Array methods (length, include?, flatten) in Ruby - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Ruby code using length?

Look at the array and the method used. What will be printed?

Ruby

arr = [1, 2, 3, 4, 5]
puts arr.length

A 5

B 4

C 6

D Error

💡 Hint

The length method tells how many items are in the array.

The array has 5 numbers, so length returns 5.

❓ Predict Output

intermediate

What does include? return here?

Check if the array contains the number 3.

Ruby

arr = [1, 2, 3, 4, 5]
puts arr.include?(3)

A false

B Error

C nil

D true

💡 Hint

The method include? returns true if the item is found.

Since 3 is in the array, include?(3) returns true.

❓ Predict Output

advanced

What is the output of flatten on a nested array?

This array has arrays inside. What will flatten do?

Ruby

arr = [1, [2, 3], [4, [5, 6]]]
puts arr.flatten.inspect

A [1, [2, 3], [4, [5, 6]]]

B [1, 2, 3, 4, 5, 6]

C [1, 2, 3, 4, [5, 6]]

D Error

💡 Hint

The flatten method removes all nested arrays and makes one flat array.

All nested arrays are merged into one single array with all elements.

❓ Predict Output

advanced

What is the length of the array after flattening?

Check the length of the array after using flatten.

Ruby

arr = [[1, 2], [3, 4], 5]
flat_arr = arr.flatten
puts flat_arr.length

A 5

B 3

C 4

D Error

💡 Hint

Flattening removes nested arrays, so count all individual elements.

The flattened array is [1, 2, 3, 4, 5], which has length 5.

🧠 Conceptual

expert

Which option causes an error when calling include??

One of these arrays will cause an error when calling include? with argument 3. Which one?

A arr = [1, 2, 3]; arr.include?(3)

B arr = [nil, false]; arr.include?(3)

C arr = nil; arr.include?(3)

D arr = []; arr.include?(3)

💡 Hint

Think about what happens if the variable is not an array.

Calling include? on nil causes a NoMethodError because nil has no such method.

Practice

1. What does the length method do when called on an array in Ruby?

easy

A. Returns the number of elements in the array

B. Checks if an element exists in the array

C. Flattens nested arrays into a single array

D. Removes duplicate elements from the array

Solution

1. Step 1: Understand the purpose of length

   The length method counts how many items are inside an array.

2. Step 2: Compare with other methods

   include? checks for presence, flatten merges nested arrays, so they do different things.

3. Final Answer:

   Returns the number of elements in the array -> Option A

4. Quick Check:

   length = count items [OK]

Hint: Remember length counts items, not checks or changes arrays [OK]

Common Mistakes:

• Confusing length with include? method
• Thinking length flattens nested arrays
• Assuming length removes duplicates

2. Which of the following is the correct syntax to check if the array arr includes the number 5?

easy

A. arr.include(5)

B. arr.includes?(5)

C. arr.has?(5)

D. arr.include?(5)

Solution

1. Step 1: Recall the correct method name and syntax

   The method to check presence is include? with a question mark at the end.

2. Step 2: Verify method call on array

   It should be called as arr.include?(5), not include or includes?.

3. Final Answer:

   arr.include?(5) -> Option D

4. Quick Check:

   Correct method is include? with question mark [OK]

Hint: Remember Ruby methods with ? end with a question mark [OK]

Common Mistakes:

• Omitting the question mark in include?
• Using incorrect method names like includes? or has?
• Using parentheses incorrectly

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

arr = [1, [2, 3], [4, [5]]]
puts arr.flatten.length

medium

A. 5

B. 3

C. 6

D. 7

Solution

1. Step 1: Apply flatten to the array

   The nested array [1, [2, 3], [4, [5]]] becomes [1, 2, 3, 4, 5] after flattening.

2. Step 2: Calculate the length of the flattened array

   The flattened array has 5 elements, so length returns 5.

3. Final Answer:

   5 -> Option A

4. Quick Check:

   Flattened array length = 5 [OK]

Hint: Flatten first, then count elements with length [OK]

Common Mistakes:

• Counting nested arrays as single elements
• Forgetting flatten changes nested arrays
• Miscounting elements after flatten

4. The following code is intended to check if the array nums contains the number 10, but it raises an error. What is the problem?

nums = [5, 10, 15]
if nums.include(10)
  puts "Found 10"
end

medium

A. The array should be flattened before checking

B. The method should be include? with a question mark

C. The number 10 should be a string "10"

D. The if statement syntax is incorrect

Solution

1. Step 1: Identify the method name error

   The method to check presence is include? with a question mark, not include.

2. Step 2: Understand why error occurs

   Calling include without question mark is undefined, causing an error.

3. Final Answer:

   The method should be include? with a question mark -> Option B

4. Quick Check:

   Use include? to check presence [OK]

Hint: Check method names carefully; include? needs question mark [OK]

Common Mistakes:

• Using include without question mark
• Thinking flatten is needed to check presence
• Confusing number and string types
• Miswriting if statement syntax

5. Given the nested array data = [[1, 2], [3, [4, 5]], 6], which Ruby code correctly flattens it and checks if the number 4 is included?

hard

A. data.include?(4.flatten)

B. data.include?(4).flatten

C. data.flatten.include?(4)

D. data.flatten.include(4)

Solution

1. Step 1: Flatten the nested array first

   Use data.flatten to convert nested arrays into a single-level array.

2. Step 2: Check if 4 is included in the flattened array

   Call include?(4) on the flattened array to get true or false.

3. Final Answer:

   data.flatten.include?(4) -> Option C

4. Quick Check:

   Flatten then include? is correct order [OK]

Hint: Flatten first, then check inclusion with include? [OK]

Common Mistakes:

• Calling include? before flatten
• Using include without question mark
• Trying to flatten the number 4
• Incorrect method chaining order