Lists

Lists are a data object in Python which can store multiple values and the elements need not be of same data type.

In this tutorial we shall learn about lists in great detail!

Characteristics of a list

1. A list can have multiple elements: Let us create a list containing the scores of the students. A list is created in [ ] square brackets.

Scores = [78,80,98,41,66]
type(Scores)

2. Lists can have elements of multiple 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]

Similarly an index of -2 indicates extract the second last element

Scores[-2]

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]

Index function

Let us again create our list:

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

index( ) function returns the Python index for the list element defined in the parenthesis. In our list John appears at 4th index!

Scores.index('John')

Scores.index(80)

List manipulation

List manipulation involves:

-Changing the elements

-Adding elements

-Removing elements from a list

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

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)

Sorting a list!

Let us create our list again!

Scores = [78,80,98,41,66]

We can sort the elements of a list using sort( ) command. By default Python sorts by ascending order.

Scores.sort();
Scores

To sort the list elements in descending order we define reverse = True inside sort( ) function.

Scores.sort(reverse= True);
print(Scores)

Counting

We can count the number of occurrence of an element using count( ) function.

Scores = ['Emma',78,'Rebecca',80,'John',98,'Thomas',41,'Robin',66]
Scores.append('John');
print(Scores)
Scores.count('John')

In our list John is appearing twice!