From: Robert Klemme <bob.news@...>
Date: 2004-10-06T00:19:51+09:00
Subject: Re: Table-driven dispatch


"James B Crigler" <james.b.crigler@lmco.com> schrieb im Newsbeitrag
news:4162AC3A.2BF27AC@lmco.com...
> I need to read a file into memory and parse it for information on
> demand as a kind of ad hoc query mechanism.  (The file is
> a non-tabular report from a database.)
>
> Though there are other field types, for the time being, I only
> want to consider fields that appear with a tag on a single line,
> e.g.,
>
> Title:  A Tale Of Two Cities
>
> Author:  Charles Dickens
>
> There are various classes of files representing different object
> types as reported from the database.  The tags, even for fields
> like title and author, are not consistent among report types.  (I
> don't have control of the database, so I can't fix it.)
>
> My original implementation approximated the code you'll find below.
>
> There are some annoyances in the code:
>
> 1.  The deferred lookups have to be written individually (def
>     title, def author, etc.), though they only have to be written
>     once if they belong in the base class.
>
> 2.  I pass the variable into find_tagged_one_line to see whether
>     it already has a value, but I can't assign it there.
>
> Is there a way, e.g., with missing_method, to create
> a table-driven caching dispatch on the field names that performs
> assignment in the find_... method?
>
>
> class Literature
>     Tags = {
> 'title'    => %r[^Title:\s*],
> 'author'   => %r[^Author:\s*],
> # etc
>     }
>
>     def initialize(path)
> @text = Array.new
> @tags = Tags
> @title = nil
> @author = nil
> if File.exists? path
>     File.open(path).each do |line|
> @text.push line.chomp
>     end

Btw, you don't close the file handle properly here.

How about:

class TaggedItem
  def initialize(tags)
    @tags = tags
    @values = {}
  end

  def push(tag, value)
    (key = get_key tag) and @values[key] = value
  end

  def [](key) @values[key] end
  def value?(key) @values.has_key? key end

  def method_missing(s,*a)
    super unless a.empty?
    @values[s]
  end

private
  def get_key(tag) @tags[tag] end
end


lit = TaggedItem.new(
  "Title" => :title,
  "Author" => :author
)

nov = TaggedItem.new(
  "Novel Title" => :title,
  "Novel Author" => :author,
  "Year of publication" => :year
)

all=[lit, nov]

while ( line = gets )
  line.chomp!
  if %r{^([^:]+):\s*(.*)$} =~ line
    all.each {|x| x.push( $1, $2 )}
  end
end

p all
p lit.author

Of course you can do all sorts of changes here, for example, you could
inherit OpenStruct and thus get rid of @values.

Kind regards

    robert