Python3 Dictionaries DS

Python Data Structures : Dictionaries

Dictionary

         Python dictionary is an unordered collection of items. While other compound data types have only value as an element, a dictionary has a key : value pair.

Dict Creation :

 

Eg:

        #empty dictionary

       my_dict={}

        #dictionary with integers as keys

       my_dict={1:'a',2:'b"}

       print(my_dict)

       

       #dictionary with mixed keys

       my_dict={'name':'satish',1:['abc','xy']}

       print(my_dict)

         #create empty dictionary using dict( )

       my_dict=dict()

       my_dict=dict([(1,'abc'),(2,'xy')])     #create a dict with list of tuples

       print(my_dict)

output:  {1:'a',2:'b'}

              {'name':'satish',1:['abc','xy']}

              {1:'abc',2:'xy'}   

Dict Access :

Eg:

       my_dict={'name': 'suma','year':2020}

       print(my_dict['name'])

        #if key is not present it gives KeyError

       print(my_dict['address'])

output: suma

            ----------------------------------------------------------------------

             --

             KeyError

             KeyError: 'address'

get() function:       

       #another way of acessing key

       print(my_dict.get('year')

        #if key is not present it will give None using get method

       print(my_dict.get('address')

       

output:  2020

              None

Dict Add or Modify Elements :

Eg:

       my_dict={'name':'uma','year':2020,'address':'kadapa'}

       #update name

       my_dict['name']='suma'

       print(my_dict)

        #add new key

       my_dict['degree']='B.Tech'

       print(my_dict)

output: {'name':'suma','year':2020,'address':'kadapa'}

              {'name':'suma', 'year':2020, 'address':'kadapa' ,'degree':'B.Tech'}

 

Dict Delete or Remove Element :

Eg: 

       my_dict={'name':'suma','year':2020,'address':'kadapa'}

        #remove a particular item

       print(my_dict.pop('year'))

       print(my_dict)

output:  2020

              {'name':'suma','address':'kadapa'}

Eg:

       my_dict={'name':'suma','year':2020,'address':'kadapa'}

       #remove any arbitrary key

       my_dict.popitem()            

       print(my_dict)

output:  {'name':'suma','year':2020}

Eg:

       squares={2:4,3:9,4:16,5:25}

       del squares[2]

       print(squares)

       squares.clear( )

       print(squares) 

output: {3:9,4:16,5:25}

              { } 

Eg:

       squares={2:4,3:9,4:16}

       #delete dictionary itself

       del squares

       print(squares)

output:  -----------------------------------------------------------

              --

              NameError 

              NameError: name 'square' is not  defined

Dictionary Methods :

Eg:

       squares={2:4,3:9,4:16,5:25}

       my_dict=squares.copy()

       print(my_dict)

output:  {2:4,3:9,4:16,5:25}

       #fromkeys[seq[ ,v]]  ->Return a new dictonary with keys from seq and values for all keys is same

       subject={ }.fromkeys(['math','english','social'],0)

       print(subject)

output:  {'math':0,'english':0,'social':0}

Eg:

       subjects={2:4,3:9,4:16,5:25}

       print(subject.items( ))        #return a new view of the dictionary items (key,value)

output:  dict_items([(2,4),(3,9),(4,16),(5,25)])

Eg:

       subject={2:4,3:9,4:16,5:25}

       print(subject.keys( ))            #return a new view of the dictionary keys

output:  dict_keys([2,3,4,5])

Eg:

       subject={2:4,3:9,4:16,5:25}

       print(subject.values( ))        #return a new view of the dictionary values

output:  dict_values([4,9,16,25])

   

       #get list of all available methods and attributes of dictionary

       d={ }

       print(dir(d))

 

output:  ['__class__','__contain__','__delattr__','__delitem__','__dir__','__doc__','__eq__','__format__','__gr__','__qetattribute__','__getitem__','__gt__','__hash__','__init__','__init_subclass__',''__iter__','__le__','__len__','__lt__,'__ne__','__new__','__reduce__','__reduce_ex__','__repr__','__setaattr__','__setitem__','__sizeof__','__str__','__subclasshook__','clear','copy','fromkeys','get','items','keys','pop','popitem','setdefault','update','values']

Dict Comprehension :

Eg:

       #dict comprehension are just like list comprehensions but for dictionaries

       d={'a':1,'b':2,'c':3}

       for pair in d.items( ):  

             print(pair)

output:  ('a',1)

               ('c',3)

               ('b',2)

Eg 1:

       #Creating a new dictionary with only pairs where the value is larger than 2

       d={'a':1,'b':2,'c':3,'d':4,'e':5}

       new_dict={k:v for k,v in d.items( ) if v>2}

       print(new_dict)

output:  {'c':3,'d':4,'e':5}

Eg 2:

       d={'a':1,'b':2,'c':3,'d':4}

       d={k+'c':v*2 for k,v in d.items( ) if v>2}]

       print(d)

output: {'cc':6,'dc':8,'ec':10}