From: dblack@...
Date: 2007-09-03T20:37:13+09:00
Subject: Re: assigning to hash keys when there is a default value?

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Hi --

On Mon, 3 Sep 2007, 7stud -- wrote:

> Pe=C3=B1a, Botp wrote:
>> they should be different (note, that has nothing to do w the bug dBlack
>> is pointing at) and the output should be,
>>
>> 5
>> {2=3D>5}    #<---dblack is complaining here because the ruby output is {=
}
>>
>> nil
>> {2=3D>10}
>>
>
> Yes.  That is what I am complaining about too!  Your are right: the
> second line for each section of the output should be different--I
> mispoke.  What I want to know is why the the output is {} and not
> {2=3D>5}.
>
> The author of "The Ruby Way (2nd ed)" presents the two examples as well
> as the different output for each example, yet he doesn't explain the
> reason for the different output.  He suggests that you can use ||=3D to
> selectively assign values only to keys that don't exist.  The point I
> think he was trying to make with the two examples is that when you have
> a default value for non-existent keys, all keys "exist", so ||=3D will no=
t
> create new keys with the default value.

I don't have the book at hand but the default value definitely doesn't
mean that all keys exist (or even "exist" :-)

irb(main):003:0> h =3D Hash.new(5)
=3D> {}
irb(main):004:0> h[1]
=3D> 5
irb(main):005:0> h.has_key?(1)
=3D> false

The only possible explanation I can come up with, which I don't like,
is that:

   h[2] ||=3D 10

is being treated like:

   5 =3D 5 || 10

which is just another can of worms.


David

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