Arrays and lists in Intro to Computing - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ trace

intermediate

Trace the final list after operations

Given the following steps on a list, what is the final list?

my_list = [1, 2, 3, 4]
my_list.append(5)
my_list.pop(1)
my_list.insert(2, 10)

Intro to Computing

my_list = [1, 2, 3, 4]
my_list.append(5)
my_list.pop(1)
my_list.insert(2, 10)

A [1, 2, 10, 3, 4, 5]

B [1, 10, 3, 4, 5]

C [1, 3, 4, 10, 5]

D [1, 3, 10, 4, 5]

💡 Hint

Remember that pop removes the item at the given index and append adds to the end.

Starting list: [1, 2, 3, 4]. After append(5): [1, 2, 3, 4, 5]. pop(1) removes element at index 1 (which is 2), so list becomes [1, 3, 4, 5]. insert(2, 10) adds 10 at index 2, resulting in [1, 3, 10, 4, 5].

🧠 Conceptual

intermediate

Understanding list indexing and slicing

What will be the output of the following slicing operation?

numbers = [10, 20, 30, 40, 50, 60]
print(numbers[1:5:2])

Intro to Computing

numbers = [10, 20, 30, 40, 50, 60]
print(numbers[1:5:2])

A [20, 30, 40, 50]

B [20, 40]

C [20, 40, 60]

D [10, 30, 50]

💡 Hint

Slicing syntax is list[start:stop:step]. It includes start index but excludes stop index.

numbers[1:5:2] means start at index 1 (20), go up to but not including index 5 (which is 60), stepping by 2. So elements at index 1 and 3 are selected: 20 and 40.

❓ Comparison

advanced

Compare list and array behavior

Which statement correctly describes a key difference between arrays and lists in programming?

A Arrays can only store elements of the same type, while lists can store elements of different types.

B Lists require fixed size at creation, arrays can grow dynamically.

C Arrays support appending elements at any position, lists do not.

D Lists are stored in contiguous memory locations, arrays are not.

💡 Hint

Think about type restrictions and flexibility of these data structures.

Arrays typically store elements of the same type for efficient memory use. Lists are more flexible and can hold different types of elements.

❓ identification

advanced

Identify the error in list operation

What error will this code produce?

my_list = [1, 2, 3]
print(my_list[5])

Intro to Computing

my_list = [1, 2, 3]
print(my_list[5])

A IndexError: list index out of range

B TypeError: list indices must be integers

C ValueError: invalid index

D SyntaxError: invalid syntax

💡 Hint

Check if the index 5 exists in the list of length 3.

The list has indices 0, 1, and 2. Accessing index 5 is out of range, causing an IndexError.

🚀 Application

expert

Determine the number of elements after nested list flattening

Given the nested list below, how many elements will the flattened list contain?

nested = [[1, 2], [3, 4, 5], [6], []]

Assuming flattening means combining all inner elements into one list.

Intro to Computing

nested = [[1, 2], [3, 4, 5], [6], []]

A 7

B 5

C 6

D 8

💡 Hint

Count all elements inside each inner list and sum them.

Inner lists have lengths 2, 3, 1, and 0 respectively. Total elements = 2 + 3 + 1 + 0 = 6.

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