Nuby - why does my #inspect misbehave?

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
attr_reader :suit, :value

protected
attr_writer :suit, :value

public
def initialize(suit = “Joker”, value = 15)
@suit = suit
@value = value
end

def inspect
puts “#{self.value} of #{self.suit}s”
end
end

class CardDeck < Card
attr_reader :size

SUITSIZE = 13

def initialize(size = 52)
@size = size

 @playing_deck = Array.new(@size) do |index|
   case index
   when *(0..12)
     Card.new("Heart", index + 1)
   end
 end
 @number_remaining = @playing_deck.length

end

def print_playing_deck
@playing_deck.each do |card|
puts card
end
# 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
######## 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
=> [, , , , ]

I don’t understand why my Card#inspect method doesn’t print anything
to stdout when there is code after the loop. Is this just a weird
artifact of irb?

BTW, running on OSX with darwinports Ruby 1.8.4.

On Mar 2, 2006, at 8:30 PM, [email protected] wrote:

attr_reader :suit, :value
def inspect
@size = size
def print_playing_deck

=>
./ex.rb:22: warning: already initialized constant SUITSIZE
4 of Hearts
5 of Hearts
=> [, , , , ]

I don’t understand why my Card#inspect method doesn’t print
anything to stdout when there is code after the loop. Is this just
a weird artifact of irb?

BTW, running on OSX with darwinports Ruby 1.8.4.

As a general rule #inspect is not supposed to print anything it is
supposed to return a string. Likewise in this case I believe your
inspect method is better named “to_s” since it gets you a string
representation of the object, not a peek into the objects internal
state. I don’t know what the cause of your bug is though. Irb will
usually print the inspect string of an evaluated expression on its
own though, and puts returns nil. So when irb does the equivalent of
p a_card its going to get back nil which is gonna be converted to the
empty string. This may be part of your problem, but first I would
suggest changing your code to match the usual ruby conventions and
secondly try running the code outside of irb.

Hi –

On Fri, 3 Mar 2006, [email protected] 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 :slight_smile:

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 :slight_smile: So that should be just:

puts @playing_deck

#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.

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 ([email protected])
Ruby Power and Light (http://www.rubypowerandlight.com)

“Ruby for Rails” chapters now available
from Manning Early Access Program! Ruby for Rails

Hi –

On Fri, 3 Mar 2006, James B. wrote:

:frowning:

?

David


David A. Black ([email protected])
Ruby Power and Light (http://www.rubypowerandlight.com)

“Ruby for Rails” chapters now available
from Manning Early Access Program! Ruby for Rails

[email protected] wrote:

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

puts array

does the iterating for you :slight_smile:

:frowning:


James B.

“Programs must be written for people to read, and only incidentally
for machines to execute.”

  • H. Abelson and G. Sussman
    (in "The Structure and Interpretation of Computer Programs)

unknown wrote:

Hi –

On Fri, 3 Mar 2006, James B. wrote:

:frowning:

?

O-(’_’ Q)

[email protected] wrote:

does the iterating for you :slight_smile:

:frowning:

?

POGSFJ

(I’ll refrain from invoking P*LS)

James