From: Wilson Bilkovich <wilsonb@...>
Date: 2006-01-12T23:51:54+09:00
Subject: Re: safe array index ?

On 1/11/06, Edward Faulkner <ef@alum.mit.edu> wrote:
> On Thu, Jan 12, 2006 at 02:40:40AM +0900, Christer Nilsson wrote:
> > Is it possible to catch index out of range ?
> > I managed to redefine Array, but I want to have both possibilities.
>
> You can use a delegator:
>
> require 'delegate'
>
> class SafeArray < DelegateClass(Array)
>   def [](index)
>     raise "index error" if index.abs >= size
>     super(index)
>   end
> end
>

I've been trying to get my head around Delegator, but I'm not seeing
what it does here.
Subclassing Array seems to work the same way.

irb session:
irb(main):001:0> class SafeArray < Array
irb(main):002:1> def [](index)
irb(main):003:2> raise "Index error" if index.abs >= size
irb(main):004:2> super(index)
irb(main):005:2> end
irb(main):006:1> end
=> nil
irb(main):007:0> a = SafeArray.new
=> []
irb(main):008:0> a.concat [1,2,3,4,5]
=> [1, 2, 3, 4, 5]
irb(main):009:0> b = Array.new
=> []
irb(main):010:0> b.concat [1,2,3,4,5]
=> [1, 2, 3, 4, 5]
irb(main):011:0> a
=> [1, 2, 3, 4, 5]
irb(main):012:0> b
=> [1, 2, 3, 4, 5]
irb(main):013:0> a[9]
RuntimeError: Index error
        from (irb):3:in `[]'
        from (irb):13
irb(main):014:0> b[9]
=> nil