* Dictionaries
*** Outline
***** Dictionaries
***** Sets
***** Arsenal Required
*** Script
    Welcome friends. 
    
    In previous tutorial we covered Lists, Tuples and related 
    functions. In this session we shall continue with Python
    data structures and cover Dictionaries and sets. We have already 
    covered some basics of Dictionaries in session on Statistics. Here
    we shall revisit those concepts and some new ones. 
    
    We give it a name and it returns a corresponding number. 
    Dictionaries are just key-value pair. For each 'key' there is
    corresponding 'value' associated with it. In lists we use indexes 
    to access elements, here we use the 'key'. 
    
    Lets start by opening IPython interpreter. 
    '{}' are used to create Python dictionaries. Lets create a dictionary say

    player = {'Mat': 134,'Inn': 233,
    'Runs': 10823, 'Avg': 52.53}
    Let's see what player contains by typing:

    print player

    Its a dictionary storing statistics of a cricket player.
    Here 'Mat', 'Inn' etc are the keys. Now in order to get the 'average' of
    this player we simply type
    print player['Avg']
    52.53

    To add a new key-value pair to this dictionary we type
    player['Name'] = 'Rahul Dravid'
    print player
    As you can see the given key-value pair has been added.
    Please note that Python dictionaries don't maintain the order
    in which the key-value pairs are stored. The order might change
    as we add new entries.

    In dictionaries Duplicate keys are overwritten, that is when we do 
    player['Mat'] = 139
    It wont create a new entry, rather it will simply overwrite previous
    value with the new one. So
    print player
    will have updated value

    As we covered in one of previous sessions 'for' can be used to iterate
    through lists. The same is possible in case of dictionaries too. We can
    iterate over them using the 'keys', for example:
    for key in player:
        print key, player[key]
    This prints the keys in the dictionary along with their corresponding 
    values. Notice that the order is not the same as we entered it.
    
    We saw how containership works in lists. There we can check if a 
    value is present in a list or not but in case of Dictionaries we
    can only check for the containership of the keys. so
    'Inn' in player
    returns True
    'Econ' in Player
    returns False as there is no such 'key'
    If you try to look or search for a 'value' it will not work.
    Dictionaries support functions to retrieve keys and values 
    such as
    player.keys()
    returns the list of all 'keys'
    player.values()
    return list of all 'values'    

    Now we shall move on to 'sets'. Sets in Python are an unordered 
    collection of unique elements. This data structure comes in handy in
    situations while removing duplicates from a sequence, and computing 
    standard math operations on sets such as intersection, union, 
    difference, and symmetric difference. 
    
    Lets start by creating a set
    f10 = set([1,2,3,5,8])
    And thats how a set is created.
    f10 is the set of Fibonacci numbers less than 10
    lets print the value of f10
    print f10

    As we mentioned earlier, these are unordered structure so order of
    elements is not maintained, and output order is different than 
    input order, just as in dictionaries. Lets create one more set, a set of
    all prime numbers less than 10
    p10 = set([2,3,5,7])
    print p10.
    
    To get union of these two sets we use the or '|' operator
    f10 | p10
    
    For intersection we use the and '&' operator:
    f10 & p10
    
    f10 - p10 gives difference between f10 and p10, that is, the set of all elements
    present in f10 but not in p10.
    The carat '^' operator gives us the symmetric difference of 2 sets. That is
    f10 union p10 minus f10 intersection p10
    f10 ^ p10

    To check if a set is the super set or a subset of another set, the greater than 
    and the lesser than operators are used
    set([2,3]) < p10
    returns True as p10 is superset of given set
    
    Similar to lists and dictionaries, sets also supports containership so
    2 in p10
    returns True as 2 is part of set p10 and 
    4 in p10
    returns False.
    
    The 'len' function works with sets also:
    len(f10) returns the length, which is 5 in this case.
    We can also use 'for' loops to iterate through a set just as with dictionaries and lists.
    
    With this we come to the end of this tutorial on Dictionaries and 
    sets. We have seen how to initialize dictionaries, how to index them using keys
    and a few functions supported by dictionaries. We then saw how to initialize
    sets, perform various set operations and a few functions supported
    by sets. Hope you have enjoyed it, Thank you.

*** Notes