select array ruby​​ - 使用select获得＆gt; = 4的容量

Question

我尝试使用select来获取容量＆gt; = 4：

class Home
  attr_reader(:name, :city, :capacity, :price)

  def initialize(name, city, capacity, price)
    @name = name
    @city = city
    @capacity = capacity
    @price = price
  end
end

homes = [
  Home.new("Nizar's place", "San Juan", 2, 42),
  Home.new("Fernando's place", "Seville", 5, 47),
  Home.new("Josh's place", "Pittsburgh", 3, 41),
  Home.new("Gonzalo's place", "Málaga", 2, 45),
  Home.new("Ariel's place", "San Juan", 4, 49)
]

我试过了：

high_capacities_homes = homes.select do |hm|
  hm.capacity >= 4
  puts high_capacities_homes
end

没有任何成功和我的想法，有人可以帮忙吗？ ：（

Answer 1

尝试

high_capacities_homes = homes.select do |hm|
  hm.capacity >= 4
end

puts high_capacities_homes

puts返回nil，因此所有块都评估为false。

Answer 2

让我们仔细看看发生了什么。执行代码时，您将获得以下内容。

high_capacities_homes = homes.select do |hm|
  hm.capacity >= 4
  puts high_capacities_homes
end





 => [] 

返回一个空数组，但为什么所有这些空行？这是因为您的代码正在为puts nil的每个元素执行homes。考虑一下：

high_capacities_homes = homes.select do |hm|
  hm.capacity >= 4
  puts "high_capacities_homes is nil: #{high_capacities_homes.nil?}"
end
high_capacities_homes is nil: true
high_capacities_homes is nil: true
high_capacities_homes is nil: true
high_capacities_homes is nil: true
high_capacities_homes is nil: true
 => [] 

当Ruby看到

时

high_capacities_homes = <anything>

她做的第一件事是创建一个局部变量high_capacities_homes并为其赋值nil。只有在select完成枚举homes后，high_capacities_homes才会为select块的返回值指定，即[]。

还有另一个问题。方法的第一行，

hm.capacity >= 4 

返回true或false，但不会对该返回值执行操作。就好像它刚刚被射入太空，永远不会被再次看到，使你的代码等同于：

high_capacities_homes = homes.select do |hm|
  puts high_capacities_homes
end

由于puts返回nil（在打印任何打印之后），为了构造数组high_capacities_homes，您的代码相当于：

high_capacities_homes = homes.select do |hm|
  nil
end

这就是为什么没有选择homes的元素的原因。

您是否希望为hm创建homes元素hm.capacity >= 4的数组（以及有关这些实例的打印信息）？如果是这样，首先选择您想要的homes元素： ¹ 。

selected_homes = homes.select { |hm| hm.capacity >= 4 }
  # => [#<Home:0x007ff81981e138 @name="Fernando's place", @city="Seville",
  #       @capacity=5, @price=47>,
  #     #<Home:0x007ff81981dda0 @name="Ariel's place", @city="San Juan",
  #       @capacity=4, @price=49>] 

请注意，Array#select会保留hm阻止homes的所有select元素{。}}。

现在，您想要打印什么？如果它是true的每个元素的实例变量的值，您可以按照以下方式执行此操作（作为示例）。

selected_homes

打印

selected_homes.each { |hm|
  puts "#{hm.name} in #{hm.city} has capacity #{hm.capacity} and price $#{hm.price}" }

^{1当然，您可以写Fernando's place in Seville has capacity 5 and price $47 Ariel's place in San Juan has capacity 4 and price $49 。}