Master Ruby Enumerable: each, map, and select

ルビー Ruby Enumerable module is the powerhouse behind expressive, functional-style iteration. Mixed into Array, Hash, Range, Set, and custom collections, it enables clean, efficient data processing. Enhance your Ruby projects with expert Rails consulting services, optimizing each, map, and select for cleaner, high-performance code.

Ruby Enumerable: The Foundation

使用方法 Ruby Enumerable, a class must define each:

ruby
class ShoppingList
  include Enumerable

  def initialize(*items)
    @items = items
  end

  def each(&block)
    @items.each(&block)
  end
end

Now ShoppingList supports all Ruby Enumerable methods.

Ruby Enumerable: each – Execute Per Item

What each Does

Runs a block for every element. Returns the original collection.

ruby
[1, 2, 3].each { |n| puts n * 2 }
# Output:
# 2
# 4
# 6
# Returns: [1, 2, 3]

When to Use Ruby Enumerable each

• Logging or printing
• Triggering side effects (email, API calls)
• Accumulating external state

ruby
total = 0
[10, 20, 30].each { |n| total += n }
puts total # => 60

Pitfall: Never use each expecting a transformed array.

ruby
# Wrong
[1, 2, 3].each { |n| n * 2 } # => [1, 2, 3]

Ruby Enumerable: map (or collect) – Transform Every Item

What map Does

Applies a block and returns a new array of results.

ruby
[1, 2, 3].map { |n| n * 2 } # => [2, 4, 6]

When to Use Ruby Enumerable map

• Converting data types
• Extracting fields
• Building new structures

Ruby Enumerable map 例

Capitalize Names

ruby
["alice", "bob"].map(&:upcase) # => ["ALICE", "BOB"]

Extract User Emails

ruby
users = [{email: "[email protected]"}, {email: "[email protected]"}]
users.map { |u| u[:email] } # => ["[email protected]", "[email protected]"]

With Index

ruby
fruits = ["apple", "banana"]
fruits.map.with_index { |f, i| "#{i + 1}. #{f}" }
# => ["1. apple", "2. banana"]

Pitfall: Don’t use map for side effects only.

ruby
# Returns [nil, nil, nil]
[1, 2, 3].map { |n| puts n }

Ruby Enumerable: 選択する (or find_all) – Filter Matching Items

What 選択する Does

Returns a new array of elements where block returns 真実.

ruby
[1, 2, 3, 4].select(&:even?) # => [2, 4]

When to Use Ruby Enumerable 選択する

• Filtering valid records
• Querying data
• Subsetting collections

Ruby Enumerable 選択する 例

Passing Scores

ruby
scores = [89, 45, 92, 61]
scores.select { |s| s >= 70 } # => [89, 92]

Active Users

ruby
users = [
  {name: "Alice", active: true},
  {name: "Bob",   active: false}
]
users.select { |u| u[:active] } # => [{name: "Alice", active: true}]

Opposite: reject

ruby
[1, 2, 3, 4].reject(&:even?) # => [1, 3]

Ruby Enumerable: Key Differences

Ruby Enumerable: Chaining for Power

Chain methods for expressive pipelines:

ruby
(1..10)
  .select(&:even?)      # [2, 4, 6, 8, 10]
  .map { |n| n ** 2 }   # [4, 16, 36, 64, 100]
  .each { |sq| puts sq }

Real-world example:

ruby
active_emails = users
  .select { |u| u[:active] && u[:age] >= 18 }
  .map { |u| u[:email] }

Ruby Enumerable: Performance Tips

One Pass > Two

ruby
# Slower: two iterations
users.map(&:age).select { |a| a > 18 }

# Faster: one iteration
users.select { |u| u[:age] > 18 }.map(&:age)

Symbol-to-Proc Shorthand

ruby
names.map(&:upcase)

Lazy Evaluation for Large/Infinite Data

ruby
(1..Float::INFINITY).lazy
  .select(&:even?)
  .map { |n| n * n }
  .first(5)
# => [4, 16, 36, 64, 100]

Ruby Enumerable: Bonus Patterns

Safe Parsing with map + compact

ruby
["1", "2", "abc"].map { |s| Integer(s) rescue nil }.compact
# => [1, 2]

Find First Match

ruby
admin = users.find { |u| u[:role] == "admin" }  # Better than select.first

Accumulate with each_with_object

ruby
[1, 2, 3].each_with_object({}) { |n, h| h[n] = n * 2 }
# => {1=>2, 2=>4, 3=>6}

Ruby Enumerable: Final Checklist

• 用途 each → side effects
• 用途 map → transform
• 用途 選択する → filter
• Chain for clarity
• Never ignore return values
• Prefer 探す over select.first
• 用途 lazy for huge/infinite streams

結論：

The Ruby Enumerable module is more than a collection of methods — it’s a philosophy of clean, readable, and efficient code. By mastering each, map、 そして 選択する, you gain the ability to:

• Express intent clearly — no more confusing loops.
• Avoid bugs — return values matter, and these methods enforce it.
• Write performant code — one-pass operations and lazy evaluation.
• Chain fluently — build powerful data pipelines in a single line.

で レールカーマをリードしている。 Ruby on Rails開発会社, we apply these principles every day to build scalable, maintainable, and elegant applications. Whether it’s filtering users, transforming datasets, or sending notifications, our developers leverage Ruby’s Enumerable power to write code that’s both expressive and efficient.

Practice these methods daily, chain them wisely, and you’ll soon write Ruby the way it was meant to be written — elegant, expressive, and effortless.

Start today: open your terminal, fire up irb, and experiment with map, select、 そして each on any array. The power — and the RailsCarma way — is in your hands.