List length and membership test in Python

Introduction

We use list length to know how many items are in a list. Membership test helps us check if an item is inside the list.

Counting how many friends you have in a list.

Checking if a grocery item is already in your shopping list.

Finding out if a word exists in a list of vocabulary words.

Making sure a number is part of a list before using it.

Verifying if a task is in your to-do list before marking it done.

Syntax

Python

my_list = [item1, item2, item3]
length = len(my_list)
if element in my_list:
    # do something

len() is a built-in function that returns the number of items in the list.

The in keyword checks if an element exists in the list and returns True or False.

Examples

Empty list has length 0 and no elements inside.

Python

my_list = []
print(len(my_list))  # Output: 0
print(5 in my_list)  # Output: False

List with one element has length 1 and membership test returns True for that element.

Python

my_list = [10]
print(len(my_list))  # Output: 1
print(10 in my_list)  # Output: True

List with multiple elements. Membership test works for first, last, and missing elements.

Python

my_list = [1, 2, 3, 4, 5]
print(len(my_list))  # Output: 5
print(1 in my_list)  # Output: True
print(5 in my_list)  # Output: True
print(6 in my_list)  # Output: False

Sample Program

This program creates a shopping list, prints its length, and checks if certain items are in the list.

Python

def main():
    shopping_list = ['milk', 'eggs', 'bread']
    print(f"Shopping list has {len(shopping_list)} items.")

    item_to_check = 'eggs'
    if item_to_check in shopping_list:
        print(f"{item_to_check} is in the shopping list.")
    else:
        print(f"{item_to_check} is NOT in the shopping list.")

    item_to_check = 'butter'
    if item_to_check in shopping_list:
        print(f"{item_to_check} is in the shopping list.")
    else:
        print(f"{item_to_check} is NOT in the shopping list.")

if __name__ == '__main__':
    main()

Important Notes

Time complexity for len() is O(1) because Python stores the list size.

Membership test in takes O(n) time because it checks each item until it finds a match or reaches the end.

Common mistake: Using == instead of in to check membership.

Use len() when you need the count of items. Use membership test to check presence before actions like removal or update.

Summary

len() tells how many items are in a list.

in checks if an item is inside the list and returns True or False.

These tools help you work with lists safely and correctly.

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