Data Structures: Lists and Dictionaries

Data Structures: Lists and Dictionaries#

So far, we’ve stored information in individual variables:

a = 'Sakib'
b = 1
c = 2.0

Maybe we want to store the age of Sakib:

sakib_age = 100

But this approach becomes impractical quickly. What if we need to store the age of everyone in a class of 50 students? We’d need 50 separate variables — a waste of memory and effort!

Instead, we can store multiple pieces of information using Python’s built-in data structures. Here’s a preview:

age = [10, 20, 32, 35, 26]

We’ve just stored 5 ages in a single variable! Let’s explore how this works.

Python’s Built-in Data Structures#

Python has four main built-in data structures:

Structure	Syntax	Description
List	`[item1, item2, ...]`	Ordered, changeable collection
Dictionary	`{key1: value1, ...}`	Key-value pairs
Tuple	`(item1, item2, ...)`	Ordered, unchangeable collection
Set	`{item1, item2, ...}`	Unordered collection of unique items

Let’s see examples of each:

List:

age = [10, 20, 32, 35, 26]

Dictionary:

students = {
    'Hermoine': 'Gryffindor',
    'Harry': 'Gryffindor',
    'Ron': 'Gryffindor',
    'Draco': 'Slytherin'
}

Tuple:

age_tuple = (10, 20, 30, 200)

We’ll focus mainly on lists and dictionaries in this chapter, as they’re the most commonly used.

Lists#

A list is an ordered collection of items. Think of it like a numbered container where you can store multiple values.

Creating Lists#

list_of_students = ['Hermoine', 'Harry', 'Ron', 'Draco']

animals = ['dog', 'cat', 'cow']

ages = [10, 20, 32, 35, 26]

Lists can contain any type of value, and you can even mix types:

anything = ['spam', 2.0, 5, [10, 20]]

Notice that last item — a list inside a list! This is called a nested list.

An empty list is created with empty brackets:

empty = []

Finding the Length of a List#

The len() function tells you how many items are in a list:

len(list_of_students)

Accessing Items (Indexing)#

Each item in a list has a position number called an index. Here’s the crucial thing to remember: indexing starts at 0, not 1!

list_of_students = ['Hermoine', 'Harry', 'Ron', 'Draco']

Index	0	1	2	3
Value	Hermoine	Harry	Ron	Draco

To access the first item:

list_of_students[0]

'Hermoine'

print(list_of_students[0])

Hermoine

To access the last item (index 3):

list_of_students[3]

'Draco'

What happens if you try to access an index that doesn’t exist?

list_of_students[4]

---------------------------------------------------------------------------
IndexError                                Traceback (most recent call last)
Cell In[17], line 1
----> 1 list_of_students[4]

IndexError: list index out of range

You get an IndexError because our list only has 4 items (indices 0-3), so index 4 doesn’t exist.

Tip

You can use negative indices to count from the end. list_of_students[-1] gives you the last item, list_of_students[-2] gives the second-to-last, and so on.

Adding Items to a List#

Use the .append() method to add an item to the end of a list:

list_of_students.append('Sakib')
list_of_students

['Hermoine', 'Harry', 'Ron', 'Draco', 'Sakib']

Changing Items (Lists are Mutable)#

Lists are mutable, meaning you can change their contents after creation. Let’s replace ‘Sakib’ with ‘Joe’:

list_of_students[4] = 'Joe'
list_of_students

['Hermoine', 'Harry', 'Ron', 'Draco', 'Joe']

Iterating Through a List#

If we want to print all the student names one by one, we could do this:

print(list_of_students[0])
print(list_of_students[1])
print(list_of_students[2])
print(list_of_students[3])
print(list_of_students[4])

Hermoine
Harry
Ron
Draco
Joe

But this is tedious! What if we had 1000 students? Instead, we use a loop:

for student in list_of_students:
    print(student)

Hermoine
Harry
Ron
Draco
Joe

Much better! The for loop automatically goes through each item in the list, one at a time. In each iteration, the variable student holds the current item.

Getting Position and Value Together#

What if you want to print both the position and the name? You can combine range() with len():

for i in range(len(list_of_students)):
    print(i + 1, list_of_students[i])

Hermoine
Harry
Ron
Draco
Joe

Here’s what’s happening:

len(list_of_students) returns 5
range(5) generates 0, 1, 2, 3, 4
In each iteration, i is the index, and list_of_students[i] is the student at that index
We print i + 1 to show human-friendly numbering (1, 2, 3… instead of 0, 1, 2…)

Note

Python also has a built-in function enumerate() that does this more elegantly. We’ll cover that later!

Dictionaries#

A dictionary (or dict) is a data structure that associates keys with values. Unlike lists, which use numeric indices, dictionaries let you use meaningful labels.

Why Dictionaries?#

Imagine we want to associate students with their houses at Hogwarts:

Hermoine	Harry	Ron	Draco
Gryffindor	Gryffindor	Gryffindor	Slytherin

We could use two separate lists:

students = ["Hermoine", "Harry", "Ron", "Draco"]
houses = ["Gryffindor", "Gryffindor", "Gryffindor", "Slytherin"]

The idea is that students[0] corresponds to houses[0], students[1] to houses[1], and so on. But this approach is fragile — if we accidentally reorder one list but not the other, everything breaks!

