From: Martin DeMello <martindemello@...>
Date: 2010-04-09T23:46:51+09:00
Subject: Re: read and write stock prices to file using arrays

On Fri, Apr 9, 2010 at 7:27 PM, Peter Miller <feurry@gmail.com> wrote:
>
> I didn't realize a hash could point to more than one object. Awesome.

It can't, but the one value a key points to can be an object of any
type, including arrays etc.. For instance, a very common ruby idiom is
to group a set of objects by using a hash of arrays. For example,
let's say you have a bunch of people, who belong to one of three
teams, red, blue or green.

data = [['john', 'red'],
  ['jane', 'green'],
  ['alice', 'red'],
  ['bob', 'blue'],
  ['tom', 'red'],
  ['dick', 'green'],
  ['harry', 'red'],
  ['bill', 'blue']]

You can collect the teams like so:

teams = {}

data.each {|name, team|
  teams[team] ||= [] # initialize with an empty array if this is the
first time you've seen the team
  teams[team] << name # append the name to the team's array
}

p teams
# => {"red"=>["john", "alice", "tom", "harry"], "green"=>["jane",
"dick"], "blue"=>["bob", "bill"]}

martin