By: Archana Shukla and Rajesh Shukla
Dictionary
Python dictionaries are a collection of some key-value pairs. Dictionaries are mutable, unordered collections with elements in the form of key:value pairs that associate keys to value. Dictionary is enclosed within curly braces {}.
Creating a dictionary
To create a dictionary following syntax is used.
Dictionary name={key1:value1,key2:value2,…….}
Example
Subject={“A”:”Accounts”,”B”:”B-Studies”,”C”:”Chemistry”}
Accessing elements of dictionary
The elements of dictionary are accessed through the keys defined in the key:value pairs.
Syntax: dictionary name[key]
>>> Subject={"A":"Accounts","B":"B-Studies","C":"Chemistry"}
>>> Subject
Output
{“A”:”Accounts”,”B”:”B-Studies”,”C”:”Chemistry”}
>>> Subject[‘B’]
Output
‘B-Studies’
Dictionary Operations
Traversing a dictionary
Traversal means to access and process each element of the dictionary. This can be done through for loop.
Example :
dic1={'m':'maths','s':'science','c':'commerce','e':'english','h':'hindi'}
for key in dic1:
print(key,":",dic1[key])
Output
m : maths
s : science
c : commerce
e : english
h : hindi
Adding elements to dictionary
We can add new elements to the dictionary. But the key should not exist in the dictionary and must be unique. If the key already exists, then the value of existing key will be changed and no new entry will be added to the dictionary.
Syntax : dictionaryname[key]=value
Example 1:
dic1={'Teena':18,'Riya':12,'Aliya':13,'Priya':17}
print(dic1)
dic1['Ravi']=20
print(dic1)
output
{‘Teena’: 18, ‘Riya’: 12, ‘Aliya’: 13, ‘Priya’: 17}
{‘Teena’: 18, ‘Riya’: 12, ‘Aliya’: 13, ‘Priya’: 17, ‘Ravi’: 20}
Example 2: if the key already exist.
dic1={'Teena':18,'Riya':12,'Aliya':13,'Priya':17}
print(dic1)
dic1['Riya']=22
print(dic1)
output:
{‘Teena’: 18, ‘Riya’: 12, ‘Aliya’: 13, ‘Priya’: 17}
{‘Teena’: 18, ‘Riya’: 22, ‘Aliya’: 13, ‘Priya’: 17}
Deleting elements from the dictionary
There are two methods for deleting elements from the dictionary:
del
We can use del command to delete a dictionary entry :
syntax: del <dictionary>[key]
dic1={'Teena':18,'Riya':12,'Aliya':13,'Priya':17}
del dic1['Priya']
print(dic1)
output :
{‘Teena’: 18, ‘Riya’: 12, ‘Aliya’: 13}
pop()
This method is also used for deleting elements from the dictionary.
Syntax: <dictionary>.pop(key)
Example :
dic1={'Teena':18,'Riya':12,'Aliya':13,'Priya':17}
dic1.pop('Aliya')
print(dic1)
output:
{‘Teena’: 18, ‘Riya’: 12, ‘Priya’: 17}
Checking for existence of the key
Membership operators ‘in’ and ‘not in’ are used to check the existence of the keys in the dictionary. These operators will return true or false on the basis of the absence or presence of the key in the dictionary.
Syntax: <key> in <dictionary> / <key> not in <dictionary>
Example :
dic1={'Teena':18,'Riya':12,'Aliya':13,'Priya':17}
print('Teena' in dic1)
output: True
dic1={'Teena':18,'Riya':12,'Aliya':13,'Priya':17}
print('Riya' not in dic1)
output : False
Dictionary Functions
| Function | Description | Example |
| Len(dict) | Returns the total number of items present in the dictionary | >>> dic1={‘Teena’:18,’Riya’:12,’Aliya’:13,’Priya’:17} >>> print(len(dic1)) 4 |
| Str(dict) | Produces a printable string representation of a dictionary | >>> dic1={‘Teena’:18,’Riya’:12,’Aliya’:13,’Priya’:17} >>> str(dic1) “{‘Teena’: 18, ‘Riya’: 12, ‘Aliya’: 13, ‘Priya’: 17}” |
| Type(variable) | Returns the type of variable passed as an argument |
Dictionary Methods
| Methods | Description | Example |
| clear() | Removes all the elements of the dictionary | |
| get() | Returns the item with the specified key | >>> emp={‘name’:’Amit’,’class’:’XII’,’rollno’:1,’sub’:’computer science’} >>> emp.get(‘name’) ‘Amit’ |
| items() | Returns the list of (key,value) tuple pairs from the dictionary | >>> emp.items() dict_items([(‘name’, ‘Amit’), (‘class’, ‘XII’), (‘rollno’, 1), (‘sub’, ‘computer science’)]) |
| keys() | Returns the list of keys from the dictionary | >>> emp.keys() dict_keys([‘name’, ‘class’, ‘rollno’, ‘sub’]) |
| values() | Returns the list of values | >>> emp.values() dict_values([‘Amit’, ‘XII’, 1, ‘computer science’]) |
| update() | Adds the key:value pairs of one dictionary to other | dic1={‘Teena’:18,’Riya’:12,’Aliya’:13,’Priya’:17} dic2={‘amit’:20,’Ali’:22,’Riya’:20} dic1.update(dic2) print(dic1) |
