From: dblack@...
Date: 2006-03-03T11:07:12+09:00
Subject: Re: Nuby - why does my #inspect misbehave?

Hi --

On Fri, 3 Mar 2006, cremes.devlist@mac.com wrote:

> I'm working through a few exercises to teach myself Ruby. One of them is the 
> creation of a deck of cards along with all the usual operations one performs 
> upon them.
>
> I override the Object#inspect method to pretty-print my card values, but it 
> doesn't always get called. Whether or not it gets called depends on the 
> **absence** of a line in #print_playing_deck. Here is my code stripped down 
> to the basics.
>
> class Card < Object

Don't inherit from Object -- it happens automatically :-)

> def inspect
>     puts "#{self.value} of #{self.suit}s"
> end

This is having a weird effect on what you get in irb, like when you
create a deck, you get:

   of s

because it's trying to print that string.  #inspect should really just
return a string, not print it out (as I think Logan also mentioned).

> def print_playing_deck
>   @playing_deck.each do |card|
>     puts card
>   end

One of the great rites of passage for new Rubyists is learning that:

   puts array

does the iterating for you :-)  So that should be just:

   puts @playing_deck

>   # WEIRD!! If I have any code after this loop, my Card#inspect is not 
> called.
>   # In this case I have a "puts" but it could even be arithmetic expressions 
> preventing
>   # the #inspect from working
>   #puts "#{@playing_deck.length} cards in deck"
> end
> end
>
> And here is the output:
>
>
> irb(main):001:0> load 'ex.rb'
> => true
> irb(main):002:0> d=CardDeck.new(5)
> of s
> =>
> irb(main):003:0> d.print_playing_deck
> #<Card:0x65b4>
> #<Card:0x63e8>
> #<Card:0x62a8>
> #<Card:0x61cc>
> #<Card:0x6168>
> 5 cards in deck
> => nil

That nil is the return value of puts, which is the last expression in
#print_playing_deck.

> ######## comment out the "puts" in #print_playing_deck and reload
> irb(main):004:0> load 'ex.rb'
> ./ex.rb:22: warning: already initialized constant SUITSIZE
> => true
> irb(main):005:0> d.print_playing_deck
> #<Card:0x65b4>
> #<Card:0x63e8>
> #<Card:0x62a8>
> #<Card:0x61cc>
> #<Card:0x6168>
> 1 of Hearts
> 2 of Hearts
> 3 of Hearts
> 4 of Hearts
> 5 of Hearts
> => [, , , , ]

Now the last expression in #print_playing is @playing_deck (the return
value of the call to #each).  To display @playing_deck, irb calls
#inspect on each element.  The side-effect of this is that "1 of
Hearts" etc. gets printed.  The return value of #inspect, however, is
nil (the value of puts), which is why you get the weird little array
of nothings at the end.


David

-- 
David A. Black (dblack@wobblini.net)
Ruby Power and Light (http://www.rubypowerandlight.com)

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