Dictionaries

Dictionary is a set of key-value pairs, where value is any hashable object. As Lists are indexed by integers, Dictionries are indexed by keys.

Creating Dictionaries

Creating an Empty Dictionary

x = {}      # {} denotes dictionary type (not set)
      x = dict()

Creating Dictionary with initial values
```
x = {'eight':8,'nine':9}
```



In [1]:

    
x = {'eight':8,'nine':9}



In [2]:

    
x['eight']









    Out[2]:





8



In [3]:

    
x[0]









    



---------------------------------------------------------------------------
KeyError                                  Traceback (most recent call last)
<ipython-input-3-1ae75c28907a> in <module>()
----> 1 x[0]

KeyError: 0

If a key is not present in Dictionary, KeyError is raised



In [4]:

    
x['zero'] = 0 # Adding a key-value to dictionary
x









    Out[4]:





{'eight': 8, 'nine': 9, 'zero': 0}



In [5]:

    
del x['nine']  # Deleting based on key
x









    Out[5]:





{'eight': 8, 'zero': 0}



In [6]:

    
'zero' in x    # Only key membership can be checked









    Out[6]:





True

Dictionary Methods

If d is a dictionary

d.keys() returns a view of d's keys
d.values() returns a view of d's values
d.items() returns a view of d's key-value pairs



In [7]:

    
x









    Out[7]:





{'eight': 8, 'zero': 0}



In [8]:

    
x.keys()









    Out[8]:





dict_keys(['zero', 'eight'])



In [9]:

    
x.values()









    Out[9]:





dict_values([0, 8])



In [10]:

    
x.items()









    Out[10]:





dict_items([('zero', 0), ('eight', 8)])



In [12]:

    
for k,v in x.items():
    print("{} = {}".format(k,v))









    



zero = 0
eight = 8

Dictionary Values can be any hashable object. This means they can be lists, tuples,... . Using Dictionaries, one can implement an Adjacency List Representation of Graph Data Structure.