From: Phrogz <gavin@...>
Date: 2007-06-16T06:30:10+09:00
Subject: Re: test to see if a variable exists

On Jun 15, 2:36 pm, "Colin Summers" <blade...@gmail.com> wrote:
> So I am stealing all these identities and storing everything in a big
> array of objects stolen_identity, and having the index be social
> security numbers:
>
> ss[34323843] = Stolen_identity.new
>
> But say I pick up a paycheck stub and it has 3428294. How do I know if
> I already have it? Does
>    if (exist ss[3428294]) then ... end
> work? How can I see if a variable exists?

Arrays (and Hashes) in Ruby are sparse - they only store the values
you actually enter. However, there are some limitations on the size of
indices for arrays:

  C:\>irb

  irb(main):001:0> ss = []
  => []

  irb(main):002:0> ss[ 884_56_2103 ] = "I HAVE YOU NOW"
  ArgumentError: index too big
        from (irb):2:in `[]='
        from (irb):2

So, you probably want to use a Hash, which has no such limitation.
With both Arrays and Hashes, supplying a key or index which has no
stored value will return a nil object:

  irb(main):003:0> ss = {}
  => {}

  irb(main):004:0> ss[ 884_56_2103 ] = "I HAVE YOU NOW"
  => "I HAVE YOU NOW"

  irb(main):005:0> ss[ 912_34_3552 ]
  => nil

In Ruby, nil and false are both non-truth values for the purposes of
boolean logic. Additionally (and slightly more cleanly), you can ask a
Hash directly if it has an entry for a specific key:

  irb(main):006:0> if ss.has_key?( 884_56_2103 ) then
  irb(main):007:1*   puts "I'm done with this guy"
  irb(main):008:1> else
  irb(main):009:1*   puts "Sweet new information!"
  irb(main):010:1> end
  I'm done with this guy