Python: Dictionaries

Dictionaries

1. Introduction to Dictionary

Definition: In python, dictionary is unordered collection of items. These items are in the form key‒value pairs.

• The dictionary contains the collection of indices called  keys and collection of values.

• Each key is associated with a single value.

The association of keys with values is called key‒value pair or item.

• Dictionary always represent the mappings of keys with values. Thus each key maps to a value.

How to create dictionary ?

• Items of the dictionary are written within the {} brackets and are separated by commas.

• The key value pair is represented using: operator. That is key: value.

• For example

  my_dictionary={1:'AAA',2:'BBB',3:'CCC'}

• The keys are unique and are of immutable types ‒ such as string, number, tuple.

• Here is a screenshot that shows how to create a dictionary

In [12]:

#creating an empty dictionary

my_dictionary = {}

my_dictionary

Out[12]: {}

In [13]:

#creating a simple dictionary with keys as integer

my_dictionary={1:'AAA',2: 'BBB',3: 'CCC'}

my_dictionary

Out[13]: {1: 'AAA', 2: 'BBB', 3: 'CCC'}

• We can also create a dictionary with mixed data type as

>>> my_dictionary= {'name':'AAA',2:89}

• We can also create a dictionary using the word dict().

For example ‒

>>> my_dictionary= dict({0:'Red', 1:'Green',2:'Blue'})

1. Accessing Values in Dictionary

We can access the element in the dictionary using the keys. Following script illustrates it

DictionaryDemo.py

student_dict={'name': 'AAA', 'roll':10}

print(student_dict['name'])

print(student_dict['roll'])

Output

AAA

10

In [14]:

student_dict={'name': 'AAA', 'roll':10}

print(student_dict['name'])

print(student_dict['roll'])

AAA

10

• Now to traverse the dictionary items we need to consider two values at a time and these are the values for both key and value.

• At a time we can assign both of these values. This is called multiple assignment.

For example ‒ Following is a python code in which we are traversing the keys and values of a dictionary in a single loop.

In [15]:

d = {'AAA':10, 'BBB':20, 'CCC':30}

for key, val in list(d.items()):      ←Traversing through dictionary

print(key, val)

AAA 10

BBB 20

CCC 30

Code explanation:

• Note that, the above loop has two iteration variables because items returns a list of tuples.

The key‒value is a tuple assignment that successively iterates through each of key value pairs in the dictionary.

2. Deleting Values in Dictionary

For removing an item from the dictionary we use the keyword del.

For example

>>> del my_dictionary[2] #deleting the item from dictionary

>>> print(my_dictionary) #display of dictionary

{0: 'Red', 1: 'Green', 3: 'Yellow'}

>>> 

3. Updating Values in Dictionary

We can update the value of the dictionary by directly assigning the value to corresponding key position.

For example ‒

>>> my dictionary = dict((0:'Red', 1:'Green',2: 'Blue')) #creation of dictionary

>>> print(my_dictionary) #display of dictionary

{0: 'Red', 1: 'Green', 2: 'Blue'}

>>> my dictionary[1]='Yellow' #updating the value at particular index

>>> print(my_dictionary) #display of dictionary

{0: 'Red', 1: 'Yellow', 2: 'Blue'} #updation of value can be verified.

>>> 

2. Basic Dictionary Operations

Various operations that can be performed on dictionary are :

1. Adding item to dictionary

We can add the item to the dictionary.

For example ‒

>>> my_dictionary= dict({0:'Red', 1:'Green', 2:'Blue'})

>>> print(my_dictionary)

{0: 'Red', 1: 'Green', 2: 'Blue'}

>>> my_dictionary [3]='Yellow' #adding the element to the dictioary

>>> print(my_dictionary)

{0: 'Red', 1: 'Green', 2: 'Blue', 3: 'Yellow'}

>>> 

2. Checking length

The len function gives the number of pairs in the dictionary.

For example‒

>>> my_dictionary=dict({0:'Red', 1:'Green', 2: 'Blue'})#creation of dictionary

>>> len(my_dictionary) # finding the length of dictionary

3                                   #meaning ‒ that there are 3 items in dictionary

>>> 

3. Iterating through dictionary

For iterating through the dictionary we can use the for loop.

The corresponding key and values present in the dictionary can be displayed using this for loop.

The illustrative program is as follows ‒

LoopDemo.py

d={'Red':10, 'Blue':42, 'Orange':21}

for i in d:

print(i,d[i])

Output

Red 10

Blue 42

Orange 21

3. Built in Dictionary Functions

Following are some commonly used methods in dictionary.

method  : Purpose

clear: Removes all items from dictionary.

copy() : Returns a copy of dictionary.

fromkeys( ): Creates a new dictionary from given sequence of elements and values provided by user.

get(key[,d]): Return the value of key. If key doesnot exit, return d (defaults to None).

items() : Return a new view of the dictionary's items (key, value).

