关于ruby中的数组的问题

Question

I have a class called Product , which contains a name , price and count .

In another class called Shop (including the class Product ), I initialize with an empty array. Then a add product to this array using the method push .

The problem happens to the method to_s of in the Shop class :

def to_s
  string = "Products in the shop:\n"
  @list.each do |list|
    #### how can i get access to the attributes of product like @list.name or something like that ?  I have tried loads of different methods but i cant get access to my attributes. (the array doesnt recognize the product class)
    puts @list.name
  end

If I create a Product without using Array , I can access to the attributes - I guess the problem comes because of the Array ...

Answer 1

So first of all, your blocks don't match up.

You want:

def to_s
  string = "Products in the shop:\n"
  @list.each do |item|
    string << item.name + "\n"     # not @list.name! @list is an Array. item is an element of your list.
  end
  string # you could also say: return string
end

You don't want to use puts because that'll send it to the console -- when you're trying to store it in a String called string . You want to make sure you return that, too.

You don't have to call your block parameter item . You could call it list , but that would be confusing, because you have another variable called @list , and the block parameter is not a list at all -- just an item from the array.

Note that this is not the most efficient way to accomplish this. The preferred way is like so:

def to_s
  products_list = @list.map do |item|
    item.name
  end
  "Products in the shop:\n" + products_list.join("\n")
end

Answer 2

Each element in @list is yielded to the each block as list .

Assuming @list is an Enumerable with Product objects, you should be able to do list.name to get at the name attribute inside the block.

Answer 3

I'd prefer map and join over each and << (just a preference), and braces ( {} ) over do end because I like to imply a return value is important by using braces, and a do end block is there for side effects, eg

def to_s
  "Products in the shop:\n" << @list.map{|item| item.name}.join("\n")
end

I also wouldn't use puts because to_s should return a string, not output a string.