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