Flat_map for nested flattening in Ruby - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to flatten the nested arrays using flat_map.

Ruby

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

Drag options to blanks, or click blank then click option'

Aselect

Bflat_map

Ceach

Dmap

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'map' only returns nested arrays without flattening.

Using 'each' does not return a new array.

Using 'select' filters elements but does not flatten.

The flat_map method maps and flattens one level of nested arrays, producing a single flat array.

2fill in blank

medium

Complete the code to flatten and double each number in the nested arrays.

Ruby

nested = [[1, 2], [3, 4], [5, 6]]
result = nested.[1] { |arr| arr.map { |n| n * 2 } }
puts result.inspect

Drag options to blanks, or click blank then click option'

Aflat_map

Bmap

Ceach

Dselect

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'map' results in nested arrays after doubling.

Using 'each' does not return a new array.

Using 'select' filters elements but does not transform or flatten.

flat_map applies the block to each sub-array, then flattens the result into a single array.

3fill in blank

hard

Fix the error in the code to flatten nested arrays of strings into a single array of characters.

Ruby

words = [["hi", "there"], ["ruby", "code"]]
chars = words.[1] { |word_arr| word_arr.map(&:chars) }
puts chars.flatten.inspect

Drag options to blanks, or click blank then click option'

Aflat_map

Bmap

Ceach

Dselect

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'map' returns nested arrays requiring extra flattening.

Using 'each' does not return a new array.

Using 'select' filters but does not flatten.

Using flat_map here flattens one level after mapping, so you don't need to call flatten separately.

4fill in blank

hard

Fill both blanks to flatten and convert nested arrays of numbers to strings.

Ruby

nested = [[1, 2], [3, 4], [5, 6]]
result = nested.[1] { |arr| arr.[2](&:to_s) }
puts result.inspect

Drag options to blanks, or click blank then click option'

Aflat_map

Bmap

Ceach

Dselect

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'map' instead of 'flat_map' on the outer array keeps nesting.

Using 'each' instead of 'map' inside does not transform elements.

flat_map flattens the outer array after mapping, and map converts each number to a string.

5fill in blank

hard

Fill all three blanks to flatten nested arrays and select only even numbers as strings.

Ruby

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

Drag options to blanks, or click blank then click option'

Aflat_map

Bselect

Ceven?

Dmap

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'map' instead of 'flat_map' keeps nested arrays.

Using 'map' instead of 'select' filters incorrectly.

Using wrong condition instead of 'even?'.

flat_map flattens the array, select filters even numbers, and even? checks if a number is even.

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