Dictionary

 INTRODUCTION:  Dictionary 

• A dictionary is like a list, but more in general. In a list, index value is an integer, while in a dictionary index value can be any other data type and are called keys. The key will be used as a string as it is easy to recall. 
• A dictionary is an extremely useful data storage construct for storing and retrieving all key value pairs, where each element is accessed (or indexed) by a unique key. However, dictionary keys are not in sequences and hence maintain no left-to right order.

• It is an un-ordered collection of items where each item consist of a key and a value. 
• It is mutable (can modify its contents ) but Key must be unique and immutable.
• Dictionary is also known as associative array or mapping or hashes . 

Key-value pair

We can refer to a dictionary as a mapping between a set of indices (which are called keys) and a set of values. Each key maps a value. The association of a key and a value is called a key-value pair.

Syntax:

my_dict = {'key1': 'value1','key2': 'value2','key3': 'value3'…'keyn': 'valuen'}

Note: Dictionary is created by using curly brackets(ie. {}).

Example

A={1:"one",2:"two",3:"three"}

print A

output: 

{1: 'one', 2: 'two', 3: 'three'}

Creating A Dictionary 

It is enclosed in curly braces {} and each item is separated from other item by a comma(,). Within each item, key and value are separated by a colon (:). 

Example: -

dict = {‘Subject': ‘Informatic Practices', 'Class': ‘11'}

Accessing List Item

dict = {'Subject': 'Informatics Practices', 'Class': 11} 
print(dict) 

print ("Subject : ", dict['Subject']) 
print ("Class : ", dict.get('Class'))

OUTPUT

{'Class': '11', 'Subject': 'Informatics Practices'} 
('Subject : ', 'Informatics Practices') 
('Class : ', 11) 

Iterating Through A Dictionary

Following example will show how dictionary items can be accessed through loop.

Example: - 

dict = {'Subject': 'Informatics Practices', 'Class': 11} 
for i in dict:

     print(dict[i])

OUTPUT

 Informatics Practices 11

Updating Dictionary Elements 

We can change the individual element of dictionary. 

Example: -

dict = {'Subject': 'Informatics Practices', 'Class': 11} 
dict['Subject']='computer science' 

print(dict) 

OUTPUT

{'Class': 11, 'Subject': 'computer science'}


Deleting Dictionary Elements

del, pop() and clear() statement are used to remove elements from the dictionary. 

del 

Example: -

dict = {'Subject': 'Informatics Practices', 'Class': 11} 
print('before del', dict) 
del dict['Class'] # delete single element 
print('after item delete', dict) 

del dict #delete whole dictionary 
print('after dictionary delete', dict) 


Output

('before del', {'Class': 11, 'Subject': 'Informatics Practices'}) 
('after item delete', {'Subject': 'Informatics Practices'}) 
('after dictionary delete', )

• pop() method is used to remove a particular item in a dictionary. 
• clear() method is used to remove all elements from the dictionary. 

Example: -

dict = {'Subject': 'Informatics Practices', 'Class': 11} 
print('before del', dict)

dict.pop('Class') 
print('after item delete', dict) 

dict.clear() 
print('after clear', dict) 

Output

('before del', {'Class': 11, 'Subject': 'Informatics Practices'}) 
('after item delete', {'Subject': 'Informatics Practices'}) 
('after clear', {})


Built-in Dictionary Functions 

• len(dict) Gives the total length of the dictionary. It is equal to the number of items in the dictionary. 
• str(dict) Return a printable string representation of a dictionary 
• type(variable) If variable is dictionary, then it would return a dictionary type.

Some more  examples of Dictionary are:- 

• Dict1= { } # this is an empty dictionary without any element. 
• 
  
• DayofMonth= { January”:31, ”February”:28, ”March”:31, ”April”:30, ”May”:31, ”June”:30, ”July”:31, ”August”:31, ”September”:30, ”October”:31, ”November”:30, ”December”:31} 
• 
  
• FurnitureCount = { “Table”:10, “Chair”:13, “Desk”:16, “Stool”:15, “Rack”:15 } 

• By above examples you can easily understand about the keys and their values. 
• One thing to be taken care of is that keys should always be of immutable type.

