From: Robert Klemme <shortcutter@...>
Date: 2009-12-10T04:50:10+09:00
Subject: Re: convert string into a variable object

On 12/08/2009 09:57 PM, Walton Hoops wrote:
> 
>> -----Original Message-----
>> From: Robert Klemme [mailto:shortcutter@googlemail.com]
>>
>> 2009/12/8 David A. Black <dblack@rubypal.com>:
>>> HI --
>>>
>>> On Tue, 8 Dec 2009, Steve Wilhelm wrote:
>>>
>>>> Toon Willems wrote:
>>>>> On 07 Dec 2009, at 21:35, Ad Ad wrote:
>>>>>
>>>>>>> to do.
>>>>>> doesnt require the  creation of any sort of a container variable.
>>>>>> --
>>>>>> Posted via http://www.ruby-forum.com/.
>>>>>>
>>>>> A much better way to do this would be to store the city names in a
>> hash.
>>>>> your code would then be:
>>>>>
>>>>> File.open(@file). each do |x|
>>>>>  @hash[x] = true
>>>>> end
>>>> Would it not be better to convert to symbols? That is:
>>>>
>>>> File.open(@file). each do |x|
>>>>  @hash[x.strip.to_sym] = true
>>>> end
>>>>
>>>> if @hash[:toronto]
>>>> or
>>>> if @hash[:"palo alto"]
>>>>
>>>> or
>>>>
>>>> if @hash[city.to_sym]
>>> In addition to the garbage collection issue that David M. pointed
>> out,
>>> symbols seem an odd choice to me for city names. I always think of
>>> symbols as appropriate for label-like functionality and values taken
>>> from a limited set. Strings are a better choice for general
>>> representation of text.
>> I would certainly use String for city names.  Hard encoding city names
>> in classes which would be regenerated over and over again is not a
>> good solution.  Basically, you want to stick all city names as keys in
>> a Hash and work from there.  The mere fact that Ruby is able to
>> generate local, instance or global variables at runtime does not mean
>> it is a proper means in all cases.  The use case at hand does not call
>> for meta programming - it just needs a data structure which can work
>> with appropriate key values.  A Hash seems like a good fit here.
> 
> Now perhaps I'm missing something, but the impression I got was that 
> while the contents of the file changes daily, he doesn't want to remove
> old cities. Rather he wants to ensure he's not creating duplicates. My 
> solution (to the problem as I understand it) would be to use sets, but 
> if he's not actually wanting to remove anything, I don't think Strings
> vs. Symbols should matter.
> 
> Example of set solution:
> irb(main):001:0> require 'set'
> => true
> irb(main):002:0> cities=Set.new
> => #<Set: {}>
> irb(main):003:0> cities << 'Austin, Texas'
> => #<Set: {"Austin, Texas"}>
> irb(main):004:0> cities << 'Boring, Oregon'
> => #<Set: {"Austin, Texas", "Boring, Oregon"}>
> irb(main):005:0> cities << 'Boise, Idaho'
> => #<Set: {"Austin, Texas", "Boring, Oregon", "Boise, Idaho"}>
> irb(main):006:0> cities << 'Boring, Oregon'
> => #<Set: {"Austin, Texas", "Boring, Oregon", "Boise, Idaho"}>
> irb(main):008:0> cities.include? 'Boring, Oregon'
> => true
> irb(main):009:0>
> 
> Is there something I'm missing?

Maybe.  I was talking about the approach to generate variable names. 
City names in this case are part of the domain data.  They are read over 
and over again and apparently the task of the program is to manage city 
names somehow.

Variable names on the other hand are part of the application structure. 
  You'd rather have a variable @city_name here than a variable 
@new_york.  It does not make much sense to regenerate classes over and 
over again (at least that seems to be what OP wants to do) does not 
really make sense.

The question what is added or removed which you discussed is a 
completely different one.  Whether a Set or a Hash is a more appropriate 
data structure depends on the use case which I haven't seen much detail 
of.  (Maybe I'm missing something as well.)

Kind regards

	robert


-- 
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