add exercise unsing matrix module made in create funtion chapter

source/Modules_and_Packages.rst

@@ -3,3 +3,49 @@
 ********************
 Modules and Packages
 ********************
+
+Exercises
+=========
+
+
+Exercise
+--------
+
+Implement a matrix and functions to handle it.
+choose the data structure of your choice.
+The API (**A**\ pplication **P**\ rogramming **I**\ nterface) to implemet is the following:
+
+         
+.. literalinclude:: _static/code/matrix.py
+   :linenos:
+   :language: python
+
+:download:`matrix.py <_static/code/matrix.py>` .  
+
+Exercise
+--------
+
+Write a program that calculates the similarity of 2 RNA sequences.
+
+* To compute the simalirity you need to parse a file containing the similarity matrix.
+* The similarity of the 2 sequences is the sum of base similarities. 
+  so you have to compare the first base of to sequence and use the matrix to get the similarity
+  from the similarity table, on so on for all bases then sum these similarities.
+  
+.. note::
+   as we  don't yet see how to read a file, we provide a list of strings that represents the file
+   as we can get them if we read that file.
+   
+::
+
+   lines = iter(['  A G C U\n'
+                 'A 1.0 0.5 0.0 0.0\n',
+                 'G 0.5 1.0 0.0 0.0\n',
+                 'C 0.0 0.0 1.0 0.5\n',
+                 'U 0.0 0.0 0.5 1.0\n'])
+
+.. literalinclude:: _static/code/similarity.py
+   :linenos:
+   :language: python
+
+:download:`similarity.py <_static/code/similarity.py>` .        
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