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|
* 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
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