Python dictionary

Dictionary is a set of unordered, changeable and indexed key value pairs.

Each key and value are separated by colon : and each pair of key value is separated by comma ,

Declaring a dictionary

my_dict={1:'Alex',2:'Ronald'}
print(my_dict)

Output

{1: 'Alex', 2: 'Ronald'}

Creating dictionary by using fromkeys()

x=('a','b','c')
y=('value1')
my_dict=dict.fromkeys(x,y)
print(my_dict)

Output is here

{'a': 'value1', 'b': 'value1', 'c': 'value1'}

Getting the value by setdefault()

my_dict={'a':'Alex','b':'Ronald','c':'Ronn'}
x=my_dict.setdefault('b')
print(x)

Output is

Ronald

If the key is not found, then it will insert the data

my_dict={'a':'Alex','b':'Ronald','c':'Ronn'}
x=my_dict.setdefault('d','Ravi')
print(my_dict)

Output is here

{'a': 'Alex', 'b': 'Ronald', 'c': 'Ronn', 'd': 'Ravi'}

Number of keys present by len()

my_dict={'a':'Alex','b':'Ronald'}
print(len(my_dict))

Output

my_dict={'a':['Alex',30],'b':['Ronald',40],'c':['Ronn',50]}
print(len(my_dict))

Output

Length of elements within the key

my_dict={'a':['Alex',30],'b':['Ronald',40],'c':['Ronn',50]}
print(len(my_dict['a']))

Output

Displaying elements based on position is not possible as dictionary is unindexed. This will give error message.

my_dict={'a':'Alex','b':'Ronald'}
print(my_dict[0])

Above lines will generate error

Displaying value through key

my_dict={'a':'Alex','b':'Ronald'}
print(my_dict['b'])

Output

Ronald

Using get()

my_dict={'a':'Alex','b':'Ronald'}
print(my_dict.get('a'))

Output

Alex

get() takes one optional parameter to return of the key is not found.

my_dict={'a':'Alex','b':'Ronald'}
print(my_dict.get('c','Not found')) # Not found

Displaying all items by looping

We used for loop to display all keys of the dictionary.

my_dict={'a':'Alex','b':'Ronald'}
for i in my_dict:
 print(i)

Output is here

a
b

To display values by using for loop

my_dict={'a':'Alex','b':'Ronald'}
for i in my_dict:
 print(my_dict[i])

Output is here

Alex
Ronald

Using items() we can list both keys and values

my_dict={'a':'Alex','b':'Ronald'}
for i,j in my_dict.items():
 print(i,j)

Output

a Alex
b Ronald

Display only values by values()

my_dict={'a':'Alex','b':'Ronald'}
for i in my_dict.values():
 print(i)

Output

Alex
Ronald

Searching key by if condition check

my_dict={'a':'Alex','b':'Ronald'}
if 'b' in my_dict:
 print("Yes, it is there ")
else:
 print("No, not there")

Output

Yes, it is there

Searching value by using values()

my_dict={'a':'Alex','b':'Ronald'}
if 'Ronald' in my_dict.values():
 print("Yes, it is there ")
else:
 print("No, not there")

Output is here

Yes, it is there

Getting Key by using Value of a dictionary

But this requirement is not for which dictionary is usually used. If possible then while creating the dictionary, key and value can be changed.

However here is the solution for getting key by using value.

my_dict={'a':'Alex','b':'Ronald'}
for i,j in my_dict.items():
    if j=='Ronald':
         print(i)

Output

Add , remove , update elements

Add item

my_dict={'a':'Alex','b':'Ronald'}
my_dict['c']='Ronn'
print(my_dict)

Output is here

{'a': 'Alex', 'b': 'Ronald', 'c': 'Ronn'}

Add item by using update() method

my_dict={'a':'Alex','b':'Ronald','c':'Ronn'}
my_dict.update({'d':'John'})
print(my_dict)

Output is here

{'a': 'Alex', 'b': 'Ronald', 'c': 'Ronn', 'd': 'John'}

We can update any value by using update()

my_dict={'a':'Alex','b':'Ronald','c':'Ronn'}
my_dict.update('b','Ravi')
print(my_dict)

{'a': 'Alex', 'b': 'Ravi', 'c': 'Ronn'}

Note: We can't keep duplicate keys in a dictionary, however duplicate values are allowed.

