List length and membership test in Python - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this code?

Consider the following Python code. What will be printed?

Python

fruits = ['apple', 'banana', 'cherry', 'date']
print(len(fruits))
print('banana' in fruits)
print('grape' in fruits)

A

4
True
False

B

3
True
False

C

4
False
True

D

3
False
True

💡 Hint

Remember, len() counts all items in the list. The 'in' keyword checks if an item exists.

The list has 4 items, so len(fruits) is 4. 'banana' is in the list, so True. 'grape' is not, so False.

❓ Predict Output

intermediate

What is the length of the list after modification?

What will be the output of the following code?

Python

numbers = [1, 2, 3, 4, 5]
numbers.append(6)
numbers.remove(2)
print(len(numbers))

A 7

B 6

C 4

D 5

💡 Hint

Appending adds one item, removing deletes one item.

Original list has 5 items. Append adds one (6), remove deletes one (2), so total remains 5.

❓ Predict Output

advanced

What does this code print?

Analyze the code and select the correct output.

Python

items = ['pen', 'pencil', 'eraser', 'pen']
print(items.count('pen'))
print('pen' in items)
print(len(items))

A

2
True
4

B

1
True
3

C

2
False
4

D

1
False
3

💡 Hint

count() returns how many times an item appears.

'pen' appears twice, so count is 2. 'pen' is in the list, so True. List length is 4.

❓ Predict Output

advanced

What error does this code raise?

What error will this code produce when run?

Python

my_list = [10, 20, 30]
print(my_list[3])

A ValueError

B KeyError

C IndexError

D TypeError

💡 Hint

List indices start at 0 and go up to length-1.

Index 3 is out of range for a list of length 3, causing IndexError.

🧠 Conceptual

expert

How many items are in the resulting list?

What is the length of the list after this code runs?

Python

a = [1, 2]
b = a
b.append(3)
print(len(a))

A 2

B 3

C 1

D Error

💡 Hint

Remember that lists are mutable and variables can reference the same list.

Both 'a' and 'b' point to the same list. Appending via 'b' changes 'a' too, so length is 3.

Practice

1.

What does the len() function do when used with a list in Python?

easy

A. It removes the last item from the list.

B. It checks if an item is in the list.

C. It adds a new item to the list.

D. It returns the number of items in the list.

Solution

1. Step 1: Understand the purpose of len()

   The len() function counts how many elements are inside a list.

2. Step 2: Compare with other options

   Checking membership uses in, adding uses append(), removing uses pop().

3. Final Answer:

   It returns the number of items in the list. -> Option D

4. Quick Check:

   len() counts items [OK]

Hint: Remember: len() counts items, in checks presence [OK]

Common Mistakes:

• Confusing len() with in operator
• Thinking len() adds or removes items
• Mixing up list methods with functions

2.

Which of the following is the correct syntax to check if the number 5 is in the list nums?

nums = [1, 3, 5, 7]

easy

A. if 5 inside nums:

B. if nums in 5:

C. if 5 in nums:

D. if nums contains 5:

Solution

1. Step 1: Recall correct membership syntax

   Python uses in to check if an item is inside a list: item in list.

2. Step 2: Check each option

   Only if 5 in nums: matches correct syntax; others are invalid Python.

3. Final Answer:

   if 5 in nums: -> Option C

4. Quick Check:

   Use in for membership test [OK]

Hint: Use 'item in list' to check membership [OK]

Common Mistakes:

• Writing 'inside' or 'contains' instead of 'in'
• Reversing order like 'list in item'
• Using invalid keywords for membership

3.

What will be the output of this code?

fruits = ['apple', 'banana', 'cherry']
print(len(fruits))
print('banana' in fruits)
print('orange' in fruits)

medium

A. 3\nTrue\nFalse

B. 3\nFalse\nTrue

C. Error\nTrue\nFalse

D. 3\nTrue\nTrue

Solution

1. Step 1: Calculate length of the list

   The list has 3 items: 'apple', 'banana', 'cherry', so len(fruits) is 3.

2. Step 2: Check membership for 'banana' and 'orange'

   'banana' is in the list, so 'banana' in fruits is True. 'orange' is not, so 'orange' in fruits is False.

3. Final Answer:

   3 True False -> Option A

4. Quick Check:

   len=3, banana in list=True, orange in list=False [OK]

Hint: Count items with len(), check presence with in [OK]

Common Mistakes:

• Confusing True/False for membership
• Miscounting list length
• Expecting syntax error for correct code

4.

Find the error in this code snippet:

items = [10, 20, 30]
if 20 in items
    print("Found 20")

medium

A. Missing colon ':' after the if condition.

B. Wrong use of 'in' operator.

C. List should be defined with parentheses, not brackets.

D. print statement syntax is incorrect.

Solution

1. Step 1: Check syntax of if statement

   Python requires a colon ':' at the end of the if condition line.

2. Step 2: Identify the missing colon

   The code has if 20 in items without a colon, causing a syntax error.

3. Final Answer:

   Missing colon ':' after the if condition. -> Option A

4. Quick Check:

   if statements need ':' [OK]

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

Common Mistakes:

• Forgetting colon after if condition
• Misusing 'in' operator syntax
• Using parentheses instead of brackets for lists

5.

You have a list numbers = [2, 4, 6, 8]. You want to check if the list has exactly 4 items and if the number 5 is not in the list. Which code correctly does this?

hard

A. if len(numbers) = 4 and 5 not in numbers:
print("Condition met")

B. if len(numbers) == 4 and 5 not in numbers:
print("Condition met")

C. if len(numbers) == 4 or 5 in numbers:
print("Condition met")

D. if len(numbers) == 4 and 5 in numbers:
print("Condition met")

Solution

1. Step 1: Check list length condition

   Use len(numbers) == 4 to test if list has exactly 4 items.

2. Step 2: Check membership condition

   Use 5 not in numbers to confirm 5 is not in the list.

3. Step 3: Combine conditions correctly

   Use and to require both conditions true together.

4. Final Answer:

   if len(numbers) == 4 and 5 not in numbers: print("Condition met") -> Option B

5. Quick Check:

   Length check and membership negation combined with and [OK]

Hint: Use 'len() == number' and 'item not in list' with and [OK]

Common Mistakes:

• Using single '=' instead of '==' for comparison
• Using or instead of and for both conditions
• Checking for 5 in list instead of not in