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+We shall now look at a few more datatypes available in Python, namely,
+tuples, dictionaries and sets. Let us start with tuples. 
+
+Tuples
+======
+
+We shall learn
+
+ * what are tuples
+ * their similarities and dissimilarities with lists
+ * why are they needed
+
+Let's get started by defining a tuple. A tuple is defined by enclosing
+parentheses around a sequence of items seperated by commas. It is similar to
+defining a list except that parentheses are used instead of square brackets.
+
+::
+
+    t = (1, 2.5, "hello", -4, "world", 1.24, 5)
+    t
+
+defines a tuple. 
+
+It is not always necessary to use parenthesis around a ``tuple``
+
+::
+    a = 1, 2, 3
+    a
+    (1, 2, 3)
+
+    b = 1, 
+    b
+    (1,)
+
+The items in the tuple are indexed using numbers and can be accessed by using
+their position.
+
+::
+
+    t[3]
+
+prints -4 which is the fourth item of the tuple.
+
+::
+
+    t[1:5:2]
+
+prints the corresponding slice
+
+This is the behaviour similar as to lists. But the difference can be seen
+when we try to change an element in the tuple.
+
+::
+
+    t[2] = "Hello"
+
+We can see that, it raises an error saying tuple does not support item
+assignment. Tuples are immutable, and cannot be changed after creation.
+
+Then, what's the use of tuples?
+
+We shall understand that soon. But let us look at a simple problem of
+swapping values.
+
+``a = 5`` and ``b = 7``. swap the values of a and b
+
+We define the two values
+
+::
+
+    a = 5
+    b = 7
+
+    a
+    b
+
+Traditionally, we swap them using a temporary variable. 
+
+::
+
+    temp = a
+    a = b
+    b = temp
+
+    a
+    b
+
+The Python way, would be 
+
+::
+
+    a
+    b
+
+    a, b = b, a
+
+    a
+    b
+
+We see that the values are swapped. This idiom works for different data-types
+also.
+
+::
+
+    a = 2.5
+    b = "hello"
+
+    a
+    b
+
+Moreover this type of behaviour is something that feels natural and you'd
+expect to happen.
+
+This is possible because of the immutability of tuples. This process is
+called tuple packing and unpacking.
+
+Let us first see what is tuple packing. Type
+
+::
+
+    5,
+
+What we see is a tuple with one element.
+
+::
+
+    5, "hello", 2.5
+
+Now it is a tuple with three elements.
+
+So when we are actually typing two or more elements seperated by commas,
+those elements are packed into a tuple.
+
+When you type
+::
+
+    a, b = b, a
+
+First the values of b and a are packed into a tuple on the right side and then
+unpacked into the variables a and b.
+
+Immutability of tuples ensures that the values are not changed during the
+packing and unpacking.
+
+That brings us to the end of our discussion of tuples. Let us now look at
+dictionaries. 
+
+Dictionaries
+============
+
+A dictionary in general, are designed to be able to look up meanings of
+words. Similarly, the Python dictionaries are also designed to look up for a
+specific key and retrieve the corresponding value. Dictionaries are data
+structures that provide key-value mappings. Dictionaries are similar to lists
+except that instead of the values having integer indexes, dictionaries have
+keys or strings as indexes. 
+
+We shall now look at creating dictionaries, accessing elements of
+dictionaries, checking for presence of elements and iterating over the
+elements. 
+
+Let us start by creating an empty dictionary, type the following in
+your IPython interpreter.
+
+::
+
+    mt_dict = {}    
+
+Notice that curly braces are used define dictionaries.
+
+Now let us see how to create a non-empty dictionary,
+
+::
+
+    extensions = {'jpg' : 'JPEG Image', 
+                  'py' : 'Python script', 
+                  'html' : 'Html document', 
+                  'pdf' : 'Portable Document Format'}
+
+Notice that each key-value pair is separated by a comma, and each key and
+value are separated using a colon.
+
+Here, we defined four entries in the dictionary extensions. The keys are
+``jpg``, ``py``, ``html``, and ``pdf``.
+
+Simply type,
+
+::
+
+    extensions
+
+in the interpreter to see the content of the dictionary. Notice that in
+dictionaries the order cannot be predicted and you can see that the values
+are not in the order that we entered in.
+
+Like in lists, the elements in a dictionary can be accessed using the
+index, here the index is the key. Try,
+
+::
+
+    print extensions['jpg']
+
+It printed JPEG Image. And now try,
+
+::
+
+    print extensions['zip']
+
+As expected it gave us an error. Obviously, our dictionary didn't have any
+key 'zip', and that's what the error message says.