Remove item by using pop() by giving key as input. Before removing we added one element having key c.

my_dict={'a':'Alex','b':'Ronald'}
my_dict['c']='Ronn'
my_dict.pop('b')
print(my_dict)

Output

{'a': 'Alex', 'c': 'Ronn'}

Removing last item by using popitem()

my_dict={'a':'Alex','b':'Ronald','c':'Ronn'}
my_dict.popitem()
print(my_dict)

Output

{'a': 'Alex', 'b': 'Ronald'}

Removing a particular key value pair by using del() and the key

my_dict={'a':'Alex','b':'Ronald','c':'Ronn'}
del my_dict['b']
print(my_dict)

Output

{'a': 'Alex', 'c': 'Ronn'}

If we don’t mention the key in above code then dictionary will be deleted. This code will generate error message.

my_dict={'a':'Alex','b':'Ronald','c':'Ronn'}
del my_dict
print(my_dict)

Error message is here

name 'my_dict' is not defined

We can remove all elements of the dictionary by using clear() method.

my_dict={'a':'Alex','b':'Ronald','c':'Ronn'}
my_dict.clear()
print(my_dict)

Output is here ( all elements are removed)

{}

Copying a dictionary by using copy() method.

my_dict={'a':'Alex','b':'Ronald','c':'Ronn'}
my_dict1=my_dict.copy()
print(my_dict1)

Output is here

{'a': 'Alex', 'b': 'Ronald', 'c': 'Ronn'}

Copying a dictionary by using dict() method

my_dict={'a':'Alex','b':'Ronald','c':'Ronn'}
my_dict1=dict(my_dict)
print(my_dict1)

Output

{'a': 'Alex', 'b': 'Ronald', 'c': 'Ronn'}

Collecting keys from the dictionary by using keys()

my_dict={'a':'Alex','b':'Ronald','c':'Ronn'}
my_keys=my_dict.keys()
print(my_keys)

Output is here

dict_keys(['a', 'b', 'c'])

We can display all elements

my_dict={'a':'Alex','b':'Ronald','c':'Ronn'}
my_keys=my_dict.keys()
for i in my_keys:
    print(i)

Output is here

a
b
c

Get maximum or minimum element based on Keys or Values of a dictionary.

Merging of two Dictionary

Using update

my_dict1={'a':'Alex','b':'Ronald','c':'Ronn'}
my_dict2={'d':'Rabi','b':'Kami','c':'Loren'}
my_dict1.update(my_dict2)
print(my_dict1)# {'a': 'Alex', 'b': 'Kami', 'c': 'Loren', 'd': 'Rabi'}

my_dict1={'a':'Alex','b':'Ronald','c':'Ronn'}
my_dict2={'d':'Rabi','b':'Kami','c':'Loren'}
my_out={**my_dict1,**my_dict2}
print(my_out) # {'a': 'Alex', 'b': 'Kami', 'c': 'Loren', 'd': 'Rabi'}

Dictionary containing List

We can store multiple elements by using List and store them in a dictionary. Check this code , the outputs are also written within comments.

my_dict={'a':['Alex',30],'b':['Ronald',40],'c':['Ronn',50]}
print(my_dict['a'][0]) # Output : Alex
print(my_dict['c'][1]) # Output : 50
print(my_dict['a'][:1])# Output : ['Alex']
print(my_dict['a'][:2])# Output : ['Alex', 30]

We can add more elements like one more list to this dictionary

my_dict={'a':['Alex',30],'b':['Ronald',40],'c':['Ronn',50]}
print(my_dict['a'][0]) # Output : Alex
print(my_dict['c'][1]) # Output : 50

my_dict.update({'d':['Ravi',60]})
print(my_dict)

Getting unique elements and counting the occurrence by using Counter.

Exercise on Dictionary
Ask for Name and mark of three students
Create a dictionary with this and print them
Following details of the student are to be stored.

Name ( this will be key of the dictionary )
Mark in three subjects i.e Physics , Chemistry , Math
Attendance in number of days

Create a dictionary and store three student details as mentions above.

Print the dictionary
Display all students name and attendance
Create one csv( comma separated value ) file with id ( integer) , name(string), mark1(integer ), mark2(integer),mark3(integer), attendance(integer)
Read the file and using the data create one dictionary. The first column id of the csv file should be the key and rest of the data should be value of the dictionary.
One sample csv file is kept here with some data. Same can be used.
```
1,Ravi,20,30,40,120
2,Raju,30,40,50,130
3,Alex,40,50,60,140
4,Ronn,50,60,70,150
```

Solution

Python

set list tuple

Dictionary in Python

Declaring a dictionary

Displaying value through key

Displaying all items by looping

Getting Key by using Value of a dictionary

Add , remove , update elements

Merging of two Dictionary

Dictionary containing List