From: Michael Morin <uzimonkey@...>
Date: 2008-08-19T15:36:31+09:00
Subject: Re: passing a variable from a block back to the method

James Dinkel wrote:
> I have several files that I have to read, check the contents, and then
> write any changes.  I overloaded the File class to help me out with
> this:
> 
> ----------------------------------------------------------
> class File
> 
>   def self.change(filename)
> 
>     # read the file, execute a block, then write the file
>       File.open(filename, 'r+') do |file|
>         lines = file.readlines
>         yield lines
>         # return if not changing anything
>         file.pos = 0
>         file.print lines
>         file.truncate(file.pos)
>       end
>   end
> end
> ----------------------------------------------------------
> 
> The method is called with:
> 
> ----------------------------------------------------------
> File.change('awesomefile') do |contents|
>   contents.collect! do |line|
>     # magic happens
>     # decide not to change anything
>   end
> end
> ----------------------------------------------------------
> 
> This is a huge waste if the files don't change (as they often don't).
> The problem I'm having is with the "# decide not to change anything" in
> the block and passing that back to the method so "# return if not
> changing anything" knows the answer.  I'm not sure if I can pass an
> instance variable around to do this?  I don't really know how that would
> work since I don't really have an instance of file to work with.
> 
> Maybe someone has some ideas?

This works well.  However, you have to be careful the last statement in 
your block doesn't accidentally evaluate to nil, so I added a true at 
the end just in case.

class File

   def self.change(filename)

     # read the file, execute a block, then write the file
     File.open(filename, 'r+') do|file|
       lines = file.readlines

       # return if not changing anything
       return if yield(lines) == false

       file.pos = 0
       file.print lines
       file.truncate(file.pos)
     end
   end
end

File.change('test.txt') do|contents|
   break false if contents.size < 5

   contents.each do|c|
     c.gsub!(/[[:digit:]]/, 'X')
   end

   true
end

You can also use catch and throw.  I think this method is a little cleaner.

class File

   def self.change(filename)
     catch(:nochanges) do
       # read the file, execute a block, then write the file
       File.open(filename, 'r+') do|file|
         lines = file.readlines

         # return if not changing anything
         return if yield(lines) == false

         file.pos = 0
         file.print lines
         file.truncate(file.pos)
       end
     end
   end
end

File.change('test.txt') do|contents|
   throw :nochanges if contents.size < 5

   contents.each do|c|
     c.gsub!(/[[:digit:]]/, 'X')
   end
end

-- 
Michael Morin
Guide to Ruby
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