+
+Well that was about creating dictionaries, now how do we add or delete items.
+
+::
+
+    extensions['cpp'] = 'C++ code'
+
+Adds a new key value pair, ``cpp : C++ code``
+
+We delete items using the ``del`` keyword
+
+::
+
+    del extension['pdf']
+
+Let us check the content of the dictionary now,
+
+::
+
+    extensions
+
+So the changes have been made. Now let us try one more thing,
+
+::
+
+    extensions['cpp'] = 'C++ source code'
+    extensions
+
+As you can see, it neither added a new thing nor gave an error, but it
+simply replaced the existing value with the new one.
+
+Now let us learn how to check if a particular key is present in the
+dictionary. For that we can use ``in``,
+
+::
+
+    'py' in extensions
+    'odt' in extensions
+
+It will return ``True`` if the key is found in the dictionary, and
+will return ``False`` if key is not present. Note that we can check
+only for container-ship of keys in dictionaries and not values.
+
+Now let us see how to retrieve the keys and values. We can use the
+method ``keys()`` for getting a list of the keys in a particular
+dictionary and the method ``values()`` for getting a list of
+values. Let us try them,
+
+::
+
+    extensions.keys()
+
+It returned the ``list`` of keys in the dictionary extensions. And now
+the values, 
+
+::
+
+    extensions.values()
+
+It returned the ``list`` of values in the dictionary.
+
+Now let us print the data in the dictionary. We can use ``for`` loop to
+iterate.
+
+::
+
+    for each in extensions.keys():
+        print each, "-->", extensions[each]
+
+
+This brings us to the end of our discussion on dictionaries. Let us now look
+at sets. 
+
+Sets
+====
+
+We shall look at, 
+
+ * sets 
+ * operations on sets
+
+Sets are collections of unique elements. ``set`` datastructure in Python
+provides an implementation of this. 
+
+Lets look at how to input sets.
+
+::
+ 
+    a_list = [1, 2, 1, 4, 5, 6, 2]
+    a = set(a_list)
+    a
+     
+We can see that duplicates are removed and the set contains only unique
+elements.
+
+::
+
+    f10 = set([1, 2, 3, 5, 8])
+    p10 = set([2, 3, 5, 7])
+
+* f10 is the set of fibonacci numbers from 1 to 10.
+* p10 is the set of prime numbers from 1 to 10.
+
+Various operations that we do on sets are possible here also.
+
+The | (pipe) character stands for union
+
+::
+
+    f10 | p10
+
+gives us the union of f10 and p10
+
+The & (ampersand) character stands for intersection.
+
+::
+
+    f10 & p10
+
+gives the intersection
+
+similarly,
+
+::
+
+    f10 - p10
+
+gives all the elements that are in f10 but not in p10
+
+::
+
+    f10 ^ p10
+
+is all the elements in f10 union p10 but not in f10 intersection p10. In
+mathematical terms, it gives the symmectric difference.
+
+Sets also support checking of subsets.
+
+::
+
+    b = set([1, 2])
+    b < f10
+
+gives a ``True`` since b is a proper subset of f10.
+
+Similarly,
+::
+
+    f10 < f10
+
+gives a ``False`` since f10 is not a proper subset.
+
+Where as, 
+
+::
+
+    f10 <= f10
+
+gives ``True`` since every set is a subset of itself.
+
+Sets can be iterated upon just like lists and tuples. 
+
+::
+
+    for i in f10:
+        print i,
+
+prints the elements of f10.
+
+The length and containership check on sets is similar as in lists and tuples.
+
+::
+
+    len(f10)
+
+shows 5. And
+
+::
+    
+    1 in f10
+    4 in f10
+
+prints ``True`` and ``False`` respectively
+
+The order in which elements are organised in a set is not to be relied upon.
+There is no ordering of elements of a set. Sets do not support indexing and
+hence slicing and striding do not make sense, either. 
+
+Here's an example that shows the use of sets. 
+
+Given a list of marks, marks = [20, 23, 22, 23, 20, 21, 23] list all the
+duplicates
+
+Duplicates marks are the marks left out when we remove each element of the 
+list exactly one time.
+
+::
+
+    marks = [20, 23, 22, 23, 20, 21, 23] 
+    marks_set = set(marks)
+    for mark in marks_set:
+        marks.remove(mark)
+
+    # we are now left with only duplicates in the list marks
+    duplicates = set(marks)
+
+This brings us to the end of our discussion on sets. 
+.. 
+   Local Variables:
+   mode: rst
+   indent-tabs-mode: nil
+   sentence-end-double-space: nil
+   fill-column: 77
+   End:
+
+