Ruby Academy · Lesson

Lazy Enumerators

Lazy evaluation.

Lazy Enumerators is a free Ruby Academy lesson on CoddyKit — lesson 3 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the Ruby Academy learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.

Lazy Evaluation

Normally Enumerable methods process the entire collection at once. A lazy enumerator processes elements one at a time, only as needed. This lets you work with huge or even infinite sequences.

result = (1..Float::INFINITY).lazy.map { |n| n * 2 }.first(5)
p result

The Problem with Eager Methods

Calling map on an infinite range without lazy would never finish, because it tries to build a complete result array first.

# Eager version would hang. Lazy completes instantly:
first_three = (1..Float::INFINITY).lazy.select(&:even?).first(3)
p first_three

Creating a Lazy Enumerator

Call .lazy on any enumerable to get a lazy version. Chained methods then become lazy too.

lazy = (1..10).lazy
puts lazy.class

Nothing Runs Until Forced

Lazy chains build a recipe but do not execute. A terminal method like first, to_a, or force triggers the work.

chain = (1..Float::INFINITY).lazy.map { |n| n * n }
puts "Nothing computed yet"
p chain.first(4)

Forcing Results

force (alias to_a) realizes a bounded lazy chain into an array.

result = (1..20).lazy.select(&:odd?).first(5)
p result
all = (1..6).lazy.map { |n| n * 10 }.force
p all

Chaining Lazily

Multiple lazy operations are interleaved per element rather than run as separate full passes. Each value flows through the whole chain before the next begins.

result = (1..Float::INFINITY)
  .lazy
  .map { |n| n * n }
  .select { |n| n.even? }
  .first(4)
p result

take and take_while

Lazy take and take_while stop pulling values once their condition is met.

result = (1..Float::INFINITY).lazy.take_while { |n| n < 6 }.to_a
p result

Infinite Sequences

Lazy enumerators shine with generators. Pair them with Enumerator to model endless streams.

naturals = Enumerator.new do |y|
  n = 1
  loop { y << n; n += 1 }
end
p naturals.lazy.map { |x| x * 3 }.first(5)

Memory Efficiency

Because lazy chains never materialize the full intermediate collection, they save memory when filtering large data down to a few results.

big = (1..1_000_000).lazy
first_match = big.select { |n| n % 7 == 0 && n % 11 == 0 }.first
puts first_match

When to Use Lazy

Use lazy when:

The source is infinite or very large.
You only need a few results.
You want to avoid big intermediate arrays.

For small collections, eager methods are simpler and fast enough.

fizz = (1..Float::INFINITY).lazy.select { |n| n % 3 == 0 }.first(5)
p fizz

Putting It Together

Lazy enumerators defer work:

.lazy makes a chain lazy.
Work runs only when a terminal method pulls values.
Perfect for infinite or huge sequences.

squares = (1..Float::INFINITY).lazy.map { |n| n * n }
p squares.select(&:odd?).first(4)

Quick Check

Test your understanding of lazy enumerators.

Recap

You learned lazy enumerators:

.lazy defers computation.
Terminal methods (first, to_a, force) trigger work.
Each value flows through the whole chain on demand.
Ideal for infinite or large sequences.

p (1..Float::INFINITY).lazy.select(&:even?).map { |n| n + 1 }.first(3)

Frequently asked questions

Is the “Lazy Enumerators” lesson free?

Yes — the full text of “Lazy Enumerators” is free to read here on the web, and the Ruby Academy course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the Ruby Academy course, upgrade to CoddyKit PRO.

What will I learn in “Lazy Enumerators”?

Lazy evaluation. 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; this is — lesson 3 of 4, so you can start here or from the beginning and move at your own pace.

How long does the “Lazy Enumerators” lesson take?

Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.

Can I write and run code in this Ruby Academy lesson?

Yes. Every Ruby Academy lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.

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