Ruby Academy · Lesson

Array and Hash Patterns

Destructure data.

Lesson 2 of 413 steps

Array and Hash Patterns is a free Ruby Academy lesson on CoddyKit. This is lesson 2 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.

Array Patterns

An array pattern matches by position and length. Each element pattern is checked against the corresponding element.

case [1, 2, 3]
in [1, 2, 3]
  puts 'exact match'
end

Binding Array Elements

Use identifiers to capture each position into a variable.

case [10, 20]
in [x, y]
  puts(x + y)
end

Length Matters

By default an array pattern requires the exact length. A 2-element pattern will not match a 3-element array.

case [1, 2, 3]
in [a, b]
  puts 'two'
in [a, b, c]
  puts 'three'
end

The Splat for the Rest

A splat *rest captures any number of remaining elements into an array, making length flexible.

case [1, 2, 3, 4]
in [first, *rest]
  puts(first)
  p rest
end

Splat in the Middle

The splat can sit between fixed elements to grab the head and tail while ignoring the middle.

case [1, 2, 3, 4, 5]
in [first, *, last]
  puts("#{first} ... #{last}")
end

Hash Patterns

A hash pattern matches by keys. It checks that the given keys exist and their values match; extra keys are allowed.

case { name: 'Ada', age: 36 }
in { name: 'Ada' }
  puts 'found Ada'
end

Binding Hash Values

Capture a value by giving the key a binding name. Writing just the key binds to a variable of the same name.

case { name: 'Bob', age: 25 }
in { name: String => n, age: Integer => a }
  puts("#{n} is #{a}")
end

Shorthand Key Binding

Omitting the pattern after a key binds a variable named after the key automatically.

case { city: 'Istanbul' }
in { city: }
  puts(city)
end

Requiring No Extra Keys

Hash patterns ignore extra keys by default. Add **nil to require that only the listed keys are present.

case { a: 1 }
in { a: 1, **nil }
  puts 'exactly one key'
end

Nested Patterns

Patterns nest freely, letting you destructure deep structures in a single branch.

data = { user: { name: 'Ada', tags: ['admin', 'dev'] } }
case data
in { user: { name:, tags: [first_tag, *] } }
  puts("#{name}: #{first_tag}")
end

Matching Parsed JSON

Pattern matching pairs beautifully with parsed JSON or CSV data, pulling out exactly the fields you need.

require 'json'
resp = JSON.parse('{"status":"ok","items":[1,2,3]}', symbolize_names: true)
case resp
in { status: 'ok', items: }
  puts(items.sum)
end

Quick Check

By default, will the hash pattern in { a: 1 } match the hash { a: 1, b: 2 }?

Answer it in the interactive lesson

Recap: Array and Hash Patterns

You can destructure collections:

Array patterns match by position and length
*rest captures remaining elements
Hash patterns match by key, allowing extras
Shorthand { key: } binds same-named variables
**nil forbids extra keys; patterns nest deeply

case { id: 1, items: [9, 8] }
in { items: [head, *] } then puts head
end

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Is the “Array and Hash Patterns” lesson free?

Yes — any 3 lessons of the Ruby Academy learning path, including “Array and Hash Patterns”, 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 “Array and Hash Patterns”?

Destructure data. 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.

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