Built-in functions in Dictionary

SN	Function	Function Details and working
1.	len( )	Returns the length of the Dictionary(key-value pair will be count as 1 ) Len(Mydict)
2.	dict( )	Creates Dictionary X=dict(name=”sunil”,age=30,country=”India”
3.	keys( )	Returns all available keys x.keys() output: dict.keys([‘name’,’age’,’country’])
4.	values()	Returns all the available values. x.values( ) output: dict_values([‘sunil’,30,’India’])
5.	get( )	The get() method returns the value of the item with the specified key Mydict ={'Name': 'Raj', 'Age': 15, 'Class': 12,'Totalmarks':450} print(Mydict.get('Class')) print(Mydict['Name'])
6.	update():-	updates the dictionary with the elements from another dictionary objector from an iterable of key/value pairs mydict = {'Africa':200,'australia':300,'England':400} mydict.update({'China':500})
7.	del()	The del keyword can be used to in-place delete the key that is present inthe dictionary in Python. test_dict = {"Arushi": 22, "Mani": 21, "Haritha": 21} removes Mani del test_dict['Mani']
8.	pop()	method removes the specified item from the dictionary and return thecorresponding value. Mydict =        {'Name': 'Raj', 'Age': 15, 'Class': 12, 'Totalmarks':450} Mydict.pop('Name')
9.	popitem():-	The popitem() method removes the item that was last inserted into thedictionary. Mydict =        {'Name': 'Raj', 'Age': 15, 'Class': 12, 'Totalmarks':450} K= Mydict.popitem() print("Last item of dictionary = ", K)
10.	fromkeys()	The dict.fromkeys() method creates a new dictionary from the given iterable (string, list, set, tuple) as keys and with the specified value. keys = ('Mumbai','Bangalore','Chicago','New York') value = 'city' dictionary = dict.fromkeys(keys, value)
11.	copy( )	copy() method returns a copy (shallow copy) of the original dictionary original={'Name': 'Raj', 'Age': 15, 'Class': 12, 'Totalmarks':450} new = original.copy() print('Orignal: ', original) print(‘New:’,new)
12.	setdefault( )	The setdefault() method returns the value of a key (if the key is in dictionary). If not, it inserts key with a value to the dictionary. romanNums = {'I':1, 'II':2, 'III':3, 'IV':4, 'V':5 } value = romanNums.setdefault('I') print("The return value is: ", value) value = romanNums.setdefault('VI') print("The return value is: ",value) print("Updated dictionary: ",romanNums)
13.	max( ) & min( )	Used to find maximum and minimum respectively fromthe dictionary. my_dict = {'x':500, 'y':5874, 'z': 560} key_max = max(my_dict.keys(), key=(lambda k: my_dict[k])) key_min = min(my_dict.keys(), key=(lambda k: my_dict[k])) print('Maximum Value: ',my_dict[key_max]) print('Minimum Value: ',my_dict[key_min])
14.	clear()	removes all items from the dictionary. Mydict ={'Name': 'Raj', 'Age': 15, 'Class': 12, 'Totalmarks':450} Mydict.clear() print(Mydict)
15.	sorted( )	The sorted() function returns a sorted list of the specified iterable object dict = {6:'George' ,2:'John' ,1:'Potter' ,9:'Micheal' ,7:'Robert' ,8:'Gayle' } b = sorted(dict.keys()) print("Sorted keys",b) c = sorted(dict.items()) print("Sorted Values",c)

To print a Dictionary in a beautify manner, we need to import json module. After that following syntax of dumps ( ) will be used.

MORE EXERCISES 

1. Write the output for the following Python codes.

A={1:100,2:200,3:300,4:400,5:500}

print A.items()

print A.keys()

print A.values()

Output

[(1, 100), (2, 200), (3, 300), (4, 400), (5, 500)]

[1, 2, 3, 4, 5]

[100, 200, 300, 400, 500]

2. write a program to enter names of employees and their salaries as input and store them in a dictionary. Here n is to input by the user.

