Dictionary in Pyhton

In python, a dictionary is a mixed collection of elements. Unlike other collection data types such as a list or tuple, the dictionary type stores a key along with its element. The keys in a Python dictionary is separated by a colon ( : ) while the commas work as a separator for the elements. The key value pairs are enclosed with curly braces { }.

Syntax of defining a dictionary:

Dictionary_Name = { Key_1: Value_1,
Key_2:Value_2,
……..
Key_n:Value_n
}

Key in the dictionary must be unique case sensitive and can be of any valid Python type.

Creating a Dictionary

# Empty dictionary
Dict1 = { }
# Dictionary with Key
Dict_Stud = { RollNo: 1234, Name:Murali, Class:XII, Marks:451}

Dictionary Comprehensions

In Python, comprehension is another way of creating dictionary. The following is the syntax of creating such dictionary.

Syntax

Dict = { expression for variable in sequence [if condition] }

The if condition is optional and if specified, only those values in the sequence are evaluated using the expression which satisfy the condition.

Example

Dict = { x : 2 * x for x in range(1,10)}
Output of the above code is
{1: 2, 2: 4, 3: 6, 4: 8, 5: 10, 6: 12, 7: 14, 8: 16, 9: 18}

Accessing, Adding, Modifying and Deleting elements from a Dictionary

Accessing all elements from a dictionary is very similar as Lists and Tuples. Simple print function is used to access all the elements. If you want to access a particular element, square brackets can be used along with key.

Example : Program to access all the values stored in a dictionary

MyDict = { 'Reg_No': '1221',
			'Name' : 'Tamilselvi',
			'School' : 'CGHSS',
			'Address' : 'Rotler St., Chennai 112' }
print(MyDict)
print("Register Number: ", MyDict['Reg_No'])
print("Name of the Student: ", MyDict['Name'])
print("School: ", MyDict['School'])
print("Address: ", MyDict['Address'])

Output:

{'Reg_No': '1221', 'Name': 'Tamilselvi', 'School': 'CGHSS', 'Address': 'Rotler St., Chennai 112'}
Register Number: 1221
Name of the Student: Tamilselvi
School: CGHSS
Address: Rotler St., Chennai 112

Note that, the first print statement prints all the values of the dictionary. Other statements are printing only the specified values which is given within square brackets.

In an existing dictionary, you can add more values by simply assigning the value along with key. The following syntax is used to understand adding more elements in a dictionary.

dictionary_name [key] = value/element

Example : Program to add a new value in the dictionary

MyDict = { 'Reg_No': '1221',
			'Name' : 'Tamilselvi',
			'School' : 'CGHSS', 'Address' : '
			Rotler St., Chennai 112'}
print(MyDict)
print("Register Number: ", MyDict['Reg_No'])
print("Name of the Student: ", MyDict['Name'])
MyDict['Class'] = 'XII - A' # Adding new value
print("Class: ", MyDict['Class']) # Printing newly added value
print("School: ", MyDict['School'])
print("Address: ", MyDict['Address'])

Modification of a value in dictionary is very similar as adding elements. When you assign a value to a key, it will simply overwrite the old value.

In Python dictionary, del keyword is used to delete a particular element. The clear( ) function is used to delete all the elements in a dictionary. To remove the dictionary, you can use del keyword with dictionary name.

Syntax:

# To delete a particular element.
del dictionary_name[key]
# To delete all the elements
dictionary_name.clear( )
# To delete an entire dictionary
del dictionary_name

Example : Program to delete elements from a dictionary and finally deletes the dictionary.

Dict = {'Roll No' : 12001, 'SName' : 'Meena', 'Mark1' : 98, 'Marl2' : 86}
print("Dictionary elements before deletion: \n", Dict)
del Dict['Mark1'] # Deleting a particular element
print("Dictionary elements after deletion of a element: \n", Dict)
Dict.clear() # Deleting all elements
print("Dictionary after deletion of all elements: \n", Dict)
del Dict
print(Dict) # Deleting entire dictionary

Output:

Dictionary elements before deletion:

{'Roll No': 12001, 'SName': 'Meena', 'Mark1': 98, 'Marl2': 86}

Dictionary elements after deletion of a element:

{'Roll No': 12001, 'SName': 'Meena', 'Marl2': 86}

Dictionary after deletion of all elements:

{ }

Traceback (most recent call last):

File "E:/Python/Dict_Test_02.py", line 8, in <module>

print(Dict)

NameError: name 'Dict' is not defined

Difference between List and Dictionary

(1) List is an ordered set of elements. But, a dictionary is a data structure that is used for matching one element (Key) with another (Value).

(2) The index values can be used to access a particular element. But, in dictionary key represents index. Remember that, key may be a number of a string.

(3) Lists are used to look up a value whereas a dictionary is used to take one value and look up another value.