A dictionary makes this relationship explicit:

students = {
    "Hermoine": "Gryffindor",
    "Harry": "Gryffindor",
    "Ron": "Gryffindor",
    "Draco": "Slytherin"
}

Accessing Values#

To get a value from a dictionary, you use its key:

print(students['Hermoine'])

Gryffindor

print(students['Draco'])

Slytherin

Think of it like looking up a word in a real dictionary — you search for the word (key) and get its definition (value).

Iterating Through a Dictionary#

You can loop through a dictionary’s keys:

for name in students:
    print(name)

Hermoine
Harry
Ron
Draco

Notice this only prints the keys. To get the values too:

for name in students:
    print(name, students[name])

Hermoine Gryffindor
Harry Gryffindor
Ron Gryffindor
Draco Slytherin

Let’s make the output more readable:

for name in students:
    print(name, "lives in", students[name])

Hermoine lives in Gryffindor
Harry lives in Gryffindor
Ron lives in Gryffindor
Draco lives in Slytherin

Adding More Information#

What if we want to store more data about each student — not just their house, but also their gender?

Name	House	Gender
Hermoine	Gryffindor	Female
Harry	Gryffindor	Male
Draco	Slytherin	Male

We can create a list of dictionaries — each dictionary represents one student with all their attributes:

students_info = [
    {'name': 'Hermoine', 'house': 'Gryffindor', 'gender': 'female'},
    {'name': 'Harry', 'house': 'Gryffindor', 'gender': 'male'},
    {'name': 'Draco', 'house': 'Slytherin', 'gender': 'male'}
]

Now we have a list called students_info containing three dictionaries. Each dictionary has three key-value pairs.

for student in students_info:
    print(
        student['name'],
        'lives in', student['house'],
        'and is', student['gender']
    )

Hermoine lives in Gryffindor and is female
Harry lives in Gryffindor and is male
Draco lives in Slytherin and is male

This structure — a list of dictionaries — is extremely common in data analysis. It’s essentially a table where each dictionary is a row, and each key is a column.

Adding Items to a Dictionary#

To add a new key-value pair, simply assign to a new key:

students['sakib'] = 'Winchester'
students

{'Hermoine': 'Gryffindor',
 'Harry': 'Gryffindor',
 'Ron': 'Gryffindor',
 'Draco': 'Slytherin',
 'sakib': 'Winchester'}

Changing Values#

You can update an existing value the same way:

students['Draco'] = 'Winchester'
students

{'Hermoine': 'Gryffindor',
 'Harry': 'Gryffindor',
 'Ron': 'Gryffindor',
 'Draco': 'Winchester',
 'sakib': 'Winchester'}

Draco has transferred to Winchester!

Tuples (Brief Overview)#

A tuple is like a list, but immutable — once created, you cannot change it:

coordinates = (10, 20)

Tuples are useful when you have data that shouldn’t be modified. We’ll see them more when we work with pandas.

Summary: Lists vs Dictionaries#

Feature	List	Dictionary
Syntax	`[item1, item2, ...]`	`{key1: value1, ...}`
Access	By index number `list[0]`	By key `dict['key']`
Order	Ordered	Ordered (Python 3.7+)
Best for	Sequential data, collections	Named data, lookups

Exercises#

Exercise 8

Exercise 1: List Operations

Create a list called fruits containing: apple, banana, cherry.

Add “orange” to the end of the list
Change “banana” to “blueberry”
Print the length of the list
Print each fruit on a separate line using a loop

Solution to Exercise 8

fruits = ['apple', 'banana', 'cherry']

# Add orange
fruits.append('orange')

# Change banana to blueberry
fruits[1] = 'blueberry'

# Print length
print(len(fruits))

# Print each fruit
for fruit in fruits:
    print(fruit)

Exercise 9

Exercise 2: Create a Dictionary

Create a dictionary called prices that maps the following items to their prices:

milk: 1.50
bread: 2.00
eggs: 3.25

Then print each item and its price in the format: “milk costs £1.50”

Solution to Exercise 9

prices = {
    'milk': 1.50,
    'bread': 2.00,
    'eggs': 3.25
}

for item in prices:
    print(f"{item} costs £{prices[item]}")

Exercise 10

Exercise 3: Student Records

Create a list of dictionaries representing 3 students. Each student should have:

name
age
course

Then write a loop that prints each student’s information in a nice format.

Solution to Exercise 10

students = [
    {'name': 'Alice', 'age': 22, 'course': 'Economics'},
    {'name': 'Bob', 'age': 25, 'course': 'Data Science'},
    {'name': 'Carol', 'age': 23, 'course': 'Business'}
]

for student in students:
    print(f"{student['name']} is {student['age']} years old, studying {student['course']}")

Exercise 11

Exercise 4: Sum and Average

Given the list scores = [85, 92, 78, 90, 88]:

Calculate the sum of all scores
Calculate the average (mean) score
Find the highest score
Find the lowest score

Hint: You can use a loop, or Python has built-in functions sum(), max(), and min().

Solution to Exercise 11

scores = [85, 92, 78, 90, 88]

# Method 1: Using built-in functions
total = sum(scores)
average = total / len(scores)
highest = max(scores)
lowest = min(scores)

print(f"Sum: {total}")
print(f"Average: {average}")
print(f"Highest: {highest}")
print(f"Lowest: {lowest}")

# Method 2: Using a loop (for sum)
total_loop = 0
for score in scores:
    total_loop = total_loop + score
print(f"Sum (loop): {total_loop}")