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]:

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]:

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.