Check if a List Includes an Element

1. The Challenge: List Membership Testing

Create a function check(lst, el) that checks if a list lst contains an element el. Return True if present, otherwise False.

2. Why It Matters

Searching for items in lists is a daily requirement in permission checking, tag filtering, and lookup validation.

3. The Python Solution

Use Python's built-in in membership operator:

def check(lst, el):
    return el in lst

# Testing our function
print(check([1, 2, 3, 4, 5], 3))        # Output: True
print(check([1, 1, 2, 1, 1], 3))        # Output: False
print(check(["a", "b", "c"], "b"))      # Output: True
print(check([], 5))                      # Output: False

4. How the in Operator Works

For lists, the in operator performs a linear search in O(N) time, scanning elements sequentially until a match is found. For O(1) instant lookups on large datasets, use a Python set().

5. Common Mistakes

• Manual For Loop Overuse: Writing for item in lst: if item == el: return True is unnecessary! Python's in operator does this natively in 1 clean line.

6. Next Steps

Use in for membership checks. Next, let's reverse a list in Python!