Collections of Data: Lists, Tuples, and Dictionaries

Why We Need Collections of Data

First Steps With Lists

A list is an ordered collection of items. You write a list with square brackets `[]`, and items are separated by commas.

Here is how to create and use a simple shopping list:

```python

Create a list of strings

shopping_list = ["milk", "bread", "eggs"]

print(shopping_list)

print("Number of items:", len(shopping_list))

Access items by index (positions start at 0)

print("First item:", shopping_list[0])

print("Second item:", shopping_list[1])

print("Last item:", shopping_list[2])

Change an item

shopping_list[1] = "brown bread"

print("Updated list:", shopping_list)

```

Run this code and watch:

• How the list is printed
• How `len()` tells you how many items are inside
• How changing one position updates the list

Indexes and Negative Indexes

Common List Methods: append, insert, pop

Lists are mutable, which means you can change them after you create them. Here are three very common list methods:

• `append(item)` – add an item to the end of the list
• `insert(index, item)` – add an item at a specific position
• `pop(index)` – remove and return the item at a position (by default, removes the last item)

Try this code:

```python

friends = ["Alex", "Jordan", "Taylor"]

print("Start:", friends)

1. Add to the end

friends.append("Sam")

print("After append:", friends)

2. Insert at position 1

friends.insert(1, "Riley")

print("After insert at index 1:", friends)

3. Remove last item with pop()

last_friend = friends.pop()

print("Popped:", last_friend)

print("After pop:", friends)

4. Remove by index (position 0)

first_friend = friends.pop(0)

print("Removed first:", first_friend)

print("Final list:", friends)

```

Watch how the list changes step by step.

Practice: Fix the List

Try this thought exercise. You do not need to run code, but you can if you want.

You start with this list of tasks:

```python

tasks = ["wake up", "brush teeth", "go to school"]

```

You want the list to become:

```python

["wake up", "brush teeth", "eat breakfast", "go to school", "do homework"]

```

Your challenge: Write the Python lines (in your head or on paper) to:

1. Add "eat breakfast" between "brush teeth" and "go to school".
2. Add "do homework" to the end of the list.

Hints:

• Use `insert(index, item)` to put something in the middle.
• Use `append(item)` to add something at the end.

Check yourself:

A possible solution is:

```python

tasks.insert(2, "eat breakfast")

tasks.append("do homework")

```

Think: Why is the index `2` for "eat breakfast"?

Tuples: Lists You Cannot Change

Quick Check: Lists vs Tuples

Test your understanding of lists and tuples.

Dictionaries: Storing Key–Value Pairs

Iterating Over Dictionary Keys and Values

You often want to loop through everything in a dictionary.

Useful patterns:

• `for key in dict:` – loop over keys
• `for key, value in dict.items():` – loop over keys and values together

Example:

```python

user = {

"name": "Aisha",

"age": 15,

"email": "aisha@example.com"

}

print("Keys only:")

for key in user:

print(key)

print("\nKeys and values:")

for key, value in user.items():

print(key, "->", value)

```

This will show each key and the value it points to, like a small table.

Mini Design Challenge: Choose the Right Collection

For each situation, decide whether a list, tuple, or dictionary fits best. Think first, then check suggested answers.

1. You want to store the 3D coordinates of a point: x, y, z.
2. You want to store all the scores from a test for 30 students, but you do not care about names, just the numbers.
3. You want to store information about a book: title, author, year, and ISBN.
4. You want to store the days of the week, which never change.

Suggested answers:

1. Tuple – a small, fixed group of related numbers.
2. List – an ordered, changeable sequence of scores.
3. Dictionary – key–value pairs like `"title"`, `"author"`, etc.
4. Tuple (or list) – but tuple makes sense because the days do not change.

Review: Key Terms

Use these flashcards to review the main ideas from this module.