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Python | Data Structure Or Data Type (Dictionary)

Data Type Of Python

In this session we will see the new data type of python which is Dictionary.

4) Dictionary:- Dictionary is a unordered collection of mutable data type. It contains key value pair enclosed in curly braces {}. Dictionary are hashable by keys.
Dictionary keys should be unique and key should be immutable. That means we can use string, tuple, frozenset as a dictionary key but not list, dictionary, or other mutable types.
  • Empty dictionary:- 
    D = {}

  • Dictionary with multiple elements:-
    D = { 'a':10, 'b':20, 'c':30 }
    Here the part before colon is the key and the part after colon is value like this dictionary has key value pair.

  • Iteration:- 
    D = { 'a':10, 'b':20, 'c':30 }
    For i in D:
           print(i)
    => 'a'
         'b'
         'c'
    In the dictionary only keys are hashable not values therefor when you iterate dictionary you get only keys.

  • Hashable:- 
    Dictionary are hashable by keys.
    For example:-
    D = { 'a':10, 'b':20, 'c':30 }
    print(D['a']) => 10
    It return the value of the respected key.

  • Updation:- 
    D = { 'a':10, 'b':20, 'c':30 }
    D['b'] = 'Hi' 
    => D ={ 'a':10, 'b':'Hi', 'c':30 }
    Here the value of key b is update.

  • Membership:-
    print('a' in D) => True
    print('d' in D) => False
Methods Of Dictionary:- 
  1. clear():- It removes all items from the dictionary.
    For example:-
    D = { 'a':10, 'b':20, 'c':30 }
    print(D.clear()) => {}

  2. get(key,default=none):- It return the value for a key if key is in the dictionary else it return none.
    For example:-
    D = { 'a':10, 'b':20, 'c':30 }
    print(D.get('a')) => 10
    print(D.get('e')) => none

  3. setdefault(key,default=none):- It insert the key with a value of default if key is not in the dictionary. It return value for the dictionary if the key is in the dictionary otherwise it return none.
    For example:-
    D = { 'a':10, 'b':20, 'c':30 }
    print(d.setdefault('a','Hi')) => 10
    print(d.setdefault('e')) => None
    print(d.setdefault('f','python')) => 'python'
    Here f is not the member of dictionary D when you use setdefault method the this key is add to the dictionary.

  4. pop(key,default):- It remove first specified key and return the corresponding value. If key is not found it return default otherwise raised KeyError.
    For example:-
    D = { 'a':10, 'b':20, 'c':30 }
    print(D.pop('a')) => 10
    print(D.pop('e', 'not present')) => 'not present'
    print(D.pop('f')) => KeyError

  5. popitem():- It remove and return only arbitary key,value pair as a tuple but raises key error if dictionary is empty.
    For example:-
    D = { 'a':10, 'b':20, 'c':30 }
    print(D.popitem()) => ('a',10)
    print(D.popitem()) => ('c',30)

  6. update(dict):- It update the dictionary with the key and value pair from another dictionary it overwrite the existing keys.
    For example:-
    D = { 'a':10, 'b':20, 'c':30 }
    D1 = {'a':'Hi', 'd':24, 'e':21}
    print(D.update(D1)) => D={'a':'Hi', 'b':20, 'c':30, 'd':24, 'e':21}
    Here in dictionary D and D1 a is the common key so in the output the value of a in dictionary D1 is overwrite the value of a in dictionary D.

  7. fromkeys(iterable,value=none):- It create a new dictionary with keys from iterable and values set to value which is default is none.
    For example:-
    D1 = {}
    S = 'Python'
    print(D1.fromkeys(S)) => { 'P':none, 'y':none, 't':none, 'h':none, 'o':none, 'n':none}
    Here String S is iterate and store in dictionary as a key and there value becomes none because we does not give any value to it. Now we another example with the value.
    print(D1.fromkeys('abc',10)) => {'a':10, 'b':10, 'c':10}
    Here we give the value 10 so it goes to the every key of the dictionary.

  8. keys():- It return the list of dictionary keys.
    For example:-
    D = { 'a':10, 'b':20, 'c':30 }
    print(D.keys()) => ['a', 'b', 'c']     
    Here a, b and c are the keys of dictionary D.

  9. values():- It return a list of dictionary values.
    For example:-
    D = { 'a':10, 'b':20, 'c':30 }
    print(D.values()) => [10, 20, 30]
    Here 10, 20 and 30 are the values of dictionary D.

  10. items():- It return list of two tuple as a key value pair.
    For example:-
    D = { 'a':10, 'b':20, 'c':30 }
    print(D.items()) => [('a',10), ('b', 20), ('c', 30)]

These all are the methods of dictionary.

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