The Comparable Module

Comparable is a Ruby mixin that turns a single <=> method you write into the full set of comparison operators — < , <= , == , > , >= , plus between? and clamp .

Learn The Comparable Module in our free Ruby course — a beginner-friendly interactive lesson with worked examples, a practice exercise and a quick reference.

Part of the free Ruby course at LearnCodingFast — hands-on lessons with examples you run in your browser, plus practice exercises and a quick quiz.

In this lesson you'll make your own classes orderable and sortable by defining just one method.

What You'll Learn in This Lesson

1️⃣ Define <=> and include Comparable

The spaceship operator <=> answers one question: is the left object less than ( -1 ), equal to ( 0 ), or greater than ( 1 ) the right? Define it, include Comparable , and every comparison operator appears for free. A neat trick is to delegate to an Array of fields, whose own <=> compares element by element.

2️⃣ between? and clamp Come Free

Beyond the operators, Comparable hands you two practical helpers. between?(low, high) checks whether an object falls inside a range, and clamp(low, high) pulls an out-of-range value back to the nearest boundary — perfect for limiting volume, brightness, or a temperature reading.

3️⃣ Sorting Custom Objects

Because Array#sort , min , and max all call <=> , defining it once makes your objects sortable everywhere. Keep <=> in natural ascending order for clarity, then reverse at the call site if you want descending. sort_by is often clearer still when you just want to sort on one attribute.

🎯 Your Turn

The Book class is almost there — it just needs the comparison operator. Replace the ___ with the operator that returns -1 , 0 , or 1 .

Build a Money class that stores cents, includes Comparable, and demonstrates sort, min, max, between?, and clamp. Run with ruby money.rb .

📋 Quick Reference — Comparable

Practice quiz

What must the spaceship operator <=> return?

  • true or false
  • Always an Integer 0..100
  • -1, 0, or 1 (or nil if uncomparable)
  • The two objects in an array

Answer: -1, 0, or 1 (or nil if uncomparable). <=> returns -1 (less), 0 (equal), 1 (greater), or nil when the objects can't be compared.

What single method must you define to get all comparison operators from Comparable?

  • <=>
  • ==
  • compare
  • <

Answer: <=>. Define <=> and include Comparable; the module derives <, <=, ==, >, >=, between?, and clamp.

If you define <=> but forget to include Comparable, calling obj1 < obj2 will:

  • Work fine
  • Return nil
  • Compare by object id
  • Raise NoMethodError because < isn't defined

Answer: Raise NoMethodError because < isn't defined. <=> alone doesn't create <; you must include Comparable to derive the operators.

What does [2, 1] <=> [2, 10] return?

  • 1
  • -1
  • 0
  • nil

Answer: -1. Arrays compare element by element: 2 == 2, then 1 < 10, so the result is -1.

Which TWO helper methods does Comparable provide beyond the operators?

  • between? and clamp
  • map and select
  • sort and reverse
  • min and max

Answer: between? and clamp. Comparable gives you between?(low, high) and clamp(low, high) for free.

Temp.new(40).clamp(Temp.new(18), Temp.new(30)) returns the temperature:

  • 40
  • 18
  • 30
  • nil

Answer: 30. clamp pulls an out-of-range value back to the nearest boundary; 40 clamps down to the high, 30.

Why does Array#sort work on your custom objects once they're Comparable?

  • sort reads instance variables directly
  • sort calls <=> under the hood to order elements
  • sort uses == only
  • It doesn't — you must write your own sort

Answer: sort calls <=> under the hood to order elements. sort, min, and max all rely on <=>, so defining it once makes objects sortable everywhere.

The recommended way to sort custom objects in DESCENDING order is to:

  • Reverse the logic inside <=>
  • Define a separate sort_desc method
  • Use min instead of max
  • Keep <=> ascending and reverse at the call site (e.g. array.sort.reverse)

Answer: Keep <=> ascending and reverse at the call site (e.g. array.sort.reverse). Keep <=> in natural ascending order and reverse at the call site, so max stays intuitive.

How do Comparable and Enumerable differ?

  • They are the same module under two names
  • Comparable orders one object vs another (<=>); Enumerable traverses a collection (each)
  • Comparable is for arrays only
  • Enumerable needs <=>, Comparable needs each

Answer: Comparable orders one object vs another (<=>); Enumerable traverses a collection (each). Comparable orders objects via <=>; Enumerable traverses collections via each.

Why store money as cents (integers) when making it Comparable?

  • Cents sort alphabetically
  • Comparable rejects floats
  • Integers compare cleanly without floating-point drift
  • It uses less memory than any other approach

Answer: Integers compare cleanly without floating-point drift. Comparable units like cents compare exactly, avoiding float rounding surprises.