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Ruby Map and Select

The beginner course walked arrays with each and built new arrays by hand. map, select and reject do those jobs in one line each.

Map builds a new array

map runs the block on each number. The block doubles each number, and map collects the results into a new array.

numbers = [1, 2, 3, 4]
doubled = numbers.map { |n| n * 2 }
p doubled

Select and reject split a list

select keeps the numbers that pass the test. reject keeps the numbers that fail the same test.

numbers = [1, 2, 3, 4, 5, 6]
evens = numbers.select { |n| n.even? }
odds = numbers.reject { |n| n.even? }
p evens
p odds

Map with an index

with_index adds a second block value. The second value counts the items, starting at zero.

names = ["Ana", "Ben", "Cid"]
labels = names.map.with_index { |name, i| "#{i + 1}. #{name}" }
p labels

A chain of select and map

A chain runs the calls from left to right. select first keeps the long words, and map then makes each kept word uppercase.

words = ["cat", "horse", "dog", "elephant"]
result = words.select { |w| w.length > 3 }.map { |w| w.upcase }
p result

Example

prices = [10, 25, 8, 40]
sale = prices.select { |p| p >= 10 }.map { |p| p / 2 }
p sale

Key points

  • map runs a block on every item.
  • map returns a new array of the block results.
  • The original array stays unchanged.
  • select keeps the items where the block is true.
  • reject keeps the items where the block is false.
  • map.with_index hands the block each item and its index.
  • Calls chain, so select can feed its result into map.

In this example, select keeps the prices of 10 or more. map then halves each kept price. The printed array holds the three sale prices.

Try it Yourself

Edit the code, then press Run.

Running is simulated

Exercise

What does [2, 4, 6].map { |n| n + 1 } return?