Dictionary- Key: Value Pairs In Python Programing

 

Dictionaries  

Represents a group of elements arranged in the form of key-value pairs.  The key and its value are separated by a colon, Unordered mutable collection of key-value pairs .

Creating Dictionaries 

1. Initializing dictionary- all the key:value pairs of a dictionary are written collectively

s={'Name': 'Anand', 'AdmnNo': 2367, 'Class': 11, 'Per': 92.7}

2. Adding key:value pairs to an empty dictionary 

s={ }  s=dict()  s['Name']='Anand'  s['AdmnNo']=2367  s['Class']=11  s['Per']=92.7  print(s) Out[1]: {'Name': 'Anand', 'AdmnNo': 2367, 'Class': 11, 'Per': 92.7}

3. Creating a dictionary from name:value pairs 

(i) Specify key:value pairs as keyword arguments to dict() function 

s=dict(Name='Anand',AdmnNo=2367,Class=11, Per=92.7)  print(s) Out[1]: {'Name': 'Anand', 'AdmnNo': 2367, 'Class': 11, 'Per': 92.7}

(ii) Specify comma separated key:value pairs 

s=dict({'Name':'Anand','AdmnNo':2367,'Class': 11,'Per':92.7})  print(s)  Out[1]: {'Name': 'Anand', 'AdmnNo': 2367, 'Class': 11, 'Per': 92.7}

(iii) Specify keys separately and corresponding values separately 

s=dict(zip(('Name','AdmnNo','Class','Per'),('Anand',2367,11,92.7)))  print(s)  Out[1]: {'Name': 'Anand', 'AdmnNo': 2367, 'Class': 11, 'Per': 92.7}

(iv) Specify key:value pairs separately in the form of sequence 

s=dict([['Name','Anand'],['AdmnNo',2367],['Class',11],['Per',92.7]]) print(s)  Out[1]: {'Name': 'Anand', 'AdmnNo': 2367, 'Class': 11, 'Per': 92.7}

s=dict((['Name','Anand'],['AdmnNo',2367],['Class',11],['Per',92.7]))  print(s) Out[2]: {'Name': 'Anand', 'AdmnNo': 2367, 'Class': 11, 'Per': 92.7}

Accessing elements of a dictionary 

• While in lists, the elements are accessed through their index, in dictionaries elements are accessed through their keys.

• A dictionary operation that takes a key and finds the corresponding value is called lookup. 

student= { 'Name':'Anand', 'AdmnNo':2367, 'Class':12, 'Per': 92.7 }  print(student['Name'])  Out[1]:‘Anand’ print(student['Per'])  Out[2]:92.7

• Attempting to access a key that does not exist causes an error. 

print(student['RollNo']) KeyError : 'RollNo'

• As Python dictionaries are unordered, key order is not guaranteed in Python. 

• Keys act like indexes to access values from a dictionary. 

Accessing Keys or Values Simultaneously 

<dictionary>.keys() : method to view all keys in one go 

<dictionary>.values() : method to view all values in one go 

print(student) Out[1]: {'Name': 'Anand', 'AdmnNo': 2367, 'Class': 12, 'Per': 92.7}  print(student.keys())  Out[2]: dict_keys(['Name', 'AdmnNo', 'Class', 'Per']) print(student.values())  Out[3]: dict_values(['Anand', 2367, 11, 92.7])

Characteristics of a Dictionary

Dictionaries like lists are mutable and that is the only similarity they have with lists. Otherwise, dictionaries are different types of data structures with following characteristics.

1.Unordered Set

 A dictionary is an unordered set of key:value pairs. Its value can contain references to any type of object .You cannot tell the order or position of the key:value pairs in a dictionary there is no index associated.

2.Not a sequence

Unlike a string, list and tuple, a dictionary is not a sequence because it is an unordered set of elements. The sequences are indexed by a range of ordinal numbers. Hence, they are ordered, but a dictionary is an unordered collection.

3.Indexed by Keys,Not Numbers

Dictionaries are indexed bike keys. According to python, a key can be “any non table type”. Since strings and numbers are not mutable you can use them as a key in a dictionary. And if a tuple contains immutable objects such as integers or strings etc., then only it can also be used as a key. But the values in a dictionary can be of any type, and types can be mixed within one dictionary. 

4. Keys must be Unique

Each of the keys within a dictionary must be unique. Since keys are used to identify values in a dictionary, there cannot be duplicate keys in a dictionary. However, two unique keys can have same values. 

5.Dictionaries are Mutable

Like lists, dictionaries are also mutable. We can change the value of a certain key “in place” using the assignment statement as per syntax 

     <dictionary>[< key>] = <value>

6.Internally Stored as Mapping

Internally, the key:value pairs of a dictionary are associated with one another with some internal functions (called hash-functions). This way of linking is called mapping.