Introduction to Dictionaries
a set of key: value pairs
student={ ‘Rollno’:101 , ‘Name’ : ‘Abc’ }
The pairs are separated by comma ( , ) and all the pairs are inserted into { } brackets. The name of the dictionary is student.
If the key is provided we can get its value.
Duplicate keys are not allowed , if same keys are entered, the old key will be replaced by new value
Keys are immutable :
we can use number, string or tuples as keys.
we cannot use list or directories as keys
Operations on Dictionaries
1.Length of dictionary
n = len(student)
2.Modify the existing values
student[‘Name’]= ‘xyz’
3.Delete the dictionary pair
del student[‘Rollno’]
To view dictionary
print(student)
Check if key exist
‘Name’ in student // it returns true
‘Marks’ in student // it returns false
Dictionary Methods
Remove all key value pairs
student.clear()
Copy all elements
st = student.copy()
Create a new dictionary from specified keys and values
keys = {‘a', ‘b', ‘c', ‘d', ‘e' }
value = 1
st.fromkeys( keys, value )
Get value associated with the key
student.get(‘Rollno’) // it returns 101
Get key value pairs
student.items()
Get sequence of all keys
student.keys()
Get sequence of all values
student.values()
Adds all elements of dictionary x_students to students.
student.update(x_students)
Removes the key and its value from student and returns the value
student.pop(‘Rollno’)
If key is found it returns the value and if not found the key value pair is stored
student.setdefault(‘Marks’, 10)
It inserts a new key value pair returns the value 10
To print all values using loop
for s in student:
print(s)
Converting List into Dictionary
To convert list to dictionary there are two steps:
Create zip class object by passing the two list to zip function.
The second step is convert the zip object into a dictionary, using dict() function.
Suppose we have two lists :
subject = [ “maths”, “science”, “english“ , “computer” }
marks = [ 45, 50, 42, 46 ]
Zip function converts sequences into zip class object
z = zip(subject, marks )
The Dict function converts the zip object to dictionary.. The 0th element becomes key and 1st element becomes value and so on…
d = dict(z)
for k in d:
print( k, d[k])
Passing dictionaries to functions
Passing a dictionary to a function by passing name of the directory as parameter
Defining the function
def myfun(std)
for i, j in std.items():
print( i, “ ”, j )
Calling the function
student={ ‘Rollno’:101 , ‘Name’ : ‘Abc’ }
myfun( student ) // here dictionary student is passed as a parameter
Example:
#program of dictionary
student={'rollno':1,'name':'abc'}
n=len(student)
print("No of key value pair is",n)
student['result']=70.0
n=len(student)
print("No of key value pair is",n)
print(student)
student['result']=75.0
print(student)
del student['result']
print(student)
if( 'result' not in student):
print("Result key is not in student")
#key should be unique
#dictionary methods
s1=student.copy()
print(s1)
print(student)
keys={'a', 'b', 'c', 'd','e' }
value = 1
s=dict.fromkeys( keys,value)
print('new dictionary', s)
print('it returns the associated value of the key ',student.get('name'))
tp=student.items()
print('items method returns associated key and value pairs', tp)
print("All the keys from dictionary", student.keys())
print("All the values from dictionary", student.values())
s.update(student)
print("update s from student",s)
s.pop('a')
print(s)
print('To print all the keys from student')
for st in student:
print(st)
print('To print all the values from student')
for st in student:
print(student[st])
print('To print all the key -values from student')
for st in student:
print(st,"-",student[st])
#list to dictionary conversion
subject = [ 'maths', 'science', 'english','computer']
marks = [ 45, 50, 42, 46 ]
z = zip(subject, marks )
d=dict(z)
print(d)
Output:
'No of key value pair is', 2)
('No of key value pair is', 3)
{'name': 'abc', 'rollno': 1, 'result': 70.0}
{'name': 'abc', 'rollno': 1, 'result': 75.0}
{'name': 'abc', 'rollno': 1}
Result key is not in student
{'name': 'abc', 'rollno': 1}
{'name': 'abc', 'rollno': 1}
('new dictionary', {'a': 1, 'c': 1, 'b': 1, 'e': 1, 'd': 1})
('it returns the associated value of the key ', 'abc')
('items method returns associated key and value pairs', [('name', 'abc'), ('rollno', 1)])
('All the keys from dictionary', ['name', 'rollno'])
('All the values from dictionary', ['abc', 1])
('update s from student', {'a': 1, 'c': 1, 'b': 1, 'e': 1, 'd': 1, 'rollno': 1, 'name': 'abc'})
{'c': 1, 'b': 1, 'e': 1, 'd': 1, 'rollno': 1, 'name': 'abc'}
To print all the keys from student
name
rollno
To print all the values from student
abc
1
To print all the key -values from student
('name', '-', 'abc')
('rollno', '-', 1)
{'maths': 45, 'science': 50, 'computer': 46, 'english': 42}
