Python Dictionaries & Dictionaries Programs

Dictionaries in Python:

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Basic Types in Python :

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a=10                       #int
b="hello"              #str
c=2.5                      #float
e=bytearray()       #bytearray
f=(1,2,3)                #tuple
g=[1,2,3]               #list
h={1,2,3}              #set
i={}                       #dict
j=b"sample"       #bytes
k=None               #empty
=====================

What are Dictionaries?
Dictionary is a Python built-in data type that is used to store collections of unordered key-value pairs.

Why Dictionaries?
Dictionaries offer the association of keys to their corresponding values. Each key refers to one (and only one) value in the dictionary. This not only facilitates storing and processing of data, but also enriches the whole process by enabling the user to store data in pairs. This greatly enhances searching and indexing operations.

What are the Features of Dictionaries?

• Dictionaries are collections of key-value pairs.
• Dictionaries are unordered: unlike lists that have values stored in a certain order based on the array index, dictionaries don’t maintain one order for its data items.
• Data items are accessed by keys, not by indexes.

Syntax for Initializing Dictionaries
A dictionary name is an identifier (just like variable names, list names, and function names). Dictionaries are assigned values using the normal assignment operators ‘=’.

D1 = {1 : ‘IT’, 2 : ‘Sales’, 3 : ‘Finance’, 4 : ‘HR’ }

From the above statement, we can notice that:

• Dictionaries are delimited by curly braces.
• A key comes before its corresponding value.
• Keys and values are separated by colons ‘:’
• Like lists, pairs are separated from each other’s by commas ‘,’

 Dictionary is an unordered set of key and value pair.

·      It is an container that contains data, enclosed within curly braces.

·      The pair i.e., key and value is known as item.

·      The key passed in the item must be unique.

·      The key and the value is separated by a colon(:). This pair is known as item. Items are separated from each other by a comma(,).

·      Different items are enclosed within a curly brace and this forms Dictionary.

·      Dictionaries are optimized to retrieve values when the key is known.

 Example:

         data={100:'Manish' ,101:'Vijay' ,102:'Rahul'}  

     print (data)   

   

 Output:

{100: 'Manish', 101: 'Vijay', 102: 'Rahul'}  

·      Dictionary is mutable i.e., value can be updated.

·      Key must be unique and immutable. Value is accessed by key.

·      Value can be updated while key cannot be changed.

·      Dictionary is known as Associative array since the Key works as Index and they are decided by the user.

Example:

plant={}  

plant[1]='Ravi'  

plant[2]='Manoj'  

plant['name']='Hari'  

plant[4]='Om'  

print (plant[2])  

print (plant['name'])  

print (plant[1])  

print (plant)   

Output:

       Manoj

       Hari

       Ravi

      {1: 'Ravi', 2: 'Manoj', 4: 'Om', 'name': 'Hari'}

      

How to create a dictionary?

# empty dictionary

my_dict = {}

# dictionary with integer keys

my_dict = {1: 'apple', 2: 'ball'}

# dictionary with mixed keys

my_dict = {'name': 'John', 1: [2, 4, 3]}

# using dict()

my_dict = dict({1:'apple', 2:'ball'})

# from sequence having each item as a pair

my_dict = dict([(1,'apple'), (2,'ball')])

Accessing Values

Since Index is not defined, a Dictionaries value can be accessed by their keys.

Syntax:

[key]

Example:

data1={'Id':100, 'Name':'Suresh', 'Profession':'Developer'}

data2={'Id':101, 'Name':'Ramesh', 'Profession':'Trainer'}

print "Id of 1st employer is",data1['Id']

print "Id of 2nd employer is",data2['Id']

print "Name of 1st employer:",data1['Name']

print "Profession of 2nd employer:",data2['Profession']

Output:

Id of 1st employer is 100

Id of 2nd employer is 101

Name of 1st employer is Suresh

Profession of 2nd employer is Trainer

Updation

The item i.e., key-value pair can be updated. Updating means new item can be added. The values can be modified.

