Rails - Sep 2012 - ruby koans don't understand the principle sandwhich code

hello, 

Im still working on ruby koans.
Now I have to do some sandwhich code.

The exercise looks like this :

require File.expand_path(File.dirname(__FILE__) + ''/edgecase'')

class AboutSandwichCode < EdgeCase::Koan

  def count_lines(file_name)
    file = open(file_name)
    count = 0
    while line = file.gets
      count += 1
    end
    count
  ensure
    file.close if file
  end

  def test_counting_lines
    assert_equal 4, count_lines("example_file.txt")
  end

  # ------------------------------------------------------------------

  def find_line(file_name)
    file = open(file_name)
    while line = file.gets
      return line if line.match(/e/)
    end
  ensure
    file.close if file
  end

  def test_finding_lines
    assert_equal "test\n", find_line("example_file.txt")
  end

  # ------------------------------------------------------------------
  # THINK ABOUT IT:
  #
  # The count_lines and find_line are similar, and yet different.
  # They both follow the pattern of "sandwich code".
  #
  # Sandwich code is code that comes in three parts: (1) the top slice
  # of bread, (2) the meat, and (3) the bottom slice of bread.  The
  # bread part of the sandwich almost always goes together, but
  # the meat part changes all the time.
  #
  # Because the changing part of the sandwich code is in the middle,
  # abstracting the top and bottom bread slices to a library can be
  # difficult in many languages.
  #
  # (Aside for C++ programmers: The idiom of capturing allocated
  # pointers in a smart pointer constructor is an attempt to deal with
  # the problem of sandwich code for resource allocation.)
  #
  # Consider the following code:
  #

  def file_sandwich(file_name)
    file = open(file_name)
    yield(file)
  ensure
    file.close if file
  end

  # Now we write:

  def count_lines2(file_name)
    file_sandwich(file_name) do |file|
      count = 0
      while line = file.gets
        count += 1
      end
      count
    end
  end

  def test_counting_lines2
    assert_equal 4, count_lines2("example_file.txt")
  end

  # ------------------------------------------------------------------

  def find_line2(file_name)
    # Rewrite find_line using the file_sandwich library function.
  end

  def test_finding_lines2
    assert_equal __, find_line2("example_file.txt")
  end

  # ------------------------------------------------------------------

  def count_lines3(file_name)
    open(file_name) do |file|
      count = 0
      while line = file.gets
        count += 1
      end
      count
    end
  end

  def test_open_handles_the_file_sandwich_when_given_a_block
    assert_equal __, count_lines3("example_file.txt")
  end

end


But I don''t get the principle.

Can anyone explain this to me ?

Roelof



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> But I don''t get the principle.
There is no such principle as the Sandwich principle.  The principle is 
Don''t Repeat Yourself(DRY).  So if you find yourself writing the same
or
similar code over and over again, try to figure out a way to extract the 
repeated part into a method.  That way you can write the code once in 
the method, and then call the method when needed.

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