Reduce/inject for accumulation in Ruby - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Ruby code using inject?

Consider the following Ruby code that uses inject to accumulate values. What will be printed?

Ruby

numbers = [1, 2, 3, 4]
result = numbers.inject(0) { |sum, n| sum + n * 2 }
puts result

A 24

B 10

C 20

D 16

💡 Hint

Remember that inject starts with the initial value and adds each element multiplied by 2.

The inject starts at 0. Then it adds 1*2=2, then 2*2=4, then 3*2=6, then 4*2=8. Sum is 2+4+6+8=20.

❓ Predict Output

intermediate

What does this inject code return?

What is the value of result after running this Ruby code?

Ruby

words = ['a', 'b', 'c']
result = words.inject('') { |acc, w| acc + w.upcase }
puts result

A abc

B Error

C aBc

D ABC

💡 Hint

Each word is converted to uppercase before adding to the accumulator.

The accumulator starts as an empty string. Each word is upcased and concatenated: '' + 'A' + 'B' + 'C' = 'ABC'.

❓ Predict Output

advanced

What is the output of this inject with a hash accumulator?

Given this Ruby code, what will be printed?

Ruby

arr = [1, 2, 3, 4]
result = arr.inject(Hash.new(0)) do |hash, n|
  hash[n % 2] += 1
  hash
end
puts result

A {0=>2, 1=>2}

B {0=>1, 1=>3}

C {0=>3, 1=>1}

D TypeError

💡 Hint

Count how many numbers are even (key 0) and odd (key 1).

Numbers 2 and 4 are even (key 0), count 2. Numbers 1 and 3 are odd (key 1), count 2.

❓ Predict Output

advanced

What does this inject code output when accumulating arrays?

What will be the output of this Ruby code?

Ruby

arrays = [[1, 2], [3, 4], [5]]
result = arrays.inject([]) { |acc, arr| acc + arr }
puts result.inspect

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

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

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

D TypeError

💡 Hint

The + operator concatenates arrays in Ruby.

Starting with empty array, each sub-array is concatenated: [] + [1,2] = [1,2], then + [3,4] = [1,2,3,4], then + [5] = [1,2,3,4,5].

❓ Predict Output

expert

What is the final value of result after this complex inject?

Analyze this Ruby code using inject with a nested block and determine the final value of result.

Ruby

data = [2, 3, 4]
result = data.inject({}) do |hash, n|
  hash[n] = (1..n).inject(1) { |prod, x| prod * x }
  hash
end
puts result.inspect

A NoMethodError

B {2=>2, 3=>6, 4=>24}

C {2=>2, 3=>3, 4=>4}

D {2=>1, 3=>6, 4=>24}

💡 Hint

The inner inject calculates factorial for each number.

For each n, inner inject calculates factorial: 2! = 2, 3! = 6, 4! = 24. These are stored as values in the hash with keys n.