Flat_map for nested flattening in Ruby - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of flat_map with nested arrays

What is the output of this Ruby code using flat_map to flatten nested arrays?

Ruby

nested = [[1, 2], [3, 4], [5]]
result = nested.flat_map { |arr| arr }
puts result.inspect

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

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

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

D [[1, 2], [3, 4], [5]]

💡 Hint

Remember that flat_map maps and then flattens one level.

The flat_map method applies the block to each element and then flattens the result by one level. Here, each element is an array, and the block returns it as is, so the result is a single flattened array with all elements.

❓ Predict Output

intermediate

Using flat_map with transformation

What is the output of this Ruby code that uses flat_map to transform and flatten nested arrays?

Ruby

nested = [[1, 2], [3, 4], [5]]
result = nested.flat_map { |arr| arr.map { |x| x * 2 } }
puts result.inspect

A [2, 4, 6, 8, 10]

B [[2, 4], [6, 8], [10]]

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

D [[1, 4], [9, 16], [25]]

💡 Hint

The block doubles each number, then flat_map flattens the result.

The inner map doubles each number in the sub-arrays. Then flat_map flattens the resulting arrays into one array.

🔧 Debug

advanced

Result of incorrect flat_map usage with each

What is the output of this Ruby code when using flat_map incorrectly?

Ruby

nested = [[1, 2], [3, 4], [5]]
result = nested.flat_map do |arr|
  arr.each do |x|
    x * 2
  end
end
puts result.inspect

A SyntaxError: unexpected end-of-input

B NoMethodError: undefined method `flat_map' for Array

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

D TypeError: no implicit conversion of nil into Array

💡 Hint

Check what the block returns for each element when using flat_map.

The each method iterates but returns the original sub-array arr. The x * 2 expressions are evaluated but their values are discarded. Thus, the block returns arr, and flat_map flattens the nested array without transformation into [1, 2, 3, 4, 5].

🧠 Conceptual

advanced

Difference between map and flat_map

Which statement best describes the difference between map and flat_map in Ruby?

A <code>map</code> flattens arrays; <code>flat_map</code> only transforms elements without flattening.

B <code>map</code> transforms elements and returns an array of arrays; <code>flat_map</code> transforms and flattens one level.

C <code>map</code> and <code>flat_map</code> are identical in behavior.

D <code>flat_map</code> only works on flat arrays; <code>map</code> works on nested arrays.

💡 Hint

Think about what happens after the transformation in each method.

map applies the block and returns an array of the results, which can be nested arrays. flat_map applies the block and then flattens the result by one level, combining nested arrays into a single array.

❓ Predict Output

expert

Complex nested flattening with flat_map

What is the output of this Ruby code that uses flat_map twice to flatten a deeply nested array?

Ruby

nested = [[[1, 2], [3]], [[4, 5]], [6]]
result = nested.flat_map { |arr| arr.flat_map { |x| x } }
puts result.inspect

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

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

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

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

💡 Hint

Each flat_map flattens one level after mapping.

The first flat_map maps each sub-array and flattens one level, then the inner flat_map flattens the next level. Together they flatten the nested arrays into a single flat array.