Ruby Academy · Lesson

reduce and each_with_object

Aggregate collections.

Lesson 4 of 413 steps

reduce and each_with_object is a free Ruby Academy lesson on CoddyKit. This is lesson 4 of 4. Any 3 lessons of this learning path are free to read in full — after that, CoddyKit PRO unlocks every lesson, plus hands-on practice with a built-in code editor and a 24/7 AI tutor. It is part of the Ruby Academy learning path, and your progress syncs across the web and the CoddyKit app. The Ruby Academy course includes 4 lessons in total.

What Is reduce?

reduce (alias inject) combines all elements into a single value. The block receives an accumulator and the current element.

sum = [1, 2, 3, 4].reduce(0) { |acc, n| acc + n }
puts sum

How the Accumulator Works

The accumulator carries the running result between iterations. Whatever the block returns becomes the next accumulator.

[1, 2, 3].reduce(0) do |acc, n|
  puts "acc=#{acc} n=#{n}"
  acc + n
end

reduce Without a Start

Without an initial value, the first element becomes the accumulator and iteration starts from the second.

puts [10, 20, 30].reduce { |acc, n| acc + n }

reduce with a Symbol

For simple operators, pass a symbol like :+ or :* instead of a block.

puts [1, 2, 3, 4].reduce(:+)
puts [1, 2, 3, 4].reduce(:*)

Finding a Maximum

reduce can compute things beyond sums, such as the largest value.

nums = [3, 9, 2, 7]
max = nums.reduce { |acc, n| n > acc ? n : acc }
puts max

Building a String

The accumulator can be any type, including a string you build up.

letters = ["r", "u", "b", "y"]
word = letters.reduce("") { |acc, c| acc + c.upcase }
puts word

What Is each_with_object?

each_with_object is like reduce but you give it a mutable object that is returned automatically. You change the object inside the block.

result = [1, 2, 3].each_with_object([]) do |n, arr|
  arr << n * 2
end
puts result.inspect

Building a Hash

each_with_object is perfect for building hashes because you do not need to return the object each time.

words = ["apple", "pear"]
lengths = words.each_with_object({}) do |w, h|
  h[w] = w.length
end
puts lengths

Counting Occurrences

Combine a default hash with each_with_object to tally items.

votes = ["a", "b", "a", "a"]
tally = votes.each_with_object(Hash.new(0)) do |v, h|
  h[v] += 1
end
puts tally

reduce vs each_with_object

Use reduce when each step returns a new value (sums, products). Use each_with_object when you mutate one container (arrays, hashes).

puts [1, 2, 3].reduce(:+)
puts [1, 2, 3].each_with_object([]) { |n, a| a << n }.inspect

The tally Shortcut

For counting, Ruby has a built-in tally method that does the work for you.

puts ["a", "b", "a"].tally

Quick Check

Check aggregation skills.

Answer it in the interactive lesson

Recap

You can now aggregate collections:

reduce/inject with accumulator, start value, or symbol
each_with_object for building arrays and hashes
tally for quick counts
choose reduce for new values, each_with_object for mutable containers

Next course: numbers and math.

puts (1..5).reduce(:*)

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Frequently Asked Questions

Is the “reduce and each_with_object” lesson free?

Yes — any 3 lessons of the Ruby Academy learning path, including “reduce and each_with_object”, are free to read in full here on the web. After that, CoddyKit PRO unlocks every lesson, plus interactive practice with a built-in code editor and a 24/7 AI tutor. The Ruby Academy course includes 4 lessons in total.

What will I learn in “reduce and each_with_object”?

Aggregate collections. You practise Ruby Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

Do I need any experience to start Ruby Academy?

No prior experience is required. Ruby Academy on CoddyKit is structured for beginners through advanced learners, so you can start here or from the beginning and move at your own pace. This is lesson 4 of 4.

How long does the “reduce and each_with_object” lesson take?

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