')

medium

A. The else block is not indented properly.

B. The break statement is outside the loop.

C. The while condition is invalid.

D. The print statement inside the loop is missing parentheses.

Solution

1. Step 1: Check indentation of else block

   The else block must be aligned with the while statement, but here it is not indented properly.

2. Step 2: Verify other parts

   Break is inside the loop, while condition is valid, and print uses parentheses correctly.

3. Final Answer:

   The else block is not indented properly. -> Option A

4. Quick Check:

   Else must align with while, indentation error [OK]

Hint: Else must align with while, check indentation [OK]

Common Mistakes:

• Misplacing else inside loop body
• Confusing break placement
• Ignoring indentation errors

5.

Consider this code:

n = 5
while n > 0:
    if n == 3:
        break
    print(n)
    n -= 1
else:
    print('Loop completed')

What will be the output and why?

hard

A. 5
4
Because break stops loop, else does not run.

B. 5
4
3
Loop completed
Because else always runs.

C. 5
4
3
Because break is inside if, else runs anyway.

D. Loop completed
Because else runs before loop.

Solution

1. Step 1: Trace loop iterations and break

   n starts at 5, prints 5 and 4. When n == 3, break stops the loop immediately.

2. Step 2: Understand else block behavior

   Because the loop was stopped by break, the else block does not run, so 'Loop completed' is not printed.

3. Final Answer:

   5 4 Because break stops loop, else does not run. -> Option A

4. Quick Check:

   Break skips else, so only 5 and 4 print [OK]

Hint: Break skips else; else runs only if no break [OK]

Common Mistakes:

• Assuming else runs even after break
• Printing 3 inside loop
• Thinking else runs before loop