Question on hashes and init block

ruby-talk
1

Hi all,

I’m using the trick someone mentioned here a few weeks ago to attach a
block to a hash so I can return an error if someone tries to access a
key which does not exist:

a = Hash.new { |key,value|
raise “Missing key [#{key}] => [#{value}]”
}
puts a[“yay”]

Strangely, this results in the following:

missing [] => [yay]
nil

In other words, I try to access a key, and the attached block complains
that that key doesn’t exist, but it treats it as a value instead.

I can work around this, obviously, because it’s consistent, but can
someone explain this to me? Is this behaviour going to change? Am I just
not understanding what is going on?

Thanks,
Luke

···

–
“What’s the good of having mastery over cosmic balance and knowing the
secrets of fate if you can’t blow something up?”
– Terry Pratchett, “Reaper Man”

2

a = Hash.new { |key,value|

                   ^^^^^^^^^

This is wrong, it's

   a = Hash.new {|hash, key| }

svg% ruby -e 'a = Hash.new {|h,k| raise "#{h.inspect} -- #{k}"}; a[1] = 2; a[2]'
-e:1: {1=>2} -- 2 (RuntimeError)
        from -e:1:in `call'
        from -e:1:in `default'
        from -e:1:in `'
        from -e:1
svg%

Guy Decoux

3

Ah, ok. That makes much more sense. :slight_smile: I see now.

Thank you.

Luke

···

On Sun, 11 Jan 2004, ts wrote:

a = Hash.new { |key,value|
^^^^^^^^^

This is wrong, it’s

a = Hash.new {|hash, key| }

–
Automatic door locks are good for…
a. security
b. convenience
c. messing with the heads of people trying to get in