From: William James <w_a_x_man@...>
Date: 2007-01-21T05:30:10+09:00
Subject: Re: Proper way to either create or append to an array?

Phrogz wrote:
> Robert Klemme wrote:
> > Since you're using a Hash the block constructor is the most elegant
> > solution IMHO:
> >
> > def initialize
> >    @foo = Hash.new {|h,k| h[k]=[]}
> >    ...
> > end
>
> To be clear for those unfamiliar with this: any time you ask for the
> value of a key that doesn't exist, the block will be called (which
> creates a new empty array). This means you can never see this
> particular Hash has a key, since:
>   if @foo[ :bar ]
> will always succeed, and create a (possibly unwanted) array instance in
> the process.

What if foo[:bar] has been assigned a value of false?
That method of checking for the presence of a key is
too crude even for awk.  Note below that the mere
attempt to access the value of a key creates an
entry for that key if it doesn't already exist.

BEGIN {
  SUBSEP = "^"
  a[22,"yes"] = 88
  a[33] = 99

  # The wrong way.
  if ( a["bar"] )
    print "How did 'bar' get here?"
  # The right way.
  if ( "foo" in a )
    print "'foo' too?"
  # Any unauthorized entries?
  for (key in a)
    print key
}

---  output  -----
22^yes
33
bar