From: William James <w_a_x_man@...>
Date: 2006-03-26T20:08:54+09:00
Subject: Re: deciphering poignant guide chapter 4 example

john_sips_tea@yahoo.com wrote:
> Hi,
>
> I'm reading through the poignant guide, and am a bit stuck
> at the end of chapter 4
> ( http://poignantguide.net/ruby/chapter-4.html ). It's
> regarding the kitty_toys example:
>
> #!/usr/bin/ruby
>
> kitty_toys =
>     [:shape => 'sock', :fabric => 'cashmere'] +
>     [:shape => 'mouse', :fabric => 'calico'] +
>     [:shape => 'eggroll', :fabric => 'chenille']
>
> kitty_toys.sort_by { |toy| toy[:shape] }.each do |toy|
>     puts "Blixy has a #{ toy[:shape] } made of #{ toy[:fabric] }"
> end
>
> and I've actually got three sticking points with it:
>
> 1. For one thing, I don't understand the point of the ":shape"
> syntax. I don't understand why the author doesn't just write
> the string "shape" instead. What exactly is a "Symbol" object
> for?
>
> The poinant guide says it's just "words that look just like
> variables". And "Symbols are lightweight strings." But that
> doesn't help me much. The PickAxe 2nd ed, in chapter 3 says,
> "The construct :artist is an expression that returns a Symbol
> object corresponding to artist.", so, I can understand that
> (i.e. that there's some Symbol class and we're getting an
> instance of it by using that notation), but I'm still not
> getting the point... why not just use strings? What does
> having "Symbols" buy the programmer?
>
> 2. Next up, why is
>
> kitty_toys =
>     [:shape => 'sock', :fabric => 'cashmere'] +
>     [:shape => 'mouse', :fabric => 'calico'] +
>     [:shape => 'eggroll', :fabric => 'chenille']
>
> supposed to be shorthand for
>
> kitty_toys = [
>     {:shape => 'sock', :fabric => 'cashmere'},
>     {:shape => 'mouse', :fabric => 'calico'},
>     {:shape => 'eggroll', :fabric => 'chenille'}
> ]
>
> How does that work? (Hmm... what does adding Arrays
> in Ruby mean anyway? In Python it concatenates.)
> Why does this shorthand exist? Hmm,.. it doesn't seem
> to be saving much finger typing...
>
> 3. Finally, at the end of that example given in the poignant
> guide:
>
> #!/usr/bin/ruby
>
> # ... Create kitty_toys as shown above, then
>
> kitty_toys.sort_by { |toy| toy[:shape] }.each do |toy|
>     puts "Blixy has a #{ toy[:shape] } made of #{ toy[:fabric] }"
> end
>
> how does that "kitty_toys.sort_by" line work? I believe that
> braces and "do ... end" are equivalent, so it looks to me like
> there's some kind of "loop-in-a-loop" going on, as in:

No, kitty_toys.sort_by { |toy| toy[:shape] } simply produces an
array that's sorted by the shape of each toy.  Then he
iterates through the array and prints each toy.

>
> # Warning, Python code follows:
> for i in range(1, 8):
>     for j in range(1, 4):
>         print i, j
>
> Is there a more verbose way of writing that kitty_toys snippet
> to make it a bit more obvious what's going on? I mean, I guess
> the sort_by method is probably looking for something to sort
> kitty_toys on, and we're telling it to use what it finds in
> toy[:shape] for each hash it iterates over, but then, is that
> next "each" looping over items in a given hash, or ... gah. I'm
> not getting it. :)
>
> Thanks,
> ---John

Here's another way:

kitty_toys =
  [ { :shape, 'sock',     :fabric, 'cashmere'},
    { :shape, 'mouse',    :fabric, 'calico'},
    { :shape, 'eggroll',  :fabric, 'chenille'}
  ]

puts kitty_toys.sort_by { |toy| toy[:shape] }.map { |toy|
    "Blixy has a #{ toy[:shape] } made of #{ toy[:fabric] }"
}