Example:

data1={'Id':100, 'Name':'Suresh', 'Profession':'Developer'}

data2={'Id':101, 'Name':'Ramesh', 'Profession':'Trainer'}

data1['Profession']='Manager'

data2['Salary']=20000

data1['Salary']=15000

print (data1)

print (data2)

Output:

{'Salary': 15000, 'Profession': 'Manager','Id': 100, 'Name': 'Suresh'}

{'Salary': 20000, 'Profession': 'Trainer', 'Id': 101, 'Name': 'Ramesh'}

Deletion

del statement is used for performing deletion operation.

An item can be deleted from a dictionary using the key.

Syntax:

del [key]

Example:

data={100:'Ram', 101:'Suraj', 102:'Alok'}

del data[102]

print data  

del data

print data   #will show an error since dictionary is deleted.

Output:

{100: 'Ram', 101: 'Suraj'}

Traceback (most recent call last):

          File "C:/Python27/dict.py", line 5, in

           print data

NameError: name 'data' is not defined

Functions and Methods

Python Dictionary supports the following Functions:

Functions:

1) len(dictionary)

Example:

data={100:'Ram', 101:'Suraj', 102:'Alok'}

print (data)

print (len(data))

Output:

{100: 'Ram', 101: 'Suraj', 102: 'Alok'}

3

2) str(): produces a printable string representation

     of  a dictionary.

Syntax

Following is the syntax for str() method −

str(dict)

Example:

#!/usr/bin/python3

dict = {'Name': 'Manni', 'Age': 7, 'Class': 'First'}

print ("Equivalent String : %s" % str (dict))

Output:

Equivalent String : {'Name': 'Manni', 'Age': 7, 'Class': 'First'}

3) type(variable):Returns the type of the passed variable. If passed variable is dictionary, then it would return a dictionary type.

Syntax

Following is the syntax for type() method −

type(dict)

Example:

#!/usr/bin/python3

dict = {'Name': 'Manni', 'Age': 7, 'Class': 'First'}

print ("Variable Type : %s" %  type (dict))

Output:

Variable Type : <type 'dict'>

Methods:

1)dict.clear():Removes all elements of dictionary dict

Example:

#!/usr/bin/python3

dict = {'Name': 'Zara', 'Age': 7}

print ("Start Len : %d" %  len(dict))

dict.clear()

print ("End Len : %d" %  len(dict))

Output:

Start Len : 2

End Len : 0

2)dict.copy():Returns a shallow copy of dictionary dict

Example:

#!/usr/bin/python3

dict1 = {'Name': 'Manni', 'Age': 7, 'Class': 'First'}

dict2 = dict1.copy()

print ("New Dictionary : ",dict2)

Output:

New dictionary :  {'Name': 'Manni', 'Age': 7, 'Class': 'First'}

3)dict. fromkeys():Create a new dictionary with keys from seq and values set to value.

Syntax

Following is the syntax for fromkeys() method −

dict.fromkeys(seq[, value]))

Parameters

• seq − This is the list of values which would be used for dictionary keys preparation.
• value − This is optional, if provided then value would be set to this value

Return Value

This method returns the list.

Example:

#!/usr/bin/python3

seq = ('name', 'age', 'sex')

dict = dict.fromkeys(seq)

print ("New Dictionary : %s" %  str(dict))

dict = dict.fromkeys(seq, 10)

print ("New Dictionary : %s" %  str(dict))

Output:

New Dictionary : {'age': None, 'name': None, 'sex': None}

New Dictionary : {'age': 10, 'name': 10, 'sex': 10}

4)dict. get():For key key, returns value or default if key not in dictionary

Syntax

Following is the syntax for get() method –

dict.get(key, default=None)

Parameters

·       key − This is the Key to be searched in the dictionary.

·       default − This is the Value to be returned in case key does not exist.

Return Value

This method return a value for the given key. If key is not available, then returns default value None.

Example:

#!/usr/bin/python3

dict = {'Name': 'Zara', 'Age': 27}

print ("Value : %s" %  dict.get('Age'))

print ("Value : %s" %  dict.get('Sex', "NA"))

Output:

Value : 27

Value : NA

5)dict.items(): Returns a list of dict's (key, value) tuple pairs

Example:

#!/usr/bin/python

dict = {'Name': 'Zara', 'Age': 7}

print ("Value : %s" %  dict.items())

Output:

Value : [('Age', 7), ('Name', 'Zara')]

6)dict.keys():Returns list of dictionary dict's keys

Example:

#!/usr/bin/python

dict = {'Name': 'Zara', 'Age': 7}

print ("Value : %s" %  dict.items())

Output:

Value : dict_keys(['Name', 'Age'])

7)dict. setdefault():The method setdefault() is similar to get(), but will set dict[key] = default if key is not already in dict.

