Choosing the right data structure in Intro to Computing - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a data structure?

A data structure is a way to organize and store data so it can be used efficiently, like how a bookshelf organizes books for easy finding.

beginner

Why is choosing the right data structure important?

Choosing the right data structure helps your program run faster and use less memory, just like choosing the right container helps you carry things easily.

beginner

Which data structure is best for fast lookup by key?

A dictionary (or map) is best for fast lookup by key, like a phone book where you find a number by a person's name quickly.

beginner

When should you use a list (array)?

Use a list when you want to keep items in order and access them by position, like a line of people waiting where you know who is first or last.

beginner

What is a real-life analogy for a stack data structure?

A stack is like a stack of plates: you add or remove plates only from the top, following last-in, first-out (LIFO) order.

Which data structure is best for storing unique items without order?

A Set

B List

C Queue

D Stack

A set stores unique items and does not keep them in order, like a collection of unique stamps.

If you need to process items in the order they arrive, which data structure should you use?

A Stack

B Tree

C Dictionary

D Queue

A queue processes items in first-in, first-out (FIFO) order, like a line at a store.

Which data structure allows fast access to elements by index?

A Set

B Dictionary

C List (Array)

D Graph

Lists or arrays allow fast access by position, like knowing the 3rd book on a shelf.

What is the main advantage of using a dictionary (map)?

A Fast lookup by key

B Stores unique items only

C Keeps items in order

D Processes items in FIFO order

Dictionaries provide fast lookup by key, like finding a contact by name.

Which data structure is like a stack of plates?

A Queue

B Stack

C Set

D List

A stack follows last-in, first-out (LIFO), like adding/removing plates from the top.

Explain how you would choose a data structure for storing a list of tasks to do in order.

Think about how you handle a to-do list or waiting line.

Describe a real-life example for when a dictionary (map) is the best data structure to use.

Think about looking up someone's phone number by their name.

Practice

1. Which data structure should you use if you want to store a list of items where order matters and duplicates are allowed?

easy

A. List or array

B. Set

C. Dictionary

D. Tuple

Solution

1. Step 1: Understand the need for order and duplicates

   Lists and arrays keep the order of items and allow duplicates, which matches the requirement.

2. Step 2: Compare with other structures

   Sets do not allow duplicates, dictionaries store key-value pairs, and tuples are immutable but also keep order.

3. Final Answer:

   List or array -> Option A

4. Quick Check:

   Order + duplicates = List/array [OK]

Hint: Use lists for ordered data with duplicates [OK]

Common Mistakes:

• Choosing set which removes duplicates
• Choosing dictionary which stores key-value pairs
• Confusing tuple immutability with order

2. Which of the following is the correct way to create an empty set in Python?

easy

A. empty_set = {}

B. empty_set = set()

C. empty_set = []

D. empty_set = ()

Solution

1. Step 1: Recall syntax for empty set

   In Python, {} creates an empty dictionary, not a set.

2. Step 2: Identify correct set creation

   Using set() creates an empty set correctly.

3. Final Answer:

   empty_set = set() -> Option B

4. Quick Check:

   Empty set = set() [OK]

Hint: Use set() to create empty sets, {} is a dict [OK]

Common Mistakes:

• Using {} which creates an empty dictionary
• Using [] which creates a list
• Using () which creates a tuple

3. What will be the output of this Python code?

data = {'apple': 3, 'banana': 5, 'orange': 2}
print(data['banana'])

medium

A. 5

B. 3

C. 'banana'

D. KeyError

Solution

1. Step 1: Understand dictionary key-value access

   In the dictionary, 'banana' is a key with value 5.

2. Step 2: Access the value for 'banana'

   Using data['banana'] returns the value 5.

3. Final Answer:

   5 -> Option A

4. Quick Check:

   Dictionary key 'banana' = 5 [OK]

Hint: Dictionary[key] returns the value for that key [OK]

Common Mistakes:

• Confusing key with value
• Expecting the key name as output
• Mistyping key causing KeyError

4. You want to store unique user IDs and quickly check if a user ID exists. Which data structure is best? The code below has an error. Find and fix it.

user_ids = []
user_ids.add(101)
user_ids.add(102)

medium

A. Use dictionary instead of list

B. Use append instead of add: user_ids.append(101)

C. Change list to set: user_ids = set()

D. No error, code is correct

Solution

1. Step 1: Identify the error in method usage

   Lists do not have an add() method; add() is for sets.

2. Step 2: Choose correct data structure for unique items

   Sets store unique items and support add() method, so change list to set.

3. Final Answer:

   Change list to set: user_ids = set() -> Option C

4. Quick Check:

   Unique items + add() = set [OK]

Hint: Use set() for unique items and add() method [OK]

Common Mistakes:

• Using add() on list causing AttributeError
• Using append() but duplicates allowed
• Choosing dictionary unnecessarily

5. You have a list of student names with possible duplicates. You want to count how many times each name appears. Which data structure is best and why?

hard

A. Use a tuple to store names immutably

B. Use a set to store unique names only

C. Use a list to store all names again

D. Use a dictionary to map names to counts

Solution

1. Step 1: Understand the need to count occurrences

   Counting requires storing each name with its count, which is a key-value pair.

2. Step 2: Choose data structure for key-value pairs

   Dictionaries store keys (names) with values (counts), perfect for this task.

3. Final Answer:

   Use a dictionary to map names to counts -> Option D

4. Quick Check:

   Counting items = dictionary [OK]

Hint: Use dictionary for counting items with keys and values [OK]

Common Mistakes:

• Using set which removes duplicates and loses counts
• Using list which doesn't map names to counts
• Using tuple which is immutable and not for counting