#Program to create a dictionary which stores names of employees and their salary

num = int(input("Enter the number of employees whose data to be stored: "))

count = 1

employee = dict() #create an empty dictionary

for count in range (n):

    name = input("Enter the name of the Employee: ")

    salary = int(input("Enter the salary: "))

    employee[name] = salary

print("\n\nEMPLOYEE_NAME\tSALARY")

  for k in employee:

        print(k,'\t\t',employee[k])

Output

Enter the number of employees to be stored: 5

Enter the name of the Employee: 'Tarun'

Enter the salary: 12000

Enter the name of the Employee: 'Amina'

Enter the salary: 34000

Enter the name of the Employee: 'Joseph'

Enter the salary: 24000

Enter the name of the Employee: 'Rahul'

Enter the salary: 30000

Enter the name of the Employee: 'Zoya'

Enter the salary: 25000

EMPLOYEE_NAME SALARY

'Tarun' 12000

'Amina' 34000

'Joseph' 24000

'Rahul' 30000

'Zoya' 25000

3. Write a program to count the number of times a character appears in a given string.

#Count the number of times a character appears in a given string

st = input("Enter a string: ")

dic = {} #creates an empty dictionary

for ch in st:

    if ch in dic: #if next character is already in dic

        dic[ch] += 1

    else:

        dic[ch] = 1 #if ch appears for the first time

for key in dic:

    print(key,':',dic[key])

Output

Enter a string: HelloWorld

H : 1

e : 1

l : 3

o : 2

W : 1

r : 1

d : 1

4. Write a program to convert a number entered by the user into its corresponding number in words. for example if the input is 876 then the output should be ‘Eight Seven Six’.

num = input("Enter any number: ") #number is stored as string

                                                                      #numberNames is a dictionary of digits and corresponding number names

numberNames = {0:'Zero',1:'One',2:'Two',3:'Three',4:'Four', 5:'Five',6:'Six',7:'Seven',8:'Eight',9:'Nine'}

result = ''

for ch in num:

     key = int(ch) #converts character to integer

     value = numberNames[key]

      result = result + ' ' + value

 print("The number is:",num)

 print("The numberName is:",result)

Output

Enter any number: 6512

The number is: 6512

The numberName is: Six Five One Two

5. Count the number of times, a character appears in a given string using a dictionary :



#initializing string 

test_str = "AMARDEEP"

# using dict.get() to get count # of each element in string

 res = {}

for keys in test_str:

res[keys] = res.get(keys, 0) + 1                 # printing result

print ("Count of all characters in GeeksforGeeksis : \n" , res)


6. Create a dictionary with names of employees, salary and access them.

Mydict = { }

while True :

     name = input("Enter employee name :-") sl = int(input("Enter employee salary :-"))

    Mydict[ name] = sl

    user = input("Do you want to quit then enter yes :-") if user == "yes" :

    break;

print(Mydict)

Short Answers type questions[1 mark]

Q1. Keys of dictionary must be
(a) antique (b)unique (c) mutable (d) integers 

Q2. We can repeat the values of Key in Dictionary?
a. True   b. False

Q3. Key – value concept is in
(a) List (b)String (c)Dictionary (d)Tuple 

Q4. What type of error is returned by the following code :
              a={'a' : "Apple", 'b' : "Banana" , 'c' : "Cat"} print(a[1])

Q5. Write the two ways to construct an empty dictionary.

Q6. Write the output of following code:
sales = {'Audi':45, 'BMW':32, 'Ferrari':12}
for x in sales:
  print(x)
Q7. Suppose a dictionary days is declared as:

days={1:"Sun", 2:"Mon", 3:"Wed"}

Write a statement in Python to change Wed to Tue.
Q.8. is used to remove all items form a particular dictionary.
Q.9. What will be the output:-

d1={‘rohit’:56,” Raina”:99}

print(“Raina” in d1)

Q10. Which of the following function create a dictionary from sequence of key - valuePairs.

                   (a) dictionary( ) (b) dict( ) (c) create( ) (d) convert( )

Short Answer Type Questions [2 marks]

Q1. Parth wants to display the value corresponding to the key “3” in dictionary given
        below. As a friend of Parth, help him to find the correct code. 

           D={1: ‘Amit’, 2: ‘Suman’, 3: ‘Ravi’, 4: ‘Anuj’}

           (a) print(D.get(3)) (b) print(D[3]) (c) Both of the above (d) None of the above 

Q2. Write Python code to convert following two list into one dictionary :-

             keys = ['Ten', 'Twenty', 'Thirty'] values = [10, 20, 30]

Q3. Print the value of key ‘physics’ from the following dictionary

     MyDict = {"class": {"student": {"name": "Mike","marks": {"physics": 70,"history": 80}}}} 

Q4. Get the key of a minimum value from the following dictionary
     My_dict = {'Physics': 82,'Math': 65,'CS': 75}

Q5. Find the output of the following python code:-

a = {}
a[1] = 1
a['1'] = 2
a[1]= a[1]+1
count = 0 

 for i in a:
    count += a[i] 

    print("count=", count)

Q6. What is the output of the following of code? 

       a = {i: i*i*i for i in range(6)}
       print (a)

Q7. What will be output of following python program :-

dict = {(3,4,8):4,(5,6,9):3}
print(dict) 

print('output:',dict[5,6,])

Q8. Find the output of the following code:-

     dictlang = {'c#': 6, 'GO': 89, 'Python':4,'Rust':10}cpydict = dictlang.copy() print(cpydict)

Q9. Find the output of the following code:-

  fruitsDict = {'Apple': 100,'Orange': 200,'Banana': 400, 'pomegranate':600 } 

  if 'Apple' in fruits Dict:
        del fruitsDict['Apple']
        print('Dict after deleting key =',fruitsDict)

Q10. Create a dictionary ‘ODD’ of odd numbers between1 and 10, where the key is the decimal                        number and the value is the corresponding number in words.

 

Long Answer Type Question[3 & 4 mark Questions]

1) Answer the following question on the given dictionary 

Employee= {'Name': 'Aman', 'Salary': 10000, 'Gender': 'Male'}

(i) Add a new key(‘City) with value “Jaipur” in Employee dictionary

(ii) Display all the keys of the Employee dictionary

(iii) Write code to delete all the items of the Employee dictionary

2) What are the differences between dictionary and list?

3) Consider the following dictionary capitals

capitals ={ "Maharashtra": "mumbai","Delhi" : "New Delhi","Uttar pradesh":"Lucknow"}

Find the output of the following statements:-

(i)    print(capitals.get("Lucknow"))

(ii)    print(capitals.keys())

(iii)   print("Delhi" in capitals)

4) Write a program to convert a number entered by the user into its corresponding number in words. for       example if the input is 876 then the output should be ‘Eight Seven Six’.

5) Python Program to Multiply All the Items in a Dictionary.

6) Write a Python program to print all unique values in a dictionary. 

    Sample Data : [{"V":"S001"}, {"V": "S002"}, {"VI": "S001"}, {"VI": "S005"},
      {"VII":"S005"}, {"V":"S009"},{"VIII":"S007"}]

    Expected Output : Unique Values: {'S005', 'S002', 'S007', 'S001', 'S009'}

7) Write a program to create a dictionary namely Mydict with 10 keys 0 to 9, each having value as 20.         Update the first and last values by adding 100 to each of them.

8) Write a python function to print sum of all items in a dictionary.

Case Based Questions

1. Mohan is student who is learning python programming. Mohan is unable to find out the output of          the following python program. Help the Mohan by finding outputof the following :-

Mydict= {'A':10,'B':20,'a':30, 'D':40}
Val_A= ''
for i in Mydict: if (i>Val_A):
     Val_A= i
     Val_B= Mydict[i]
     print(Val_A) # Line1
     print(Val_B) # Line2
     print(20 in Mydict) # Line3 

    print('D' in Mydict) # Line4 
Mylist.sort() # Line5
print(Mylist[-1]) # Line6

(i) What output does Line1 produce ?
(ii) What output does Line2 produce ?
(iii) What output does Line3 produce ?
(iv) What output does Line4 produce ?

(v) What is the return value form the list sort() function (line5)
(vi) What output does Line6 produce ?

2. Mr. Rajesh Kumar is a teacher in a school. He is doing his work manually . As a python learner         solve  the problems of Rajesh Kumar by python programs:-

(i) Create a dictionary student which ask Student roll number, Name and Marksof students and display      them in tabular format.
(ii) Display the names of those students who have secured marks more than 75.
(iii) Delete those students who have secured less than 50 marks.

