* Lists

*** Outline

***** Lists

***** Tuples

***** Arsenal Required

*** Script

    Welcome friends. 

    In previous tutorial we covered Lists and Tuples and related 

    functions. In this session we will continue with Python supported

    data structures and cover Dictionaries and sets. We have already 

    covered some basics of Dictionaries in session on Statistics. Here

    we will revisit those concepts and some new ones. 

    We give it a name and it should return a corresponding number. 

    Dictionaries are just key-value pair. For each 'key' there is

    corresponding 'value'. In lists we have indexes to access elements,

    here we have 'key'. 

    '{}' are used to create Python dictionaries. Lets start by opening 

    IPython interpreter. Lets create a dictionary say

    player = {'Mat': 134,'Inn': 233,

    'Runs': 10823, 'Avg': 52.53}

    Its dictionary storing statistics of a cricket player.

    Now to get the 'average' of this player we have to simply write

    print player['Avg']

    52.53

    To add a new key-value to this dictionary we have to something like

    player['Name'] = 'Rahul Dravid'

    print player    

    Please dont forget that Python dictionaries dont maintain the order

    in which key-value pair are stored it changes 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 new one. So

    print player

    will have updated value

    As we covered in one of previous sessions to iterate through lists 

    we use 'for'. In case of dictionaries we can iterate over them using 

    'keys' for example

    for key in player:

        print key, player[key]

    We saw how containership works in lists, there we can check if a 

    value is present in list or not, in case of Dictionaries it works

    only for 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 'value' it wont work.

    Dictionaries supports some functions to retrieve keys and values 

    like

    player.keys()

    returns the list of all 'keys'

    player.values()

    return list of all 'values'    

    Next we shall look at 'sets'. Sets in Python are unordered 

    collection of unique elements. This data structure comes 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])

    f10 is set of Fibonacci numbers less then 10

    lets print value of f10

    print f10

    As we mentioned earlier, these are unordered structure so order of

    elements are not maintained, and output order is different than 

    input order. Lets create one more 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 '|' operator

    f10 | p10

    For intersection & operator is used:

    f10 & p10

    f10 - p10 gives difference between f10 and p10, which is, elements

    present in f10 but not present in p10.

    ^ operator gives us symmetric difference that is p10 union f10 minus

    f10 intersection p10

    f10 ^ p10

    To check if a set is super set or subset greater than and lesser than

    operators can be 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.

    len function works with sets also:

    len(f10) returns the length, that is 5

    We can also use 'for' loops to iterate through a set.

    With this we come to the end of this tutorial on Dictionaries and 

    sets. We have covered some of properties of both data types and 

    functions supported by them. Thank you.

*** Notes