Indexing, Membership, and Length

Indexing, Membership, and Length

In this tutorial, we'll access items, check membership, and count values.

Many collections share common operations. Three of the most useful are indexing, membership, and length.

Indexing

Indexing gets an item by position. Lists, strings, and tuples support indexing.

names = ["Ada", "Grace", "Linus"]

print(names[0])
print(names[2])

Output:

Ada
Linus

Python starts counting at 0.

Negative indexes count from the end:

print(names[-1])

Output:

Linus

Membership

Membership checks whether a value is inside something. Use in.

names = ["Ada", "Grace", "Linus"]

print("Ada" in names)
print("Sam" in names)

Output:

True
False

This also works with strings:

print("py" in "python")

And dictionaries check keys:

student = {"name": "Ada", "score": 95}
print("score" in student)

Length

len() counts how many items there are:

print(len(names))
print(len("python"))

Output:

3
6

Common Mistake

The last index is always one less than the length:

names = ["Ada", "Grace", "Linus"]
print(len(names))  # 3
print(names[3])    # error

The valid indexes are 0, 1, and 2.

Putting Them Together

You can combine these ideas:

names = ["Ada", "Grace", "Linus"]

if len(names) > 0:
    print("First name:", names[0])

if "Grace" in names:
    print("Grace is in the list")

This checks the collection before using it, which makes your code easier to reason about.

Practice

Create a list of four cities. Print the first city, check if a city is in the list, and print the length.