From: Robert Klemme <shortcutter@...>
Date: 2012-05-01T06:10:04+09:00
Subject: Re: Question about array of hashes

On Mon, Apr 30, 2012 at 10:52 PM, Ralph Shnelvar <ralphs@dos32.com> wrote:
> I'm looking at  Page 39 of Why's (poignant) Guide to Ruby ( http://www.rubyinside.com/media/poignant-guide.pdf ) and I see
>
> [:shape => 'sock', :fabric => 'cashmere'] +
> [:shape => 'mouse', :fabric => 'calico'] +
> [:shape => 'eggroll', :fabric => 'chenille']
>
> The result of this is (irb 1.9.2)
>
> irb(main):001:0> kitty_toys =
> irb(main):002:0* [:shape => 'sock', :fabric => 'cashmere'] +
> irb(main):003:0* [:shape => 'mouse', :fabric => 'calico'] +
> irb(main):004:0* [:shape => 'eggroll', :fabric => 'chenille']
> => [{:shape=>"sock", :fabric=>"cashmere"}, {:shape=>"mouse", :fabric=>"calico"}, {:shape=>"eggroll", :fabric=>"chenille"}]
> irb(main):024:0> kitty_toys.size
> => 3
>
> That is, it is an array of 3 hashes
>
> But the lexically similar

Similar but not identical.

> irb(main):001:0> x=
> irb(main):002:0* [1, :fabric => 'cashmere'] +
> irb(main):003:0* [2, :fabric => 'calico'] +
> irb(main):004:0* [3, :fabric => 'chenille']
> => [1, {:fabric=>"cashmere"}, 2, {:fabric=>"calico"}, 3, {:fabric=>"chenille"}]
> irb(main):005:0> x.size
> => 6
>
> produces what is to me a surprisingly different result.

Why are you surprised that a different input gives a different output?

> Clearly, ruby has flattened things for x and yet ... it seems to have kept the hashes together for kitty_toys

No, it hasn't flattened anything.  Note what you have:

irb(main):007:0> [1, :fabric => 'cashmere']
=> [1, {:fabric=>"cashmere"}]

This is an Array with a Fixnum and a Hash.  And Array#+ creates a new
Array with the content of both arguments concatenated:

irb(main):010:0> [1,2] + [4,5]
=> [1, 2, 4, 5]

> I would have expected
>
> irb(main):006:0> y = [[1, {:fabric=>"cashmere"}], [2, {:fabric=>"calico"}], [3, {:fabric=>"chenille"}]]
> => [[1, {:fabric=>"cashmere"}], [2, {:fabric=>"calico"}], [3, {:fabric=>"chenille"}]]
> irb(main):007:0> y.size
> => 3

I am not sure why you expect that.  No nested Arrays are created.

> In fact, the more I stare at it, the more I think that the correct syntax should be
>
> doggy_toys =
> [{:shape => 'sock', :fabric => 'cashmere'}] +
> [{:shape => 'mouse', :fabric => 'calico'}] +
> [{:shape => 'eggroll', :fabric => 'chenille'}]
>
> Why is the kitty_toys syntax even legal?

It's Ruby's way to be nice to the user.  You can also use this for
method arguments:

irb(main):008:0> p(:foo => 1, :bar => 2)
{:foo=>1, :bar=>2}
=> {:foo=>1, :bar=>2}

And there are even Keyword arguments, which are translated to a Hash as well:

irb(main):009:0> p(foo: 3, bar: 4)
{:foo=>3, :bar=>4}
=> {:foo=>3, :bar=>4}

> I see that using the new 1.9 syntax for creating hash tables also works the same way.

Exactly.

> irb(main):018:0> birdy_toys =
> irb(main):019:0* [shape:'sock', fabric:'cashmere'] +
> irb(main):020:0* [shape:'mouse', fabric:'calico'] +
> irb(main):021:0* [shape:'eggroll', fabric:'chenille']
> => [{:shape=>"sock", :fabric=>"cashmere"}, {:shape=>"mouse", :fabric=>"calico"}, {:shape=>"eggroll", :fabric=>"chenille"}]
> irb(main):022:0> doggy_toys == birdy_toys
> => true
>
> So what are the rules for "implied hash table"?  Where would I find that documented?

I am sure "The Well-Grounded Rubyist" covers this.  Section 9.3, I believe.
http://www.amazon.com/dp/1933988657

Kind regards

robert

-- 
remember.guy do |as, often| as.you_can - without end
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