Map/collect for transformation in Ruby - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Ruby code using map?

Consider the following Ruby code that uses map to transform an array of numbers. What will be printed?

Ruby

numbers = [1, 2, 3, 4]
result = numbers.map { |n| n * 3 }
puts result.inspect

A [3, 6, 9, 12]

B [1, 2, 3, 4]

C [1, 4, 9, 16]

D nil

💡 Hint

Remember, map creates a new array by applying the block to each element.

The map method applies the block { |n| n * 3 } to each element, multiplying each number by 3. So the output is [3, 6, 9, 12].

❓ Predict Output

intermediate

What does this code output when using collect with a condition?

Look at this Ruby code using collect with a conditional inside the block. What is the output?

Ruby

arr = [5, 10, 15, 20]
result = arr.collect { |x| x > 10 ? x / 5 : x }
puts result.inspect

A [1, 2, 3, 4]

B [5, 10, 3, 4]

C [5, 10, 15, 20]

D [5, 2, 3, 4]

💡 Hint

Check how the ternary operator changes values greater than 10.

The block checks if each element is greater than 10. If yes, it divides by 5; otherwise, it keeps the original value. So 15 becomes 3, 20 becomes 4, others stay the same.

🔧 Debug

advanced

Why does this map code raise an error?

This Ruby code tries to use map to transform an array but raises an error. What is the cause?

Ruby

arr = [1, 2, 3]
result = arr.map do |x|
x * 2
end
result = result + 1
puts result.inspect

A NoMethodError: undefined method `+' for nil:NilClass

B SyntaxError: unexpected end-of-input

C TypeError: no implicit conversion of Integer into Array

D No error, outputs [3, 4, 5]

💡 Hint

Look at the line where result is added to 1.

The map returns an array, but result + 1 tries to add an integer to an array, causing a TypeError.

❓ Predict Output

advanced

What is the output of this nested map transformation?

Given this Ruby code with nested arrays and map, what will be printed?

Ruby

matrix = [[1, 2], [3, 4]]
result = matrix.map { |row| row.map { |n| n + 1 } }
puts result.inspect

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

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

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

D [2, 3, 4, 5]

💡 Hint

Each inner array is transformed by adding 1 to each element.

The outer map goes through each row (array), and the inner map adds 1 to each number. So each number increases by 1.

🧠 Conceptual

expert

How many elements are in the resulting array after this map?

What is the number of elements in the array returned by this Ruby code?

Ruby

arr = [1, 2, 3, 4, 5]
result = arr.map { |x| x if x.even? }
puts result.compact.size

A 3

B 0

C 5

D 2

💡 Hint

Remember that map keeps the same number of elements, but some may be nil.

The map returns an array with elements that are even numbers or nil for odd numbers. After compact, only even numbers remain. There are 2 even numbers (2 and 4), so size is 2.

However, the code prints result.compact.size, which is 2.