Compact for removing nil values in Ruby - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of using compact on an array with nils?

Consider the following Ruby code:

arr = [1, nil, 2, nil, 3]
result = arr.compact
puts result.inspect

What will be printed?

Ruby

arr = [1, nil, 2, nil, 3]
result = arr.compact
puts result.inspect

A [1, 2, 3]

B [1, nil, 2, nil, 3]

C [nil, nil]

D Error: undefined method 'compact'

💡 Hint

compact removes all nil values from the array.

The compact method returns a new array without any nil elements. So all nils are removed, leaving only the numbers.

❓ Predict Output

intermediate

What does compact! return when no nils are present?

Look at this Ruby code:

arr = [4, 5, 6]
result = arr.compact!
puts result.inspect

What will be printed?

Ruby

arr = [4, 5, 6]
result = arr.compact!
puts result.inspect

A []

B [4, 5, 6]

C nil

D Error: undefined method 'compact!'

💡 Hint

compact! returns nil if no changes were made.

The compact! method modifies the array in place and returns nil if no nil elements were removed.

🔧 Debug

advanced

Why does this code raise an error?

This Ruby code is intended to remove nils from a hash's values:

h = {a: 1, b: nil, c: 3}
h.compact

But it raises an error. Why?

Ruby

h = {a: 1, b: nil, c: 3}
h.compact

A compact removes keys with nil values automatically

B Hash does not have a compact method in Ruby 3.12

C compact only works on arrays, not hashes

D compact requires a block for hashes

💡 Hint

Check if Hash class supports compact method in this Ruby version.

In Ruby 2.4 and later, Hash#compact is available, but if the environment is older or the method is undefined, calling compact on a hash raises NoMethodError.

❓ Predict Output

advanced

What is the output of chaining compact with map?

What does this Ruby code print?

arr = [nil, 2, nil, 4]
result = arr.compact.map { |x| x * 3 }
puts result.inspect

Ruby

arr = [nil, 2, nil, 4]
result = arr.compact.map { |x| x * 3 }
puts result.inspect

A [6, 12]

B [nil, 6, nil, 12]

C [2, 4]

D Error: undefined method 'map' for nil

💡 Hint

compact removes nils before map multiplies each number by 3.

compact removes nils, leaving [2,4]. Then map multiplies each by 3, resulting in [6,12].

🧠 Conceptual

expert

How many items remain after compact on nested arrays?

Given this Ruby code:

arr = [1, nil, [2, nil], [nil, 3], nil]
result = arr.compact
puts result.size

What number will be printed?

Ruby

arr = [1, nil, [2, nil], [nil, 3], nil]
result = arr.compact
puts result.size

A 2

B 5

C 4

D 3

💡 Hint

compact removes only top-level nils, not inside nested arrays.

compact removes the nil elements at the top level, so the array becomes [1, [2, nil], [nil, 3]]. This has 3 elements.