Rubyのeach_with_indexメソッドでインデックス付きループを簡単にマスターする方法

array.each_with_index do |element, index|
  # 処理
end

fruits = ["apple", "banana", "cherry"]

fruits.each_with_index do |fruit, index|
  puts "#{index}: #{fruit}"
end

0: apple
1: banana
2: cherry

numbers = [10, 20, 30, 40, 50]

numbers.each_with_index do |number, index|
  if index.even?
    puts "Index #{index} has an even number: #{number}"
  else
    puts "Index #{index} has an odd number: #{number}"
  end
end

Index 0 has an even number: 10
Index 1 has an odd number: 20
Index 2 has an even number: 30
Index 3 has an odd number: 40
Index 4 has an even number: 50

values = [3, 6, 9, 12, 15]

values.each_with_index do |value, index|
  result = value * index
  puts "Value #{value} at index #{index} multiplied by index: #{result}"
end

Value 3 at index 0 multiplied by index: 0
Value 6 at index 1 multiplied by index: 6
Value 9 at index 2 multiplied by index: 18
Value 12 at index 3 multiplied by index: 36
Value 15 at index 4 multiplied by index: 60

student_scores = { "Alice" => 85, "Bob" => 92, "Charlie" => 78 }

student_scores.each_with_index do |(name, score), index|
  puts "Student #{index + 1}: #{name} scored #{score} points"
end

Student 1: Alice scored 85 points
Student 2: Bob scored 92 points
Student 3: Charlie scored 78 points

students = { "Dave" => 88, "Eva" => 76, "Frank" => 90, "Grace" => 82 }

students.each_with_index do |(name, score), index|
  if index.odd?
    puts "#{name} (Index #{index}) is a special student with #{score} points"
  else
    puts "#{name} (Index #{index}) scored #{score} points"
  end
end

Dave (Index 0) scored 88 points
Eva (Index 1) is a special student with 76 points
Frank (Index 2) scored 90 points
Grace (Index 3) is a special student with 82 points

matrix = [
  [1, 2, 3],
  [4, 5, 6],
  [7, 8, 9]
]

matrix.each_with_index do |row, row_index|
  row.each_with_index do |element, col_index|
    puts "Element at (#{row_index}, #{col_index}): #{element}"
  end
end

Element at (0, 0): 1
Element at (0, 1): 2
Element at (0, 2): 3
Element at (1, 0): 4
Element at (1, 1): 5
Element at (1, 2): 6
Element at (2, 0): 7
Element at (2, 1): 8
Element at (2, 2): 9

products = {
  "Electronics" => { "Laptop" => 1000, "Smartphone" => 700 },
  "Furniture" => { "Table" => 300, "Chair" => 100 }
}

products.each_with_index do |(category, items), cat_index|
  puts "Category #{cat_index}: #{category}"
  items.each_with_index do |(item, price), item_index|
    puts "  Item #{item_index}: #{item} costs $#{price}"
  end
end

Category 0: Electronics
  Item 0: Laptop costs $1000
  Item 1: Smartphone costs $700
Category 1: Furniture
  Item 0: Table costs $300
  Item 1: Chair costs $100

fruits = ["apple", "banana", "cherry"]

fruits.each.with_index(1) do |fruit, index|
  puts "#{index}: #{fruit}"
end

1: apple
2: banana
3: cherry

names = ["John", "Paul", "George", "Ringo"]

names.each_with_index do |name, index|
  custom_index = index * 2
  puts "Position #{custom_index}: #{name}"
end

Position 0: John
Position 2: Paul
Position 4: George
Position 6: Ringo

items = ["A", "B", "C", "D", "E"]

items.each_with_index do |item, index|
  if index.odd?
    puts "Odd Index #{index}: #{item}"
  else
    puts "Even Index #{index}: #{item}"
  end
end

Even Index 0: A
Odd Index 1: B
Even Index 2: C
Odd Index 3: D
Even Index 4: E

letters = ["A", "B", "C", "D", "E"]

letters.reverse_each.with_index do |letter, index|
  puts "Element #{letter} at reverse index -#{index + 1}"
end

Element E at reverse index -1
Element D at reverse index -2
Element C at reverse index -3
Element B at reverse index -4
Element A at reverse index -5

numbers = [10, 20, 30, 40, 50]

numbers.each_with_index do |number, index|
  reverse_index = -(numbers.size - index)
  puts "Number #{number} at reverse index #{reverse_index}"
end

