/doc/tutorials/simple_tutorial/step7_merge.rst
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- .. include:: ../../global.inc
- .. _Simple_Tutorial_7th_step:
- .. index::
- pair: @merge; Tutorial
- ###################################################################
- Step 7: Merging results back together
- ###################################################################
- * :ref:`Simple tutorial overview <Simple_Tutorial>`
- * :ref:`@merge in detail <decorators.merge>`
- .. note::
- Remember to look at the example code:
- * :ref:`Python Code for step 7 <Simple_Tutorial_7th_step_code>`
- Now that we have all the partial solutions in ``*.sums``, we can merge them
- together to generate the final answer: the variance of all 100,000 random
- numbers.
- **************************************************************************************
- Calculating variances from the sums and sum of squares of all chunks
- **************************************************************************************
- If we add up all the sums, and sum of squares we calculated previously, we can
- obtain the variance as follows::
-
- variance = (sum_squared - sum * sum / N)/N
-
- where ``N`` is the number of values
- See the `wikipedia <http://en.wikipedia.org/wiki/Algorithms_for_calculating_variance>`_ entry for a discussion of
- why this is a very naive approach!
-
- To do this, all we have to do is merge together all the values in ``*.sums``, i.e.
- add up the ``sums`` and ``sum_squared`` for each chunk. We can then apply the above (naive) formula.
-
- Merging files is straightforward in **Ruffus**:
-
- .. image:: ../../images/simple_tutorial_merge1.png
-
- .. ::
-
- ::
- @merge(step_6_calculate_sum_of_squares, "variance.result")
- def step_7_calculate_variance (input_file_names, output_file_name):
- #
- # add together sums and sums of squares from each input_file_name
- # calculate variance and write to output_file_name
- ""
- The :ref:`@merge <decorators.merge>` decorator tells *Ruffus* to take all the files from the step 6 task (i.e. ``*.sums``),
- and produced a merged file in the form of ``"variance.result"``.
-
- Thus if ``step_6_calculate_sum_of_squares`` created
- | ``1.sums`` and
- | ``2.sums`` etc.
-
- This would result in the following function call:
-
- .. image:: ../../images/simple_tutorial_merge2.png
- .. ::
- ::
-
- step_7_calculate_variance (["1.sums", "2.sums"], "variance.result")
-
- The final result is, of course, in ``"variance.result"``.