• Ekta Aggarwal

Data Structures in Python

To handle and store the data in an efficient manner python has various data structures - each having its own utility. Python has 4 types of data structures which we shall be understanding in detail:


Lists


Lists are a data object in Python which can store multiple values and the elements need not be of same data type. A list is created in [ ] square brackets.

1. A list can have multiple elements

2. Elements of different data types: Following is our list of scores - which contains student names (strings) and their scores (numeric)

Scores = ['Emma',78,'Rebecca',80,'John',98,'Thomas',41,'Robin',66]

3. A list can contain several lists! Following list contains scores of students where each element is itself a list!

list2 = [['Emma',78],['Rebecca',80],['John',98],['Thomas',41],['Robin',66]]


Subsetting lists


Understanding with the help of an example!

Scores = ['Emma',78,'Rebecca',80,'John',98,'Thomas',41,'Robin',66]

Python indexing begins from 0. Thus to get the first element of the list we write:

list_name[0] . Square brackets denote that we want to extract an element from our list.

Scores[0]




Following code will return fourth element (indexing starts from 0, thus 3 represents 4th index)

Scores[3]



Writing a semi-colon : in square brackets [ ] in front of the list name would return all the elements.

Scores[:]

In Python an index of -1 indicates extract the last element of our list!

Scores[-1]





List Slicing


Extracting multiple elements of a list is called list slicing (you are slicing down i.e. cutting down your list)

Scores[1:4] # Extract the elements from first index to third index (index 4 is excluded) i.e. positions 2 to 4. (Remember Python indexing starts from 0)




If we write [N: ] in front of a list then it tells Python to extract all the elements starting from Nth index.

Scores[1:]

If we write [:N] in front of a list then it tells Python to extract elements from the starting till the (N-1)th index (N is excluded!)

Scores[:4]




Changing the elements

By defining list_name[index] = 'New_value' we can update the values in our list

Scores[2] = 'Asif'
Scores

We can also change multiple elements of a list in one command. In the following code we are updating 0th and 1st index values as 'Lisa' and 86

Scores[0:2] = ['Lisa',86] 
Scores

Adding the elements


We can also append or add elements in our list.


Method 1: Using a + symbol we can add elements to our list.

In the following code we are adding a list ['Henry',92] at the end of our original list Scores.

Scores = Scores + ['Henry',92];
print(Scores)

Method 2: Using .append( ) command

In the following code we are adding an element "Bella" at the end of our list Scores.

Scores.append('Bella');
print(Scores)

Removing elements from a list


We can remove elements from a list using .remove( ) command:

Scores.remove('Bella');
print(Scores)

We can also insert elements by specifying the index using insert ( ) function. In the following code we have added Bella at 2nd index (i.e. 3rd position) in our list.

Scores.insert(2,'Bella');
print(Scores)

Let us remove this element for the sake of simplicity.

Scores.remove('Bella');
print(Scores)

We can also remove the elements by specifying the index using pop( ) command. In the code below we are deleting the element at 3rd index i.e. 4th position.

Scores.pop(3);
print(Scores)

If you do not specify anything in the pop( ) command then by default it removes the last element from the list.

Scores.pop();
print(Scores)

Tuples


Unlike lists tuples are the data structures where the elements cannot be altered. A tuple is created using parenthesis ( )


Scores = (78,80,98,41,66);
type(Scores)





Subsetting a tuple


A tuple can be filtered like a list. Its indexing starts from 0.

Scores = ('Emma',78,'Rebecca',80,'John',98,'Thomas',41,'Robin',66)
Scores[0]
Scores[1]







You cannot alter or change the values in a tuple after it is created. For example in the following code we are trying to replace the value Emma by Henry. If it would have been a list then we could have made this change successfully. But alas! Tuples don't allow us to change their values.

Scores['Emma'] = 'Henry'

Assigning values of tuples to multiple variables


Let us create a tuple

Scores = (78,80,98,41,66)

We can assign these scores individually to each of the variables as follows:

Emma,Rebecca, John,Thomas, Robin = Scores
Emma

Here Emma gets the value 78, Rebecca as 80, John as 98 etc.







To learn about lists in detail refer to this tutorial!


Dictionaries


Python has a special object dictionaries which store the mapping in form of key and values, where value(s) map to a single key. In this tutorial we shall learn about dictionaries in depth.

Keys in dictionaries must be unique and are case -sensitive i.e. X and x will be treated as 2 different keys.


Dictionaries are denoted by { } curly brackets.


Let us create our first dictionary with key value pairs denoting Countries and their capitals.

The mapping between keys and values are denoted by : colon .

Different key-value pairs are separated by a comma.

my_dict = {'U.S.A' : 'Washington D.C',
'U.K.' : 'London',
'Japan':'Tokyo',
'Russia':'Moscow'}

type(my_dict)




Items


Items in a python dictionary are the key-value pairs. In the above dictionary we have 4 items. Items can be retrieved by:

my_dict.items()

Keys


Keys in dictionaries are unique and are used as one to many or one to one mapping for values. In our dictionary we have defined countries as our keys.

my_dict.keys()

Values


Values in a dictionary can be a one-dimensional list or single values of any variable type (numeric or string or boolean). They can be repetitive and get mapped to keys.

In my_dict we have defined capitals as the values.

my_dict.values()

Adding an item to our dictionary


We can add a new item (key-value pair) in following syntax:

dict_name['key'] = values.


Task: Add a new key - value pair 'India : New Delhi' to our dictionary my_dict.

my_dict['India'] = 'New Delhi'
my_dict








Deleting an item from our dictionary


We can delete a key value pair from our dictionary using del keyword with following syntax:

del dictionary_name['key_to_be_deleted']


Task: Delete key 'India' from our dictionary.

del my_dict['India']
my_dict







Mapping multiple values to a single key.


In a dictionary we can map multiple values to a key. The multiple values are passed in the form of a list!

Let us create a dictionary with 2 keys: Countries and Capitals, where they values will be passed on the form of lists.

my_dict = {'Countries' : ['U.S.A.','U.K.','Japan','Russia'],
          'Capitals' : ['Washington D.C.','London','Tokyo','Moscow']}
my_dict
my_dict.items()
my_dict.keys()
my_dict.values()

To learn more about dictionaries refer to this tutorial


Sets


Sets are immutable objects in python which are created using { } curly brackets. They always have unique values.

In the following set, we are repeating Japan twice but a set removes the duplicates.

my_set = {'U.S.A.','U.K.','Russia','Japan','Japan'}
my_set






Adding a value to the set


We can add the values in a set using add( ) command.

my_set.add('India')
my_set






Removing the values from a set


Values in a set can be removed using remove( ) function.

my_set.remove('India')
my_set



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