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.

Practice

1. What does the Ruby method flat_map do when applied to an array of arrays?

easy

A. It removes duplicate elements from the array.

B. It only flattens the array without applying any block.

C. It applies a block to each element and flattens the result by one level.

D. It maps elements but keeps the nested structure intact.

Solution

1. Step 1: Understand flat_map behavior

   The flat_map method applies a block to each element of the array.

2. Step 2: Flatten the mapped results

   After mapping, it flattens the resulting array by one level, combining nested arrays into a single array.

3. Final Answer:

   It applies a block to each element and flattens the result by one level. -> Option C

4. Quick Check:

   flat_map = map + flatten(1) [OK]

Hint: Remember: flat_map = map + flatten one level [OK]

Common Mistakes:

• Thinking flat_map flattens all nested levels
• Confusing flat_map with flatten
• Assuming flat_map does not apply a block

2. Which of the following is the correct syntax to use flat_map on an array arr to double each number?

easy

A. arr.flat_map do x * 2 end

B. arr.flat_map { |x| [x * 2] }

C. arr.flat_map { x * 2 }

D. arr.flat_map(x) { x * 2 }

Solution

1. Step 1: Understand flat_map block return

   The block given to flat_map should return an array or enumerable to flatten.

2. Step 2: Check each option's block return

   arr.flat_map { |x| [x * 2] } returns an array [x * 2], which flat_map will flatten by one level, producing a flat array of doubled values.

3. Final Answer:

   arr.flat_map { |x| [x * 2] } -> Option B

4. Quick Check:

   Block must return array for flat_map flattening [OK]

Hint: Return an array inside block for flat_map to flatten [OK]

Common Mistakes:

• Returning a single value instead of an array in block
• Using incorrect block syntax
• Passing arguments incorrectly to flat_map

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

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

medium

A. [2, 4, 6, 8]

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

C. [1, 2, 3, 4]

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

Solution

1. Step 1: Inner map doubles each element

   For each sub-array, sub.map { |x| x * 2 } creates a new array with doubled values: [2,4] and [6,8].

2. Step 2: flat_map flattens one level

   flat_map combines these arrays into a single flat array: [2, 4, 6, 8].

3. Final Answer:

   [2, 4, 6, 8] -> Option A

4. Quick Check:

   flat_map flattens mapped arrays one level [OK]

Hint: flat_map flattens one level after mapping [OK]

Common Mistakes:

• Expecting nested arrays in output
• Confusing map with flat_map output
• Ignoring inner map doubling

4. Identify the error in this Ruby code snippet:

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

medium

A. flat_map cannot be called on nested arrays.

B. The variable 'result' is not assigned properly.

C. The syntax of the block is incorrect.

D. flat_map block returns integers, not arrays, so flattening fails.

Solution

1. Step 1: Analyze block return values

   The block { |x| x * 2 } returns integers, not arrays or enumerables.

2. Step 2: Understand flat_map flattening requirement

   flat_map expects the block to return arrays or enumerables to flatten one level; returning integers causes flattening to fail or unexpected results.

3. Final Answer:

   flat_map block returns integers, not arrays, so flattening fails. -> Option D

4. Quick Check:

   Block must return arrays for flat_map flattening [OK]

Hint: Block must return arrays, not single values, for flat_map [OK]

Common Mistakes:

• Returning single values instead of arrays in block
• Assuming flat_map works like map without flattening
• Misunderstanding flat_map usage on nested arrays

5. Given a nested array of words grouped by their first letter, how can you use flat_map to get a flat array of all words in uppercase?

words = [['apple', 'apricot'], ['banana', 'blueberry'], ['cherry']]
# Fill in the blank:
result = words.flat_map { |group| ________ }
puts result.inspect

hard

A. group.map { |w| w.upcase }

B. group.upcase

C. group.map { w.upcase }

D. group.flat_map { w.upcase }

Solution

1. Step 1: Map each word to uppercase inside each group

   Use group.map { |w| w.upcase } to convert each word in the sub-array to uppercase.

2. Step 2: Use flat_map to flatten the mapped arrays

   flat_map will flatten the arrays of uppercase words into a single flat array.

3. Final Answer:

   group.map { |w| w.upcase } -> Option A

4. Quick Check:

   flat_map + map for nested uppercase flattening [OK]

Hint: Use map inside flat_map block to transform and flatten [OK]

Common Mistakes:

• Calling upcase on array instead of strings
• Using flat_map inside block unnecessarily
• Returning non-array values from block