From: john_sips_tea@...
Date: 2006-03-26T16:18:53+09:00
Subject: deciphering poignant guide chapter 4 example

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:

# 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