簡介
更多列舉方法
在鳥類計數中，你已經認識了 count、any?、select、all?和map這些列舉方法。
以下回顧這些方法，並補充幾個額外的方法：
fibonacci = [0, 1, 1, 2, 3, 5, 8, 13]
fibonacci.count { |number| number == 1 } #=> 2
fibonacci.any? { |number| number > 20 } #=> false
fibonacci.none? { |number| number > 20 } #=> true
fibonacci.select { |number| number.odd? } #=> [1, 1, 3, 5, 13]
fibonacci.all? { |number| number < 20 } #=> true
fibonacci.map { |number| number * 2 } #=> [0, 2, 2, 4, 6, 10, 16, 26]
fibonacci.select { |number| number >= 5 } #=> [5, 8, 13]
fibonacci.find { |number| number >= 6 } #=> 8
# Some methods work with or without a block
fibonacci.sum #=> 33
fibonacci.sum { |number| number * number } #=> 273
# There are also methods to help with nested arrays:
animals = [ ['cat', 'bob'], ['horse', 'caris'], ['mouse', 'arya'] ]
animals.flatten #=> ["cat", "bob", "horse", "caris", "mouse", "arya"]
列舉雜湊
列舉Hash物件和列舉Array物件完全一樣，差別在於區塊會接收兩個引數：鍵和值：
pet_names = {cat: "bob", horse: "caris", mouse: "arya"}
pet_names.each { |animal, name| ... }
如果你只需要其中一個值，可以使用特殊的_符號來表示不需要的那個值。
這既能讓開發者更清楚，也是一種效能最佳化。
pet_names = {cat: "bob", horse: "caris", mouse: "arya"}
pet_names.map { |_, name| name } #=> ["bob", "caris", "arya"]
巢狀列舉
你也可以在巢狀區塊中列舉，並把方法串接在一起。
舉例來說，如果我們有一個動物雜湊的陣列，而且想取出名字較短的動物，可能會想這麼做：
pets = [
{ animal: "cats", names: ["bob", "fred", "sandra"] },
{ animal: "horses", names: ["caris", "black beard", "speedy"] },
{ animal: "mice", names: ["arya", "jerry"] }
]
pets.map { |pet|
pet[:names].select { |name| name.length <= 5 }
}.flatten.sort
#=> ["arya", "bob", "caris", "fred", "jerry"]
說明
你經營著一家線上時尚精品店。一年一度的大特賣即將到來，所以你想建立一些功能，幫你盤點庫存，確保一切都準備就緒。
庫存中的單一商品以雜湊表示，而整份庫存就是由這些雜湊組成的陣列。
[
{
name: "White Shirt",
price: 40,
quantity_by_size: {s: 3, m: 7, l: 8, xl: 4}
},
{ ... }
]
1. 回傳商品名稱的陣列
實作BoutiqueInventory.item_names，它應該回傳一份按字母順序排列的商品名稱陣列。
BoutiqueInventory.new([
{price: 65.00, name: "Maxi Brown Dress", quantity_by_size: {s: 3, m: 7, l: 8, xl: 4}},
{price: 50.00, name: "Red Short Skirt", quantity_by_size: {}},
{price: 29.99, name: "Black Short Skirt", quantity_by_size: {s: 1, xl: 4}},
{price: 20.00, name: "Bamboo Socks Cats", quantity_by_size: {s: 7, m: 2}}
]).item_names
#=> ["Bamboo Socks Cats", "Black Short Skirt", "Maxi Brown Dress", "Red Short Skirt"]
2. 回傳所有便宜的商品
實作BoutiqueInventory.cheap，它應該回傳所有價格低於 30 美元的商品。
BoutiqueInventory.new([
{price: 65.00, name: "Maxi Brown Dress", quantity_by_size: {s: 3, m: 7, l: 8, xl: 4}},
{price: 50.00, name: "Red Short Skirt", quantity_by_size: {}},
{price: 29.99, name: "Black Short Skirt", quantity_by_size: {s: 1, xl: 4}},
{price: 20.00, name: "Bamboo Socks Cats", quantity_by_size: {s: 7, m: 2}}
]).cheap
# => [
# {price: 29.99, name: "Black Short Skirt", quantity_by_size: {s: 1, xl: 4}},
# {price: 20.00, name: "Bamboo Socks Cats", quantity_by_size: {s: 7, m: 2}}
# ]
3. 回傳所有缺貨的商品
實作BoutiqueInventory.out_of_stock，它應該回傳所有沒有庫存的商品（也就是quantity_by_size為空的情況）。
BoutiqueInventory.new([
{price: 65.00, name: "Maxi Brown Dress", quantity_by_size: {s: 3, m: 7, l: 8, xl: 4}},
{price: 50.00, name: "Red Short Skirt", quantity_by_size: {}},
{price: 29.99, name: "Black Short Skirt", quantity_by_size: {s: 1, xl: 4}},
{price: 20.00, name: "Bamboo Socks Cats", quantity_by_size: {s: 7, m: 2}}
]).out_of_stock
#=> [{price: 50.00, name: "Red Short Skirt", quantity_by_size: {}}]
4. 回傳單一商品的庫存
實作BoutiqueInventory.stock_for_item(name)，依名稱回傳某個商品的庫存：
BoutiqueInventory.new([
{price: 65.00, name: "Maxi Brown Dress", quantity_by_size: {s: 3, m: 7, l: 8, xl: 4}},
{price: 50.00, name: "Red Short Skirt", quantity_by_size: {}},
{price: 29.99, name: "Black Short Skirt", quantity_by_size: {s: 1, xl: 4}},
{price: 20.00, name: "Bamboo Socks Cats", quantity_by_size: {s: 7, m: 2}}
]).stock_for_item("Black Short Skirt")
#=> {s: 1, xl: 4}
5. 回傳總庫存
實作BoutiqueInventory.total_stock，計算你倉庫中商品的總數量：
BoutiqueInventory.new([
{price: 65.00, name: "Maxi Brown Dress", quantity_by_size: {s: 3, m: 7, l: 8, xl: 4}},
{price: 50.00, name: "Red Short Skirt", quantity_by_size: {}},
{price: 29.99, name: "Black Short Skirt", quantity_by_size: {s: 1, xl: 4}},
{price: 20.00, name: "Bamboo Socks Cats", quantity_by_size: {s: 7, m: 2}}
]).total_stock
#=> 36
學習練習
