From: Douglas Seifert <doug@...>
Date: 2009-09-20T21:06:48+09:00
Subject: Re: Getting to one attribute of my marshalled array of objects

--000e0cd358d2f3f6ec04740132db
Content-Type: text/plain; charset=ISO-8859-1

Brian,

I am having another issue, though.  In order to delete a car, I need to
> number the inventory when it displays so that I can ask the user to tell
> me which car to delete by its number on the list (this is part of the
> assignment so I can't really do it another way).
>
> At first I thought I would modify the to_s on CarInventory which right
> now just iterates through the array of Cars using .each and calls
> Cars.to_s, but then I decided it would be more robust if Cars.to_s was
> aware of whether it was being called repeatedly as part of a listing or
> not, from anywhere.
>

I would not go down this road.  A car should not care if it is being
displayed as part of a listing of other cars or if it is alone.  It should
just know how to display itself.  In addition, adding a parameter to to_s is
bad ruby style: to_s has well-defined semantics in the ruby language.  I
suggest before you go any further, you pick up a copy of "Programming
Ruby".  If you can't afford the money, the first edition is available for
free on line: http://www.rubycentral.com/book/.  The book does a far better
job than I ever could of explaining how the to_s method works in the Ruby
world.


>
> So I added a parameter "def to_s(listPosition=0)" to Car.to_s and
> changed the .each call in the CarInventory.to_s to pass a position
> number.  When I run the progrma now though I get a really bizarre
> behavior...I was supposed to get this type of output with 2 cars in my
> data file (simplified because I am also formatting it to comform to
> columns, etc.):
>
>
After looking at your implementations, I see what you are trying to
accomplish.  A better idea in my opinion would be to leave the counting
logic to the CarInventory class.  It is the class that knows about a list of
cars.

I noticed also that you tried to put some logic in to line up the car
attributes in a nice column layout.  You are reinventing the wheel here,
unfortunately.  One thing you should look up right away is is the % method
of class String:

$ ri String#%
--------------------------------------------------------------- String#%
     str % arg   => new_str
------------------------------------------------------------------------
     Format---Uses _str_ as a format specification, and returns the
     result of applying it to _arg_. If the format specification
     contains more than one substitution, then _arg_ must be an +Array+
     containing the values to be substituted. See +Kernel::sprintf+ for
     details of the format string.

        "%05d" % 123                       #=> "00123"
        "%-5s: %08x" % [ "ID", self.id ]   #=> "ID   : 200e14d6"

You should do an ri Kernel#sprintf to see all the good stuff that can go in
the format string.  If you are familiar with c/c++ at all, this will be
familiar to you.

I'm not sure also in your original to_s method on the CarInventory class how
it ever worked.  The each method of class Array returns the Array itself.
Your to_s method would end up returning an array of Car objects and it might
have been pure accident that it looked ok in your command loop.

Here is a revised to_s for the CarInventory class.  It handles the indexing
problem and makes sure it returns a String object in all cases.  I would
implement Car#to_s without a parameter and use the formatting methods above
to get a nice column layout

  def to_s
   if @carArray.first
     counter = 0
     @carArray.map {|car| counter += 1; "[%3d] %s" % ]counter,
car.to_s]}.join("\n")
   else
     return "(No inventory)"
   end
 end

I am not sure how a beginner goes about accumulating the knowlege necessary
to make implementing this method easy.  There is a lot of knowledge bundled
up in this implementation of to_s: use of Array#map and Array#join.
Knowledge of String#% and by implication, Kernel#sprintf.  I have done a lot
of reading I guess.

This whole thread is reminding me of a recent blog post by Jamis Buck, a guy
who I think it is fair to say is a Ruby community luminary:

http://weblog.jamisbuck.org/2009/9/17/there-is-no-magic-there-is-only-awesome-part-1

If you ever want to get good with Ruby, you need to apply his four rules:

   1. *Know thy tools.*
   2. *Know thy languages.*
   3. *Know thy libraries.*
   4. *Know thy communities.*

Applying these rules, you would know about the % method of the String class
and about Kernel#sprintf and Array#map and Array#join.

So keep studying!  It is a high hill to climb, but once you get over the
top, your programs will reach the level of conciseness that Ruby is famous
for.

-Doug Seifert

--000e0cd358d2f3f6ec04740132db--