Array Declaration and Initialization in DSA Python - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is an array in simple terms?

An array is like a row of boxes where each box can hold a value. All boxes are the same size and arranged one after another.

beginner

How do you declare an array of 5 integers in Python?

You can declare it using a list like this: arr = [0, 0, 0, 0, 0]. This creates 5 boxes holding 0 initially.

beginner

What does 'initialization' mean when working with arrays?

Initialization means putting starting values into the array boxes right after creating them.

beginner

Show how to declare and initialize an array with values 1, 2, 3, 4, 5 in Python.

Use arr = [1, 2, 3, 4, 5]. This creates an array with 5 boxes holding those numbers.

beginner

Can arrays in Python change size after creation?

Yes, Python lists (arrays) can grow or shrink by adding or removing elements, unlike fixed-size arrays in some other languages.

Which of these is a correct way to declare an array of 3 zeros in Python?

A arr = {0, 0, 0}

B arr = (0, 0, 0)

C arr = [0, 0, 0]

D arr = [0] * 3

Option C creates a list with three zeros. Option A is a set, Option B is a tuple, and D creates a list by repeating 0 three times.

What is the index of the first element in a Python array?

A 1

B 0

C -1

D Depends on the array size

Python arrays (lists) start indexing from 0, so the first element is at index 0.

How do you create an array with values 5, 10, 15 in Python?

A arr = [5, 10, 15]

B arr = (5, 10, 15)

C arr = {5, 10, 15}

D arr = 5, 10, 15

Option A creates a list (array) with the values. B is a tuple, C is a set, and D is a tuple without parentheses.

What happens if you try to access an index outside the array size in Python?

A You get an error

B It returns None

C It returns 0

D It returns the last element

Accessing an index outside the array size causes an IndexError in Python.

Which of these is NOT a way to initialize an array in Python?

A arr = [1, 2, 3]

B arr = list((1, 2, 3))

C arr = {1, 2, 3}

D arr = [0] * 3

Option C creates a set, not a list (array). The others create lists.

Explain how to declare and initialize an array with 4 elements in Python.

Think about using square brackets and putting values inside.

Describe what happens when you try to access an array element by index in Python.

Remember how Python counts positions and what happens if you go too far.

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