Syntax

Following is the syntax for setdefault() method −

dict.setdefault(key, default = None)

Parameters

·      key − This is the key to be searched.

·      default − This is the Value to be returned in case key is not found.

Return Value

This method returns the key value available in the dictionary and if given key is not available then it will return provided default value.

Example:

#!/usr/bin/python3

dict = {'Name': 'Zara', 'Age': 7}

print ("Value : %s" %  dict.setdefault('Age', None))

print ("Value : %s" %  dict.setdefault('Sex', None))

print (dict)

Output:

Value : 7

Value : None

{'Name': 'Zara', 'Sex': None, 'Age': 7}

8)dict. update(dict2):The method update() adds dictionary dict2's key-values pairs in to dict. This function does not return anything.

Example:

#!/usr/bin/python3

dict = {'Name': 'Zara', 'Age': 7}

dict2 = {'Sex': 'female' }

dict.update(dict2)

print ("updated dict : ", dict)

Output:

updated dict : {'Sex': 'female', 'Age': 7, 'Name': 'Zara'}

9)dict. values():Returns list of dictionary dict's values

Example:

#!/usr/bin/python3

dict = {'Sex': 'female', 'Age': 7, 'Name': 'Zara'}

print ("Values : ",  list(dict.values()))

Output:

Values :  ['female', 7, 'Zara']

Question: What is the difference between a set and a dictionary?

A set is an unordered data type which has no duplicate items inside it. a dictionary arranges items as {"key": "value"} pairs. sets are very handy when comparisons are to be made since sets handle data in a very controlled way as they dont allow duplicates, but dicts are useful when we want to store a bunch a data inside a single key, for example, all the cources offered in a school could be values stored inside the key which could be the school name, this is useful if we were handling large amount data that involved working with all the schools of a region.

Dictionaries Related Programs and Questions:

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1. When does a dictionary is used instead of a list?
Dictionaries – are best suited when the data is labelled, i.e., the data is a record with field names.
Lists – are better option to store collections of un-labelled items say all the files and sub directories in a folder.
Generally Search operation on dictionary object is faster than searching a list object.

2.What are Python's dictionaries?
Python's dictionaries are kind of hash table type. They work like associative arrays or hashes found in Perl and consist of key-value pairs. A dictionary key can be almost any Python type, but are usually numbers or strings. Values, on the other hand, can be any arbitrary Python object.

3.How will you create a dictionary in python?
Dictionaries are enclosed by curly braces ({ }) and values can be assigned and accessed using square braces ([]).

dict = {}
dict['one'] = "This is one"
dict[2]     = "This is two"
tinydict = {'name': 'john','code':6734, 'dept': 'sales'}

4.How will you get all the keys from the dictionary?
Using dictionary.keys() function, we can get all the keys from the dictionary object.
print (dict.keys())   # Prints all the keys

5.How will you get all the values from the dictionary?
Using dictionary.values() function, we can get all the values from the dictionary object.
print dict.values()   # Prints all the values

6. Example of dictionary
myDict = {'manish': 'bidsar','abc': 'efg'}
print(myDict)
print(myDict['manish'])
print(myDict["manish"])

7. Write a Python script to concatenate following dictionaries to create a new one
dic = {}
dic1 = {1:10,2:20}
dic2 = {3:30,4:40}
dic3 = {5:50,6:60}
for d in (dic1, dic2, dic3):
    dic.update(d)
print(dic)

8. Add key to dictionary

dict1 = {0:10, 1:20}

dict1[2] = "30"

print(dict1)

9. Write a Python program to iterate over dictionaries using for loops

d = {"blue" : 1, "green" : 2, "yellow" : 3}

for key,value in d.items():

    print(key, value)

