Array creation methods in Ruby - Cheat Sheet & Quick Revision

Recall & Review

beginner

How do you create an empty array in Ruby?

You create an empty array by using square brackets with nothing inside: [].

beginner

What does Array.new(3) do in Ruby?

It creates a new array with 3 elements, all set to nil by default: [nil, nil, nil].

beginner

How can you create an array with 5 zeros using Array.new?

Use Array.new(5, 0). This creates an array with 5 elements, each set to 0: [0, 0, 0, 0, 0].

intermediate

What is the difference between Array.new(3, []) and Array.new(3) { [] }?

The first creates an array with 3 references to the same empty array (shared object). The second creates 3 separate empty arrays (different objects).

beginner

How do you create an array from a range in Ruby?

Use the to_a method on a range. For example, (1..5).to_a creates [1, 2, 3, 4, 5].

What does Array.new(4) return?

A An error

B [0, 0, 0, 0]

C [nil, nil, nil, nil]

D [[], [], [], []]

Array.new(4) creates an array with 4 elements, all set to nil by default.

How do you create an array of 3 empty arrays, each a separate object?

A Array.new(3, [])

B Array.new(3) { [] }

C [[], [], []]

D Both B and C

Using a block with Array.new(3) { [] } or writing [[], [], []] creates separate empty arrays.

What does (5..8).to_a produce?

A [5, 6, 7, 8]

B [5, 6, 7]

C [6, 7, 8]

D An error

The range 5..8 includes 5, 6, 7, and 8. to_a converts it to an array with these elements.

What is the result of Array.new(3, 'a')?

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

B ['a', 'a', 'a']

C [nil, nil, nil]

D An error

It creates an array with 3 elements, each set to the string 'a'.

Which method creates an empty array?

A All of the above

B Array.new(0)

C Array.new

D []

All these create an empty array in Ruby.

Explain three different ways to create arrays in Ruby and when you might use each.

Think about empty arrays, arrays with repeated values, and arrays with unique objects.

Describe the difference between Array.new(3, []) and Array.new(3) { [] } in Ruby.

Consider what happens if you change one element inside the arrays.

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