1 * Lists

     2 *** Outline

     3 ***** Lists

     4 ***** Tuples

     5 ***** Arsenal Required

     6 *** Script

     7     Welcome friends. 

     8     

     9     In previous tutorial we covered Lists and Tuples and related 

    10     functions. In this session we will continue with Python supported

    11     data structures and cover Dictionaries and sets. We have already 

    12     covered some basics of Dictionaries in session on Statistics. Here

    13     we will revisit those concepts and some new ones. 

    14     

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

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

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

    18     here we have 'key'. 

    19     

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

    21     IPython interpreter. Lets create a dictionary say

    22 

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

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

    25     Its dictionary storing statistics of a cricket player.

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

    27     print player['Avg']

    28     52.53

    29 

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

    31     player['Name'] = 'Rahul Dravid'

    32     print player    

    33     Please dont forget that Python dictionaries dont maintain the order

    34     in which key-value pair are stored it changes as we add new entries.

    35 

    36     In dictionaries Duplicate keys are overwritten, that is when we do 

    37     player['Mat'] = 139

    38     It wont create a new entry, rather it will simply overwrite previous

    39     value with new one. So

    40     print player

    41     will have updated value

    42 

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

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

    45     'keys' for example

    46     for key in player:

    47         print key, player[key]

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

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

    50     only for keys. so

    51     'Inn' in player

    52     returns True

    53     'Econ' in Player

    54     returns False as there is no such 'key'

    55     If you try to look or search 'value' it wont work.

    56     Dictionaries supports some functions to retrieve keys and values 

    57     like

    58     player.keys()

    59     returns the list of all 'keys'

    60     player.values()

    61     return list of all 'values'    

    62 

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

    64     collection of unique elements. This data structure comes handy in

    65     situations while removing duplicates from a sequence, and computing 

    66     standard math operations on sets such as intersection, union, 

    67     difference, and symmetric difference. 

    68     

    69     Lets start by creating a set

    70     f10 = set([1,2,3,5,8])

    71     f10 is set of Fibonacci numbers less then 10

    72     lets print value of f10

    73     print f10

    74 

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

    76     elements are not maintained, and output order is different than 

    77     input order. Lets create one more set of all prime numbers less than

    78     10

    79     p10 = set([2,3,5,7])

    80     print p10.

    81     

    82     To get union of these two sets we use '|' operator

    83     f10 | p10

    84     

    85     For intersection & operator is used:

    86     f10 & p10

    87     

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

    89     present in f10 but not present in p10.

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

    91     f10 intersection p10

    92     f10 ^ p10

    93 

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

    95     operators can be used

    96     set([2,3]) < p10

    97     returns True as p10 is superset of given set

    98     

    99     Similar to lists and dictionaries sets also supports containership so

   100     2 in p10

   101     returns True as 2 is part of set p10 and 

   102     4 in p10

   103     returns False.

   104     

   105     len function works with sets also:

   106     len(f10) returns the length, that is 5

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

   108     

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

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

   111     functions supported by them. Thank you.

   112 

   113 *** Notes