10. Write a Python script to merge two Python dictionaries

d1 = {"a": "100", "b": "200"}

d2 = {"x": "4", "y": "500"}

d = d1.copy()

d.update(d2)

print(d)

print(d1.keys())

print(d1)

print(d1.values())

11. Write a Python program to get the maximum and minimum value in a 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])

#ALITER

values = my_dict.values()

print(values)

min = sorted(values)[0]

max = sorted(values)[-1]

print(min)

print(max)

12. Write a Python program to multiply all the items in a dictionary

#import math

d = {'n1': 5,'n2': 2,'n3': 3}

#print(mul.dict.vlaues())

res = 1

for key in d:

    res= res*d[key]

print(res)

#ALITER

d = {'n1': 5,'n2': 2,'n3': 3}

print(d.values())

res = 1

for i in d.values():

    res=res*i

print(res)

print(d.items())

13. Receiving tuples and dictionaries in functions
def powersum(power, *args):
    total = 0
    for i in args:
        total += pow(i, power)
    return total

powersum(2, 3, 4)

14.Write a Python program to remove duplicates from Dictionary

dict = {'one': 1, 'two': 2, 'three': 3, 'four': 1}

#print(dict)

#print(dict.items())

res = {}

for key,value in dict.items():

    if value not in res.values():

        res[key] = value

print(res)

15. Write a Python program to check a dictionary is empty or not.

my_dict = {}

if not bool(my_dict):

    print("Dictionary is empty")

else:

    print("Dictionary is not empty")

16.Write a Python program to combine two dictionary adding values for common keys

from collections import Counter

d1 = {'a': 100, 'b': 200, 'c':300}

d2 = {'a': 300, 'b': 200, 'd':400}

d = Counter(d1) + Counter(d2)

print(d)

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

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)

18.Write a Python program to sort a dictionary by key.

color_dict = {'red' : 1, 'blue': 2, 'orange': 3}

print(color_dict)

print(sorted(color_dict))

for key in sorted(color_dict):

    print("%s : %s" % (key, color_dict[key]))

19.Write a Python program to sum all the items in a dictionary.

dict = {'n1': 1, 'n2': 2, 'n3': 3}

print(sum(dict.values()))

20. Write a Python script to concatenate following dictionaries to create a new one
dic = {}
dic1 = {1:10,2:20}
dic2 = {3:30,4:40}
dic3 = {5:50,6:60}
for d in (dic1, dic2, dic3):
    dic.update(d)
print(dic)

21. Add key to dictionary

dict1 = {0:10, 1:20}

dict1[2] = "30"

print(dict1)

22.Example of dictionary comprehension
from pprint import pprint as pp
country_to_capital = {'United kingdom': 'London','Brazil': 'Brazilia','Morocco': 'Rabat','Sweden': 'Stockholm'}
pp(country_to_capital.items())
capital_to_country = {capital: country for country, capital in country_to_capital.items()}
pp(capital_to_country)

23. Python Program to Add a Key-Value Pair to the Dictionary

key=int(input("Enter the key (int) to be added:"))

value=int(input("Enter the value for the key to be added:"))

d={}

d.update({key:value})

print("Updated dictionary is:")

print(d)

Output:

1:

Enter the key (int) to be added:12

Enter the value for the key to be added:34

Updated dictionary is:

{12: 34}

2:

Enter the key (int) to be added:34

Enter the value for the key to be added:29

Updated dictionary is:

{34: 29}

24. Python Program to Concatenate Two Dictionaries into One

d1={'A':1,'B':2}

d2={'C':3}

d1.update(d2)

print("Concatenated dictionary is:")

print(d1)

Output:

1:

Concatenated dictionary is:

{'A': 1, 'C': 3, 'B': 2}

25. Python Program to Check if a Given Key Exists in a Dictionary or Not

d={'A':1,'B':2,'C':3}

key=input("Enter key to check:")

if key in d.keys():

      print("Key is present and value of the key is:")

      print(d[key])

else:

      print("Key isn't present!")

Output:

1:

Enter key to check:A

Key is present and value of the key is:

1

2:

Enter key to check:F

Key isn't present!