ANSWER KEY

	Short Answer Type Questions [1 Marks]
1.	B
2.	A
3.	C
4.	KEY ERROR
5.	(i) Use of {} symbol dict()function
6.	Audi BMW Ferrari
7.	Days[3]=”Tue”
8.	clear()
9.	A
10.	True
	Short Answer Type Questions [2 Marks]
1.	D
2.	keys = ['Ten', 'Twenty', 'Thirty']values = [10, 20, 30] res_dict = dict(zip(keys, values)) print(res_dict)
3.	print(MyDict['class']['student']['marks']['physics'])
4.	print(min(My_dict, key=My_dict.get))
5.	count= 4
6.	{0: 0, 1: 1, 2: 8, 3: 27, 4: 64, 5: 125}
7.	{(3, 4, 8): 4, (5, 6, 9): 3} output: 3
8.	{'c#': 6, 'GO': 89, 'Python': 4, 'Rust': 10}
9.	Dict after deleting key = {'Orange': 200, 'Banana': 400,'pomegranate': 600}
10.	ODD = {1:'One',3:'Three',5:'Five',7:'Seven',9:'Nine'} print(ODD)
Long Answer type questions
1.		(i)                  Employee['City']= "Jaipur" print(Employee) (ii)                 Employee.keys() (iii)               Employee.clear()
2.		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.
3.		(i)  None (ii)  dict_keys(['Maharashtra', 'Delhi', 'Uttar pradesh', 'Tamil Nadu ' (iii)   True
4.		num = input("Enter any number: ") #number is stored as string#numberNames is a dictionary of digits and corresponding number#names numberNames = {0:'Zero',1:'One',2:'Two',3:'Three',4:'Four',\ 5:'Five',6:'Six',7:'Seven',8:'Eight',9:'Nine'} result = '' for ch in num: key = int(ch) #converts character to integerrvalue = numberNames[key] result = result + ' ' + value print("The number is:",num) print("The numberNameis:",result)
5.		My_dict = {'A':10, 'B':20, 'C':30} Multiply= 1 for i in My_dict: Multiply= Multiply*My_dict[i]print(Multiply)
6.		L = [{"V":"S001"}, {"V": "S002"}, {"VI": "S001"}, {"VI":"S005"}, {"VII":"S005"}, {"V":"S009"},{"VIII":"S007"}] print("Original List: ",L) u_value = set( val for dic in L for val in dic.values()) print("Unique Values: ",u_value)
7.		Mydict= dict.fromkeys(range(10), 20) Mydict[0]+=20 Mydict[9]+= 20 print(Mydict)
8.		# Function to print sum def ReturnSum(myDict): list = [] for i in myDict: list.append(myDict[i]) final = sum(list)    return final dict = {'a': 100, 'b': 200, 'c': 300} print("Sum :", ReturnSum(dict))
9.		def CountFrequency(my_list):       # Function definition# #Creating an empty dictionary freq = {} for item in my_list: if (item in  freq): freq[item] += 1 else: freq[item] = 1 for key, value in freq.items(): print ("% d : % d"%(key, value))   my_list=[1, 1, 1, 5, 5, 3, 1, 3, 3, 1, 4, 4, 4, 2, 2, 2, 2] CountFrequency(my_list)       # Function calling
10.		D1= { } i=1 Num= int (input("Enter number of entries: "))  while (i<=Num):   a= input("Enter name: ")   b= input("Enter age:")     D1[a]=b     i= i+1     L= D1.keys() for i in L: print(i, '\t', D1[i])
Case Based Question
1.			(i)          A (ii)         30 (iii)        False (iv)       True (v)         None (vi)     ('a', 30)
2.			(i) n=int(input("How many student data you want to enter ...")) Student={} for i in range(n): roll_no=int(input("Enter roll no:"))         name=input("Enter name: ") marks=int(input("Enter marks: ")) Student[roll_no]=[name,marks] print("{:<10} {:<10} {:<10}".format('Rollno','Name','Marks')) for k, v in d.items(): name, num = k, v print(name, marks)) (ii) n=int(input("Enter n: ")) d={} for i in range(n):    roll_no=int(input("Enter roll no: "))     name=input("Enter name: ")      marks=int(input("Enter marks: "))     d[roll_no]=[name,marks] for k in d:     if(d[k][1]>75):         print(d[k][0]) (iii) n=int(input("How many student data you want to enter ..."))  Student={} for i in range(n): roll_no=int(input("Enter roll no: "))  name=input("Enter name: ")  marks=int(input("Enter marks: "))  Student[roll_no]=[name,marks] for k, v in list(Student.items()):  if v[1] < 50:   del Student[k]   print("Remaining students: ", Student)