Re: black magical hash element vivification

On Mon, Dec 5, 2011 at 9:05 PM, Chad Perrin <[e-mail]> wrote:
> On Tue, Dec 06, 2011 at 01:40:17AM +0900, Jes=FAs Gabriel y Gal=E1n wrote=
:
>>
>> Using that Hash constructor, what you pass is the default value for a
>> missing key. What this means is that the hash will return that object
>> to any call in which the key is not found. *But* it won't assign that
>> default object to the key. You have to do that yourself. The other
>> effect you are seeing is that the default object is returned for all
>> missing keys, hence the {2 =3D> true} for foo[2].
>
> I find it rather surprising that assignment does not create something new
> where the default used to appear, and kind of useless in this context. =
=A0I
> know it's a special case of assignment that causes this to occur, now
> that I think about it after reading replies to my original email, but it
> seems like a strange way to implement things from a language user
> perspective.
>
> Thanks to everybody who responded.

I think the surprise comes from the fact that you are using a mutable
object, or at least an object that only makes sense when modified.
There are some cases where it makes sense to have a default value that
doesn't imply assigning it to the missing key. For example, say you
have an histogram hash, and want to calculate the average of a certain
set of values:

histogram =3D Hash.new(0)
#fill the hash
possible_keys =3D [:a, :b, :c, :d] # some of them might not be in the histo=
gram:

average =3D (possible_keys.inject(0) {|total, element| total +
histogram[element]} ) * 1.0 / possible_keys.length

Here it totally makes sense to have 0 as the default value, and not
have all possible_keys assigned in the hash when you are calculating
an average for a set of keys that might or might not be in the hash.

I think that allowing this version of the constructor along with the
one where you have full control of what to do on a missing key, gives
you all the flexibility for a lot of use cases.

By the way, for the histogram case above, when you are counting
things, you also take advantage of 0 being the default value like
this:

histogram[key] +=3D 1

Jesus.