Array creation methods in Ruby - Time & Space Complexity

Time Complexity: Array creation methods

O(n)

Understanding Time Complexity

When we create arrays in Ruby, the time it takes can change depending on how we do it.

We want to know how the time grows as the array gets bigger.

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

arr = Array.new(n) { |i| i * 2 }

This code creates an array of size n, filling each element with twice its index.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: Filling each element of the array by running the block once per element.
• How many times: Exactly n times, once for each element.

How Execution Grows With Input

As the array size n grows, the number of times the block runs grows the same way.

Input Size (n)	Approx. Operations
10	10
100	100
1000	1000

Pattern observation: The operations increase directly with the size of the array.

Final Time Complexity

Time Complexity: O(n)

This means the time to create the array grows in a straight line with the number of elements.

Common Mistake

[X] Wrong: "Creating an array with a block is instant and does not depend on size."

[OK] Correct: Each element is set by running the block, so the time grows with the number of elements.

Interview Connect

Understanding how array creation time grows helps you write efficient code and explain your choices clearly in interviews.

Self-Check

"What if we create an array by repeating the same value instead of using a block? How would the time complexity change?"

Practice

1. Which of the following is a correct way to create an empty array in Ruby?

easy

A. array()

B. []

C. {}

D. new Array()

Solution

1. Step 1: Understand Ruby array syntax

   In Ruby, arrays are created using square brackets [] for literals.

2. Step 2: Identify correct empty array syntax

   [] creates an empty array, while {} creates an empty hash, and the other options are not Ruby syntax.

3. Final Answer:

   [] -> Option B

4. Quick Check:

   Empty array = [] [OK]

Hint: Empty arrays use square brackets [] in Ruby [OK]

Common Mistakes:

• Using curly braces {} which creates a hash
• Using parentheses like array() which is invalid
• Using JavaScript-like syntax new Array()

2. Which of the following is the correct syntax to create an array with 3 elements, all initialized to zero?

easy

A. Array.new(3, 0)

B. Array.new(0, 3)

C. Array(0, 3)

D. new Array(3, 0)

Solution

1. Step 1: Recall Array.new syntax

   In Ruby, Array.new(size, default_value) creates an array with given size and fills it with the default value.

2. Step 2: Check each option

   Array.new(3, 0) correctly creates [0, 0, 0]. Array.new(0, 3) creates an empty array since size is first argument. Options C and D are invalid syntax.

3. Final Answer:

   Array.new(3, 0) -> Option A

4. Quick Check:

   Array.new(3, 0) = [0, 0, 0] [OK]

Hint: Use Array.new(size, value) to create arrays with default values [OK]

Common Mistakes:

• Confusing Array.new arguments order
• Using invalid syntax like Array(0, 3)
• Trying JavaScript style new Array()

3. What is the output of the following Ruby code?

arr = Array.new(4) { |i| i * 2 }
puts arr.inspect

medium

A. [nil, nil, nil, nil]

B. [2, 4, 6, 8]

C. [0, 2, 4, 6]

D. [0, 0, 0, 0]

Solution

1. Step 1: Understand Array.new with block

   When Array.new is called with a block, it runs the block for each index, passing the index as i.

2. Step 2: Calculate each element

   For indices 0 to 3, the block returns i * 2: 0*2=0, 1*2=2, 2*2=4, 3*2=6.

3. Final Answer:

   [0, 2, 4, 6] -> Option C

4. Quick Check:

   Array with block fills elements by index * 2 [OK]

Hint: Array.new with block uses index to set each element [OK]

Common Mistakes:

• Assuming default value 0 for all elements
• Confusing block argument meaning
• Expecting nil values without block

4. The following code is intended to create an array of 3 separate empty arrays. What is the problem?

arr = Array.new(3, [])
arr[0] << 1
puts arr.inspect

medium

A. [[1], [1], [1]]

B. [[1], [], []]

C. [[], [], []]

D. SyntaxError

Solution

1. Step 1: Understand Array.new with default object

   Using Array.new(3, []) creates 3 references to the same empty array, not 3 separate arrays.

2. Step 2: Analyze mutation effect

   When arr[0] << 1 runs, it adds 1 to the shared array, so all elements show the change.

3. Final Answer:

   [[1], [1], [1]] -> Option A

4. Quick Check:

   Same object repeated causes all elements to change [OK]

Hint: Use block with Array.new for separate objects [OK]

Common Mistakes:

• Expecting separate arrays but all share one object
• Not using block form for unique elements
• Assuming << affects only one element

5. How can you create an array of 5 hashes, each with keys :a and :b initialized to 0, ensuring each hash is a separate object?

hard

A. Array(5) { {a: 0, b: 0} }

B. Array.new(5, {a: 0, b: 0})

C. [{a: 0, b: 0}] * 5

D. Array.new(5) { {a: 0, b: 0} }

Solution

1. Step 1: Understand object references in Array.new

   Using Array.new(5, {a: 0, b: 0}) or [{a: 0, b: 0}] * 5 repeats the same hash object 5 times.

2. Step 2: Use block form for unique hashes

   Using Array.new(5) { {a: 0, b: 0} } runs the block 5 times, creating 5 separate hashes.

3. Final Answer:

   Array.new(5) { {a: 0, b: 0} } -> Option D

4. Quick Check:

   Block form creates unique objects [OK]

Hint: Use block with Array.new for unique hashes [OK]

Common Mistakes:

• Using Array.new with default hash causes shared references
• Using multiplication operator with arrays of hashes
• Trying invalid Array() constructor syntax