Arrays and lists in Intro to Computing - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is an array?

An array is a collection of items stored at contiguous memory locations. Think of it like a row of mailboxes where each mailbox holds one item and has a number (index) to find it quickly.

beginner

How is a list different from an array?

A list is like a flexible container that can hold items of different types and can grow or shrink in size. An array usually holds items of the same type and has a fixed size.

beginner

What does 'index' mean in arrays and lists?

An index is a number that shows the position of an item in an array or list. It usually starts at 0, so the first item is at index 0, the second at index 1, and so on.

intermediate

Why do arrays have fixed size?

Arrays have fixed size because they reserve a block of memory all at once. This makes accessing items fast but means you can't add more items than the size you set.

beginner

Give a real-life example of a list.

A shopping list is a good example. You can add or remove items anytime, and the list can have different types of things like fruits, bread, or cleaning supplies.

What is the starting index of an array in most programming languages?

A 1

B 0

C -1

D Depends on the language

Most programming languages start array indexes at 0, meaning the first element is at position 0.

Which of these is true about lists compared to arrays?

A Lists have fixed size

B Lists always hold only numbers

C Lists can change size dynamically

D Lists store items in random order

Lists can grow or shrink as needed, unlike arrays which usually have a fixed size.

If you want to access the third item in a list, which index do you use?

A 2

B 3

C 1

D 0

Since indexing starts at 0, the third item is at index 2.

Why might you choose an array over a list?

A When you need fast access and fixed size

B When you want to store different types of data

C When you want to add items anytime

D When you want to store items in no order

Arrays provide fast access because of fixed size and contiguous memory.

Which of these is NOT a characteristic of arrays?

A Fixed size

B Contiguous memory storage

C Indexed access

D Can hold different data types easily

Arrays usually hold items of the same data type, not different types.

Explain what arrays and lists are, using a real-life analogy.

Think about how you organize things at home or in daily life.

Describe the main differences between arrays and lists.

Focus on size and type flexibility.

Practice

1. What is the position of the first item in an array or list?

easy

A. 0

B. 1

C. Depends on the list size

D. Last position

Solution

1. Step 1: Understand list indexing

   Arrays and lists start counting positions from zero, not one.

2. Step 2: Identify the first position

   The first item is always at position 0 in the list.

3. Final Answer:

   0 -> Option A

4. Quick Check:

   First position = 0 [OK]

Hint: Remember counting starts at zero in lists [OK]

Common Mistakes:

• Thinking first position is 1
• Confusing position with size
• Assuming last position is first

2. Which of the following is the correct way to add an item 'apple' to a list named fruits in Python?

easy

A. fruits.insert('apple')

B. fruits.add('apple')

C. fruits.append('apple')

D. fruits.push('apple')

Solution

1. Step 1: Recall Python list methods

   To add an item at the end of a list, Python uses the append() method.

2. Step 2: Match method to syntax

   fruits.append('apple') correctly adds 'apple' to the list.

3. Final Answer:

   fruits.append('apple') -> Option C

4. Quick Check:

   Use append() to add items [OK]

Hint: Use append() to add items at list end [OK]

Common Mistakes:

• Using add() which is for sets
• Using push() which is JavaScript
• Using insert() without position

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

numbers = [10, 20, 30, 40]
print(numbers[2])

medium

A. 30

B. 20

C. 40

D. 10

Solution

1. Step 1: Identify the index used

   The code accesses numbers[2], which means the item at position 2.

2. Step 2: Find the item at index 2

   Positions start at 0: numbers[0]=10, numbers[1]=20, numbers[2]=30.

3. Final Answer:

   30 -> Option A

4. Quick Check:

   Index 2 value = 30 [OK]

Hint: Count from zero to find index value [OK]

Common Mistakes:

• Counting from 1 instead of 0
• Confusing index with value
• Choosing last item by mistake

4. Find the error in this Python code that tries to print the last item of a list named colors:

colors = ['red', 'green', 'blue']
print(colors[3])

medium

A. SyntaxError due to wrong brackets

B. IndexError because index 3 is out of range

C. prints 'blue' correctly

D. TypeError because colors is not a list

Solution

1. Step 1: Check list length and index

   The list has 3 items at indexes 0, 1, and 2. Index 3 is beyond the last item.

2. Step 2: Understand error type

   Accessing index 3 causes an IndexError because it is out of range.

3. Final Answer:

   IndexError because index 3 is out of range -> Option B

4. Quick Check:

   Index out of range = IndexError [OK]

Hint: Last index is length minus one [OK]

Common Mistakes:

• Using wrong index number
• Thinking index 3 is valid
• Confusing error types

5. You have a list of daily temperatures: temps = [22, 19, 24, 21, 20]. How can you create a new list with only temperatures above 20 using Python list comprehension?

hard

A. [temp > 20 for temp in temps]

B. [temp for temp in temps where temp > 20]

C. [temp if temp > 20 for temp in temps]

D. [temp for temp in temps if temp > 20]

Solution

1. Step 1: Understand list comprehension syntax

   The correct syntax is: [expression for item in list if condition].

2. Step 2: Apply condition to filter temps

   Use temp for temp in temps if temp > 20 to select temps above 20.

3. Final Answer:

   [temp for temp in temps if temp > 20] -> Option D

4. Quick Check:

   Filter with if inside comprehension [OK]

Hint: Use if after for in list comprehension [OK]

Common Mistakes:

• Placing if before for
• Using > 20 inside list without if
• Using where instead of if