Dictionary Data Structure
We can use List,Tuple and Set to represent a group of individual objects as a single entity.
If we want to represent a group of objects as key-value pairs then we should go for
Dictionary.
Eg:
rollno----name
phone number--address
ipaddress---domain name
Duplicate keys are not allowed but values can be duplicated.
Hetrogeneous objects are allowed for both key and values.
insertion order is not preserved
Dictionaries are mutable
Dictionaries are dynamic
indexing and slicing concepts are not applicable
Note: In C++ and Java Dictionaries are known as "Map" where as in Perl and Ruby it is
known as "Hash"
How to create Dictionary?
d={} or d=dict()
we are creating empty dictionary. We can add entries as follows
d[100]="sohan"
d[200]="ravi"
d[300]="shiva"
print(d) #{100: 'sohan', 200: 'ravi', 300: 'shiva'}
If we know data in advance then we can create dictionary as follows
d={100:'sohan' ,200:'ravi', 300:'shiva'}
d={key:value, key:value}
How to access data from the dictionary?
We can access data by using keys.
d={100:'sohan' ,200:'ravi', 300:'shiva'}
print(d[100]) #sohan
print(d[300]) #shiva
If the specified key is not available then we will get KeyError
print(d[400]) # KeyError: 400
We can prevent this by checking whether key is already available or not by using
has_key() function or by using in operator.
d.has_key(400) ==> returns 1 if key is available otherwise returns 0
But has_key() function is available only in Python 2 but not in Python 3. Hence
compulsory we have to use in operator.
if 400 in d:
print(d[400])
Q. Write a program to enter name and percentage marks in a dictionary and
display information on the screen
1) rec={}
2) n=int(input("Enter number of students: "))
3) i=1
4) while i <=n:
5) name=input("Enter Student Name: ")
6) marks=input("Enter % of Marks of Student: ")
7) rec[name]=marks
8) i=i+1
9) print("Name of Student","\t","% of marks")
10) for x in rec:
11) print("\t",x,"\t\t",rec[x])
12)
13) Output
14) D:\Python_classes>py test.py
15) Enter number of students: 3
16) Enter Student Name: sohan
17) Enter % of Marks of Student: 60%
18) Enter Student Name: ravi
19) Enter % of Marks of Student: 70%
20) Enter Student Name: shiva
21) Enter % of Marks of Student: 80%
22) Name of Student % of marks
23) sohan 60%
24) ravi 70 %
25) shiva 80%
How to update dictionaries?
d[key]=value
If the key is not available then a new entry will be added to the dictionary with the
specified key-value pair
If the key is already available then old value will be replaced with new value.
Eg:
1. d={100:"sohan",200:"ravi",300:"shiva"}
2. print(d)
3. d[400]="pavan"
4. print(d)
5. d[100]="sunny"
6. print(d)
7.
8. Output
9. {100: 'sohan', 200: 'ravi', 300: 'shiva'}
10. {100: 'sohan', 200: 'ravi', 300: 'shiva', 400: 'pavan'}
11. {100: 'sunny', 200: 'ravi', 300: 'shiva', 400: 'pavan'}
How to delete elements from dictionary?
del d[key]
It deletes entry associated with the specified key.
If the key is not available then we will get KeyError
Eg:
1. d={100:"sohan",200:"ravi",300:"shiva"}
2. print(d)
3. del d[100]
4. print(d)
5. del d[400]
6.
7. Output
8. {100: 'sohan', 200: 'ravi', 300: 'shiva'}
9. {200: 'ravi', 300: 'shiva'}
10. KeyError: 400
d.clear()
To remove all entries from the dictionary
Eg:
1. d={100:"sohan",200:"ravi",300:"shiva"}
2. print(d)
3. d.clear()
4. print(d)
5.
6. Output
7. {100: 'sohan', 200: 'ravi', 300: 'shiva'}
8. {}
del d
To delete total dictionary.Now we cannot access d
Eg:
1. d={100:"sohan",200:"ravi",300:"shiva"}
2. print(d)
3. del d
4. print(d)
5.
6. Output
7. {100: 'sohan', 200: 'ravi', 300: 'shiva'}
8. NameError: name 'd' is not defined
Important functions of dictionary:
1. dict():
To create a dictionary
d=dict() ===>It creates empty dictionary
d=dict({100:"sohan",200:"ravi"}) ==>It creates dictionary with specified elements
d=dict([(100,"sohan"),(200,"shiva"),(300,"ravi")])==>It creates dictionary with the given
list of tuple elements
2. len()
Returns the number of items in the dictionary
3. clear():
To remove all elements from the dictionary
4. get():
To get the value associated with the key
d.get(key)
If the key is available then returns the corresponding value otherwise returns None.It
wont raise any error.
d.get(key,defaultvalue)
If the key is available then returns the corresponding value otherwise returns default
value.
Eg:
d={100:"sohan",200:"ravi",300:"shiva"}
print(d[100]) ==>sohan
print(d[400]) ==>KeyError:400
print(d.get(100)) ==sohan
print(d.get(400)) ==>None
print(d.get(100,"Guest")) ==sohan
print(d.get(400,"Guest")) ==>Guest
3. pop():
d.pop(key)
It removes the entry associated with the specified key and returns the corresponding
value
If the specified key is not available then we will get KeyError
Eg:
1) d={100:"sohan",200:"ravi",300:"shiva"}
2) print(d.pop(100))
3) print(d)
4) print(d.pop(400))
5)
6) Output
7) sohan
8) {200: 'ravi', 300: 'shiva'}
9) KeyError: 400
4. popitem():
It removes an arbitrary item(key-value) from the dictionaty and returns it.