Keys : Return a new view of the dictionary's keys.

pop(key[,d]) : Remove the item with key and return its value or d if key is not found. If d is not provided and key is not found, raises KeyError.

popitem() : Remove and return an arbitary item (key, value). Raises KeyError if the dictionary is empty.

setdefault(key[,d]) : If key is in the dictionary, return its value. If not, insert key with a value of d and return d (defaults to none).

update([other]) : Update the dictionary with the key/value pairs from other, overwriting existing keys.

values() : Return a new view of the dictionary's values.

1. The clear method

This method removed all the items from the dictionary. This method does not take any parameter and does not return anything.

For example

>>> my_dictionary={1:'AAA',2: 'BBB',3:'CCC') # creation of dictionary

>>> print(my_dictionary) #display

{1: 'AAA', 2: 'BBB', 3: 'CCC'}

>>> my_dictionary.clear() #using clear method

>>> print(my_dictionary) #display

{}

>>> 

2. The copy method

The copy method returns the copy of the dictionary. It does not take any parameter and returns a shallow copy of dictionary.

For example

>>> my_dictionary={1:'AAA',2:'BBB', 3:'CCC'}

>>> print(my_dictionary)

{1: 'AAA', 2: 'BBB', 3: 'CCC'}

>>> new_dictionary=my_dictionary.copy()

>>> print(new_dictionary)

{1: 'AAA', 2: 'BBB', 3: 'CCC'}

>>> 

3. The fromkey method

The fromkeys() method creates a new dictionary from the given sequence of elements with a value provided by the user.

Syntax

dictionary.fromkeys(sequence[, value])

The fromkeys() method returns a new dictionary with the given sequence of elements as the keys of the dictionary. If the value argument is set, each element of the newly created dictionary is set to the provided value.

For example

>>> keys={10,20,30}

>>> values = 'Number'

>>> new_dict=dict.fromkeys(keys, values)

>>> print(new_dict)

{10: 'Number', 20: 'Number', 30: 'Number'}

>>> 

4. The get method

The get() method returns the value for the specified key if key is in dictionary. This method takes two parameters key and value. The get method can return either key or value or nothing.

Syntax

dictionary.get(key, value])

For example

>>> student={'name':'AAA', 'roll':10,'marks':98} #creation of dictionary

>>> print("Name: ",student.get('name'))

Name: AAA

>>> print("roll: ",student.get('roll'))

roll: 10

>>> print("marks: ",student.get('marks'))

marks: 98

>>> print("Address: ",student.get('address')) #this key is 'address' is not specified

                                                                     #in the list

Address: None #Hence it returns none

>>> 

5. The value method

This method returns the value object that returns view object that displays the list of values present in the dictionary.

For example

>>> my_dictionary ={'marks1':99,'marks2':96,'marks3':97}#creating dictionary

>>> print(my_dictionary.values()) #displaying values

dict_values([99, 96, 97])

>>> 

6. The pop method

The pop() method removes and returns an element from a dictionary having the given key.

Syntax

pop(key), default])

where key is the key which is searched for removing the value. And default is the value which is to be returned when the key is not in the dictionary,

For example

my_dictionary={1:'Red', 2:'Blue', 3:'Green'} #creation of dictionary

>>> val=my_dictionary.pop(1) #removing the element with key 1

>>> print("The popped element is: ",val)

The popped element is: Red

>>> val=my_dictionary.pop(5,3)

#specifying default value. When the specified key is not present in the list, then the #default value is returned.

>>> print("The popped element using default value is: ",val)

The popped element using default value is: 3 #default value is returned

>>> 

Example:1

Write a python program to sort the elements of dictionary.

Solution :

Sort.py

d={'Red':10, 'Blue':42, 'Yellow':32,'Orange':21}

myList=list(d.keys())

print(myList)

myList.sort()

print("Sorted list based on Keys")

for i in myList:

   print(i)

Output

Sorted list based in Keys

Blue

Orange

Red

Yellow

Example:2

Write a python program to create a tuple from given dictionary elements.

Solution: Using the method named item of dictionary, the list of tuples can be returned.

For example ‒

>>> d={'AAA':10,'BBB':20, 'CCC':30}

>>> t1=list(d.items())

>>> print(t1)

 [('AAA', 10), ('BBB', 20), ('CCC', 30)]

>>>