Array Declaration and Initialization in DSA Python - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare an empty list named arr.

DSA Python

arr = [1]

Drag options to blanks, or click blank then click option'

A{}

B()

C[]

Darray()

Attempts:

3 left

💡 Hint

Common Mistakes

Using curly braces {} which creates a dictionary instead of a list.

Using parentheses () which creates a tuple instead of a list.

In Python, an empty list is declared using square brackets [].

2fill in blank

medium

Complete the code to create a list numbers with elements 1, 2, and 3.

DSA Python

numbers = [1]

Drag options to blanks, or click blank then click option'

A[1, 2, 3]

B(1, 2, 3)

C{1, 2, 3}

D1, 2, 3

Attempts:

3 left

💡 Hint

Common Mistakes

Using parentheses which creates a tuple, not a list.

Using curly braces which creates a set, not a list.

Lists with elements are created using square brackets with elements separated by commas.

3fill in blank

hard

Fix the error in the code to initialize a list letters with 'a', 'b', 'c'.

DSA Python

letters = [1]

Drag options to blanks, or click blank then click option'

A['a', 'b', 'c']

B('a', 'b', 'c')

C{'a', 'b', 'c'}

D'a', 'b', 'c'

Attempts:

3 left

💡 Hint

Common Mistakes

Using parentheses which creates a tuple instead of a list.

Using curly braces which creates a set instead of a list.

Lists use square brackets. Parentheses create tuples, curly braces create sets.

4fill in blank

hard

Fill both blanks to create a list evens with even numbers from 2 to 10.

DSA Python

evens = [[1] for [2] in range(2, 11, 2)]

Drag options to blanks, or click blank then click option'

Ax

Bi

Cx*2

Attempts:

3 left

💡 Hint

Common Mistakes

Using a variable in the output that is not defined in the loop.

Confusing the loop variable name.

The list comprehension uses i as both the output expression and loop variable since the range directly generates the even numbers.

5fill in blank

hard

Fill all three blanks to create a list squares of squares of numbers from 1 to 5.

DSA Python

squares = [[1] for [2] in [3](1, 6)]

Drag options to blanks, or click blank then click option'

Anum**2

Bnum

Crange

Dsquare

Attempts:

3 left

💡 Hint

Common Mistakes

Using a function other than range for iteration.

Using a variable name not consistent in output and loop.

The list comprehension squares each number num in the range from 1 to 5.

Practice

1. Which of the following is the correct way to declare and initialize an array of integers with values 1, 2, and 3 in Python?

easy

A. arr = (1, 2, 3)

B. arr = [1, 2, 3]

C. arr = {1, 2, 3}

D. arr = int[3]

Solution

1. Step 1: Understand Python array syntax and check each option

   In Python, arrays are usually represented by lists, which use square brackets []. arr = [1, 2, 3] uses square brackets with values 1, 2, 3 which is correct. arr = (1, 2, 3) uses parentheses which create a tuple, not an array. arr = {1, 2, 3} uses curly braces which create a set, not an array. arr = int[3] is invalid syntax in Python.

2. Final Answer:

   arr = [1, 2, 3] -> Option B

3. Quick Check:

   List syntax = [1, 2, 3] [OK]

Hint: Use square brackets [] for arrays (lists) in Python [OK]

Common Mistakes:

• Using parentheses () instead of square brackets
• Using curly braces {} which create sets
• Trying to declare array size like int[3]

2. Which of the following is the correct syntax to create an empty list in Python?

easy

A. list = ()

B. list = {}

C. list = []

D. list = empty()

Solution

1. Step 1: Identify empty list syntax and evaluate options

   In Python, an empty list is created using empty square brackets []. list = [] uses [], which is correct. list = () uses parentheses which create an empty tuple. list = {} uses curly braces which create an empty dictionary. list = empty() is invalid syntax.

2. Final Answer:

   list = [] -> Option C

3. Quick Check:

   Empty list = [] [OK]

Hint: Empty list always uses [] brackets [OK]

Common Mistakes:

• Using () which creates a tuple
• Using {} which creates a dictionary
• Trying to call empty() function which doesn't exist

3. What will be the output of the following code?

arr = [0] * 4
print(arr)

medium

A. Error

B. [0, 1, 2, 3]

C. [4, 4, 4, 4]

D. [0, 0, 0, 0]

Solution

1. Step 1: Understand list multiplication and predict the output

   Multiplying a list by an integer repeats the list elements that many times. Here, [0] * 4 creates a list with four zeros. The print statement outputs the list: [0, 0, 0, 0].

2. Final Answer:

   [0, 0, 0, 0] -> Option D

3. Quick Check:

   [0] * 4 = [0, 0, 0, 0] [OK]

Hint: List * n repeats elements n times [OK]

Common Mistakes:

• Expecting a range of numbers instead of repeated zeros
• Confusing list multiplication with addition
• Thinking it causes an error

4. Find the error in the following code snippet:

arr = [1, 2, 3]
arr[3] = 4
print(arr)

medium

A. IndexError: list assignment index out of range

B. SyntaxError

C. [1, 2, 3, 4]

D. [4, 2, 3]

Solution

1. Step 1: Check list indices and analyze assignment at index 3

   List indices start at 0. For a list of length 3, valid indices are 0, 1, 2. Trying to assign arr[3] = 4 is invalid because index 3 does not exist yet, causing an IndexError.

2. Final Answer:

   IndexError: list assignment index out of range -> Option A

3. Quick Check:

   Index 3 invalid for length 3 list [OK]

Hint: List indices go from 0 to length-1 [OK]

Common Mistakes:

• Trying to assign to an index outside the list
• Confusing append with direct index assignment
• Expecting automatic list resizing

5. You want to create a 2D array (list of lists) with 3 rows and 4 columns, all initialized to zero. Which code correctly does this without causing unexpected behavior?

hard

A. arr = [[0 for _ in range(4)] for _ in range(3)]
print(arr)

B. arr = [[0]*4]*3
print(arr)

C. arr = [0]*3
for i in range(4):
arr[i] = [0]*4
print(arr)

D. arr = [[0]*3 for _ in range(4)]
print(arr)

Solution

1. Step 1: Understand 2D array creation pitfalls and analyze each option

   Using [[0]*4]*3 creates 3 references to the same inner list, causing changes in one row to affect others.

   arr = [[0]*4]*3
   print(arr)

   creates shared inner lists (bad).

   arr = [[0 for _ in range(4)] for _ in range(3)]
   print(arr)

   uses nested list comprehension creating independent inner lists (correct).

   arr = [0]*3
   for i in range(4):
       arr[i] = [0]*4
   print(arr)

   has index errors and wrong loop ranges.

   arr = [[0]*3 for _ in range(4)]
   print(arr)

   creates 4 rows and 3 columns, which is reversed.

2. Final Answer:

   arr = [[0 for _ in range(4)] for _ in range(3)] -> Option A

3. Quick Check:

   Nested list comprehension avoids shared references [OK]

Hint: Use nested list comprehension for independent inner lists [OK]

Common Mistakes:

• Using multiplication for nested lists causing shared references
• Mixing up rows and columns sizes
• Index errors in loops when initializing