Number 10 at reverse index -5
Number 20 at reverse index -4
Number 30 at reverse index -3
Number 40 at reverse index -2
Number 50 at reverse index -1

colors = ["red", "green", "blue", "yellow", "purple"]

colors.each_with_index do |color, index|
  reverse_index = -(colors.size - index)
  if reverse_index >= -3
    puts "Special treatment for #{color} at reverse index #{reverse_index}"
  end
end

Special treatment for blue at reverse index -3
Special treatment for yellow at reverse index -2
Special treatment for purple at reverse index -1

prices = [100, 200, 300, 400, 500]

# 偶数インデックスの要素のみ半額にする
prices.each_with_index do |price, index|
  if index.even?
    discounted_price = price * 0.5
    puts "Price at index #{index} after discount: #{discounted_price}"
  else
    puts "Price at index #{index}: #{price}"
  end
end

Price at index 0 after discount: 50.0
Price at index 1: 200
Price at index 2 after discount: 150.0
Price at index 3: 400
Price at index 4 after discount: 250.0

names = ["Alice", "Bob", "Charlie", "David", "Eve"]

# 中央の3つの要素のみを抽出
selected_names = []
names.each_with_index do |name, index|
  if index >= 1 && index <= 3
    selected_names << name
  end
end
puts "Selected names: #{selected_names.join(', ')}"

Selected names: Bob, Charlie, David

letters = ["A", "B", "C", "D", "E", "F", "G"]

# 偶数インデックスと奇数インデックスの要素を分離
even_indexed = []
odd_indexed = []

letters.each_with_index do |letter, index|
  if index.even?
    even_indexed << letter
  else
    odd_indexed << letter
  end
end

puts "Even-indexed letters: #{even_indexed.join(', ')}"
puts "Odd-indexed letters: #{odd_indexed.join(', ')}"

Even-indexed letters: A, C, E, G
Odd-indexed letters: B, D, F

players = ["John", "Paul", "George", "Ringo"]

ranked_players = players.map.with_index(1) do |player, index|
  "Rank #{index}: #{player}"
end

puts ranked_players.join("\n")

Rank 1: John
Rank 2: Paul
Rank 3: George
Rank 4: Ringo

colors = ["red", "green", "blue", "yellow", "purple"]

colors.each_with_index do |color, index|
  reverse_index = -(colors.size - index)
  if reverse_index >= -3
    puts "#{color} - Marked (Reverse Index: #{reverse_index})"
  else
    puts color
  end
end

red
green
blue - Marked (Reverse Index: -3)
yellow - Marked (Reverse Index: -2)
purple - Marked (Reverse Index: -1)

animals = ["cat", "dog", "elephant", "tiger", "lion", "giraffe"]

cat
elephant
lion

numbers = [1, 2, 3, 4, 5]

[0, 2, 6, 12, 20]

books = ["The Hobbit", "1984", "Brave New World", "Fahrenheit 451", "To Kill a Mockingbird"]

The Hobbit
1984
Brave New World - Featured
Fahrenheit 451 - Featured
To Kill a Mockingbird - Featured

players = ["Alice", "Bob", "Charlie", "Diana"]

Rank 1: Alice
Rank 2: Bob
Rank 3: Charlie
Rank 4: Diana

scores = [10, 20, 30, 40, 50, 60]

合計: 120

# 誤り例
fruits = ["apple", "banana", "cherry"]

fruits.each_with_index do |fruit|
  puts "#{fruit} at index"
end

wrong number of arguments (given 1, expected 2)

fruits.each_with_index do |fruit, index|
  puts "#{fruit} at index #{index}"
end

# 誤り例
str = "hello"

str.each_with_index do |char, index|
  puts "#{char} at index #{index}"
end

undefined method `each_with_index' for "hello":String

str.chars.each_with_index do |char, index|
  puts "#{char} at index #{index}"
end

numbers = [10, 20, 30]

numbers.each_with_index do |num, index|
  puts "#{num} and the next number #{numbers[index + 1]}"
end

undefined method `[]' for nil:NilClass

numbers.each_with_index do |num, index|
  if index < numbers.size - 1
    puts "#{num} and the next number #{numbers[index + 1]}"
  else
    puts "#{num} is the last element"
  end
end

# 誤り例
student_scores = { "Alice" => 85, "Bob" => 92 }

student_scores.each_with_index do |name, score, index|
  puts "#{index}: #{name} - #{score}"
end

wrong number of arguments (given 3, expected 2)

student_scores.each_with_index do |(name, score), index|
  puts "#{index}: #{name} - #{score}"
end