Array creation methods in Ruby - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Ruby code creating an array?

Consider the following Ruby code that creates an array. What will be printed?

Ruby

arr = Array.new(3, 'cat')
arr[0].upcase!
puts arr.inspect

A ["cat", "cat", "cat"]

B ["CAT", "CAT", "CAT"]

C ["CAT", "cat", "cat"]

D ["cat", "CAT", "cat"]

💡 Hint

Think about how Array.new with a single object argument shares the same object.

Using Array.new(3, 'cat') creates an array with three references to the same string object. When upcase! modifies the first element, it changes the shared string, so all elements show the change.

❓ Predict Output

intermediate

What does this Ruby array creation code output?

What will be the output of this Ruby code snippet?

Ruby

arr = Array.new(3) { 'dog' }
arr[0].upcase!
puts arr.inspect

A ["DOG", "dog", "dog"]

B ["DOG", "DOG", "DOG"]

C ["dog", "dog", "dog"]

D ["dog", "DOG", "dog"]

💡 Hint

Consider how the block form of Array.new creates new objects.

Using Array.new(3) { 'dog' } creates a new string object for each element. So modifying one element does not affect the others.

🧠 Conceptual

advanced

Which Ruby array creation method creates independent objects?

Which of the following Ruby array creation methods ensures each element is a separate object?

A Array.new(4, [])

B Array.new(4)

C Array.new(4) { [] }

D Array.new(4, nil)

💡 Hint

Think about how the block form works compared to the single object argument.

Array.new(4) { [] } calls the block 4 times, creating a new empty array each time. The other methods either share the same object or fill with nil.

❓ Predict Output

advanced

What is the output of this Ruby array creation and modification?

What will this Ruby code print?

Ruby

arr = Array.new(2) { |i| i * 2 }
arr[1] = 10
puts arr.inspect

A [0, 10]

B [0, 1]

C [0, 2]

D [2, 10]

💡 Hint

Remember the block argument is the index of the element.

The block { |i| i * 2 } creates [0, 2]. Then arr[1] = 10 changes the second element to 10.

🔧 Debug

expert

Why does this Ruby array creation code raise an error?

This Ruby code raises an error. What is the cause?

Ruby

arr = Array.new(3) { |i| i + 'a' }
puts arr.inspect

A NameError because 'i' is undefined

B SyntaxError due to missing parentheses

C No error, outputs ["0a", "1a", "2a"]

D TypeError because you cannot add an integer and a string

💡 Hint

Check the operation inside the block carefully.

The block tries to add an integer i to a string 'a', which causes a TypeError in Ruby.

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