From: "Jesús Gabriel y Galán" <jgabrielygalan@...>
Date: 2012-08-03T22:25:53+09:00
Subject: Re: learning by doing part 2 - tc game

On Fri, Aug 3, 2012 at 3:06 PM, Sebastjan H. <lists@ruby-forum.com> wrote:
> I wanted to test this on a simple case, so I wrote the following:
> ------------------------------------------
> def attack(target)
> target -= 200
> end
>
> class A
> def initialize(name, hp)
> @name = name
> @hp = hp
> end
>
> def hp()
> @hp
> end
>
> def name()
> @name
> end
>
> end
>
> players = [] << A.new("test", 400)
>
> attack(players[0].hp)
>
> puts players[0].hp
> ------------------------------------------
> The code runs, but the result is 400. As if the attack method did
> nothing.
> What am I doing wrong?

The reason is that this line:

target -= 200 is equivalent to this one:
target = target - 200

which is assigning a new value to the local variable of the method.
Once you exit the method, the new value is lost. In order for this to
work, you should pass a reference to a mutable object, so that the
effects of the method are visible outside. For example:

def attack(target)
  target.hp -= 200 # supposing target has methods #hp and #hp=
end

# later....
attack(player[0])

Hope this helps,

Jesus.