Eg:
1) d={100:"sohan",200:"ravi",300:"shiva"}
2) print(d)
3) print(d.popitem())
4) print(d)
5)
6) Output
7) {100: 'sohan', 200: 'ravi', 300: 'shiva'}
8) (300, 'shiva')
9) {100: 'sohan', 200: 'ravi'}
If the dictionary is empty then we will get KeyError
d={}
print(d.popitem()) ==>KeyError: 'popitem(): dictionary is empty'
5. keys():
It returns all keys associated eith dictionary
Eg:
1) d={100:"sohan",200:"ravi",300:"shiva"}
2) print(d.keys())
3) for k in d.keys():
4) print(k)
5)
6) Output
7) dict_keys([100, 200, 300])
8) 100
9) 200
10) 300
6. values():
It returns all values associated with the dictionary
Eg:
1. d={100:"sohan",200:"ravi",300:"shiva"}
2. print(d.values())
3. for v in d.values():
4. print(v)
5.
6. Output
7. dict_values(['sohan', 'ravi', 'shiva'])
8. sohan
9. ravi
10. shiva
7. items():
It returns list of tuples representing key-value pairs.
[(k,v),(k,v),(k,v)]
Eg:
1. d={100:"sohan",200:"ravi",300:"shiva"}
2. for k,v in d.items():
3. print(k,"--",v)
4.
5. Output
6. 100 -- sohan
7. 200 -- ravi
8. 300 -- shiva
8. copy():
To create exactly duplicate dictionary(cloned copy)
d1=d.copy();
9. setdefault():
d.setdefault(k,v)
If the key is already available then this function returns the corresponding value.
If the key is not available then the specified key-value will be added as new item to the
dictionary.
Eg:
1. d={100:"sohan",200:"ravi",300:"shiva"}
2. print(d.setdefault(400,"pavan"))
3. print(d)
4. print(d.setdefault(100,"sachin"))
5. print(d)
6.
7. Output
8. pavan
9. {100: 'sohan', 200: 'ravi', 300: 'shiva', 400: 'pavan'}
10. sohan
11. {100: 'sohan', 200: 'ravi', 300: 'shiva', 400: 'pavan'}
10. update():
d.update(x)
All items present in the dictionary x will be added to dictionary d
Q. Write a program to take dictionary from the keyboard and print the sum
of values?
1. d=eval(input("Enter dictionary:"))
2. s=sum(d.values())
3. print("Sum= ",s)
4.
5. Output
6. D:\Python_classes>py test.py
7. Enter dictionary:{'A':100,'B':200,'C':300}
8. Sum= 600
Q. Write a program to find number of occurrences of each letter present in
the given string?
1. word=input("Enter any word: ")
2. d={}
3. for x in word:
4. d[x]=d.get(x,0)+1
5. for k,v in d.items():
6. print(k,"occurred ",v," times")
7.
8. Output
9. D:\Python_classes>py test.py
10. Enter any word: mississippi
11. m occurred 1 times
12. i occurred 4 times
13. s occurred 4 times
14. p occurred 2 times
Q. Write a program to find number of occurrences of each vowel present in
the given string?
1. word=input("Enter any word: ")
2. vowels={'a','e','i','o','u'}
3. d={}
4. for x in word:
5. if x in vowels:
6. d[x]=d.get(x,0)+1
7. for k,v in sorted(d.items()):
8. print(k,"occurred ",v," times")
9.
10. Output
11. D:\Python_classes>py test.py
12. Enter any word: doganimaldoganimal
13. a occurred 4 times
14. i occurred 2 times
15. o occurred 2 times
Q. Write a program to accept student name and marks from the keyboard
and creates a dictionary. Also display student marks by taking student name
as input?
1) n=int(input("Enter the number of students: "))
2) d={}
3) for i in range(n):
4) name=input("Enter Student Name: ")
5) marks=input("Enter Student Marks: ")
6) d[name]=marks
7) while True:
8) name=input("Enter Student Name to get Marks: ")
9) marks=d.get(name,-1)
10) if marks== -1:
11) print("Student Not Found")
12) else:
13) print("The Marks of",name,"are",marks)
14) option=input("Do you want to find another student marks[Yes|No]")
15) if option=="No":
16) break
17) print("Thanks for using our application")
18)
19) Output
20) D:\Python_classes>py test.py
21) Enter the number of students: 5
22) Enter Student Name: sunny
23) Enter Student Marks: 90
24) Enter Student Name: banny
25) Enter Student Marks: 80
26) Enter Student Name: chinny
27) Enter Student Marks: 70
28) Enter Student Name: pinny
29) Enter Student Marks: 60
30) Enter Student Name: vinny
31) Enter Student Marks: 50
32) Enter Student Name to get Marks: sunny
33) The Marks of sunny are 90
34) Do you want to find another student marks[Yes|No]Yes
35) Enter Student Name to get Marks: sohan
36) Student Not Found
37) Do you want to find another student marks[Yes|No]No
38) Thanks for using our application
Dictionary Comprehension:
Comprehension concept applicable for dictionaries also.
1. squares={x:x*x for x in range(1,6)}
2. print(squares)
3. doubles={x:2*x for x in range(1,6)}
4. print(doubles)
5.
6. Output
7. {1: 1, 2: 4, 3: 9, 4: 16, 5: 25}
8. {1: 2, 2: 4, 3: 6, 4: 8, 5: 10}