26. Python Program to Generate a Dictionary that Contains Numbers (between 1 and n) in the Form (x,x*x).

n=int(input("Enter a number:"))

d={x:x*x for x in range(1,n+1)}

print(d)

Output:

1:

Enter a number:5

{1: 1, 2: 4, 3: 9, 4: 16, 5: 25}

2:

Enter a number:10

{1: 1, 2: 4, 3: 9, 4: 16, 5: 25, 6: 36, 7: 49, 8: 64, 9: 81, 10: 100}

27. Python Program to Sum All the Items in a Dictionary

d={'A':100,'B':540,'C':239}

print("Total sum of values in the dictionary:")

print(sum(d.values()))

Output:

1:

Total sum of values in the dictionary:

879

28. Python Program to Multiply All the Items in a Dictionary

d={'A':10,'B':10,'C':239}

tot=1

for i in d:   

    tot=tot*d[i]

print(tot)

Output:

1:

23900

29. Python Program to Remove the Given Key from a Dictionary

d = {'a':1,'b':2,'c':3,'d':4}

print("Initial dictionary")

print(d)

key=input("Enter the key to delete(a-d):")

if key in d:

    del d[key]

else:

    print("Key not found!")

    exit(0)

print("Updated dictionary")

print(d)

Output:

1:

Initial dictionary

{'a': 1, 'c': 3, 'b': 2, 'd': 4}

Enter the key to delete(a-d):c

Updated dictionary

{'a': 1, 'b': 2, 'd': 4}

2:

Initial dictionary

{'a': 1, 'c': 3, 'b': 2, 'd': 4}

Enter the key to delete(a-d):g

Key not found!

30. Python Program to Form a Dictionary from an Object of a Class

class A(object): 

     def __init__(self): 

         self.A=1 

         self.B=2 

obj=A() 

print(obj.__dict__)

Output:

1:

{'A': 1, 'B': 2}

31. Add key to dictionary

keys=[]

values=[]

n=int(input("Enter number of elements for dictionary:"))

print("For keys:")

for x in range(0,n):

    element=int(input("Enter element" + str(x+1) + ":"))

    keys.append(element)

print("For values:")

for x in range(0,n):

    element=int(input("Enter element" + str(x+1) + ":"))

    values.append(element)

d=dict(zip(keys,values))

print("The dictionary is:")

print(d)

Output:

1:

Enter number of elements for dictionary:3

For keys:

Enter element1:1

Enter element2:2

Enter element3:3

For values:

Enter element1:1

Enter element2:4

Enter element3:9

The dictionary is:

{1: 1, 2: 4, 3: 9}

2:

Enter number of elements for dictionary:2

For keys:

Enter element1:23

Enter element2:46

For values:

Enter element1:69

Enter element2:138

The dictionary is:

{46: 138, 23: 69}

32. Python Program to Count the Frequency of Words Appearing in a String Using a Dictionary

test_string=input("Enter string:")

l=[]

l=test_string.split()

wordfreq=[l.count(p) for p in l]

print(dict(zip(l,wordfreq)))

Output:

1.

Enter string: heloo heloo i am manish i am manish you

{'heloo': 2, 'i': 2, 'am': 2, 'manish': 2, 'you': 1}

2:

Enter string:orange banana apple apple orange pineapple

{'orange': 2, 'pineapple': 1, 'banana': 1, 'apple': 2}

33. Python Program to Create a Dictionary with Key as First Character and Value as Words Starting with that Character.

test_string=input("Enter string:")

l=test_string.split()

d={}

for word in l:

    if(word[0] not in d.keys()):

        d[word[0]]=[]

        d[word[0]].append(word)

    else:

        if(word not in d[word[0]]):

          d[word[0]].append(word)

for k,v in d.items():

        print(k,":",v)

Output:

1:

Enter string:python is my most favourite programming language in the entire world

('e', ':', ['entire'])

('f', ':', ['favourite'])

('i', ':', ['in', 'is'])

('m', ':', ['most', 'my'])

('l', ':', ['language'])

('p', ':', ['programming', 'python'])

('t', ':', ['the'])

('w', ':', ['world'])

2.

Enter string: my name is manish

m : ['my', 'manish']

n : ['name']

i : ['is']

For Quick Recap on Dictionaries  watch the below videos: