Data Structure used in Python: List

Lists:

Indexing:

We are going to take a look at lists in Python. A list is a sequenced collection of different objects such as integers, strings, and other lists as well. The address of each element within a list is called an index. An index is used to access and refer to items within a list.

To create a list, type the list within square brackets [ ], with your content inside the parenthesis and separated by commas. Let’s try it!

In [1]:

# Create a list

L = ["Narendra Modi", 10.5, 2019]

L

Out[1]:

['Narendra Modi', 10.5, 2019]

We can use negative and regular indexing with a list :

In [2]:

# Print the elements on each index

print('the same element using negative and positive indexing:\n Postive:',L[0],

'\n Negative:' , L[-3]  )

print('the same element using negative and positive indexing:\n Postive:',L[1],

'\n Negative:' , L[-2]  )

print('the same element using negative and positive indexing:\n Postive:',L[2],

'\n Negative:' , L[-1]  )

the same element using negative and positive indexing:

 Postive: Narendra Modi

 Negative: Narendra Modi

the same element using negative and positive indexing:

 Postive: 10.5

 Negative: 10.5

the same element using negative and positive indexing:

 Postive: 2019

 Negative: 2019

List Content:

Lists can contain strings, floats, and integers. We can nest other lists, and we can also nest tuples and other data structures. The same indexing conventions apply for nesting:

In [5]:

# Content List

["Narendra Modi", 10.5, 2019, [1, 2], ("A", 1)]

Out[5]:

['Narendra Modi', 10.5, 2019, [1, 2], ('A', 1)]

List Operations:

We can also perform slicing in lists. For example, if we want the last two elements, we use the following command:

In [7]:

# Operation List

L = ["Narendra Modi", 10.5,2019,"NM",1]

L

Out[7]:

['Narendra Modi', 10.5, 2019, 'NM', 1]

In [8]:

# List slicing

L[3:5]

Out[8]:

['NM', 1]

We can use the method extend to add new elements to the list:

In [10]:

# Use extend to add elements to list

L = [ "aws","azure",25]

L.extend(['ibm', 10])

L

Out[10]:

['aws', 'azure', 25, 'ibm', 10]

Another similar method is append. If we apply append instead of extend, we add one element to the list:

In [13]:

# Use append to add elements to list

L = [ "aws","azure","ibm", 10.2]

L.append(["oracle", 10])

L

Out[13]:

['aws', 'azure', 'ibm', 10.2, ['oracle', 10]]

Each time we apply a method, the list changes. If we apply extend we add two new elements to the list. The list L is then modified by adding two new elements:

In [16]:

# Use extend to add elements to list

L = [ "cloud computing","aws", 2019]

L.extend(['azure', 2020])

L

Out[16]:

['cloud computing', 'aws', 2019, 'azure', 2020]

If we append the list ['a','b'] we have one new element consisting of a nested list:

In [18]:

# Use append to add elements to list

L.append(['ibm','oracle'])

L

Out[18]:

['cloud computing', 'aws', 2019, 'azure', 2020, ['a', 'b'], ['ibm', 'oracle']]

As lists are mutable, we can change them. For example, we can change the first element as follows:

In [20]:

# Change the element based on the index

A = ["aws", 2018, 2019]

print('Before change:', A)

A[0] = 'azure'

print('After change:', A)

Before change: ['aws', 2018, 2019]

After change: ['azure', 2018, 2019]

We can also delete an element of a list using the del command:

In [21]:

# Delete the element based on the index

print('Before change:', A)

del(A[0])

print('After change:', A)

Before change: ['azure', 2018, 2019]

After change: [2018, 2019]

We can convert a string to a list using split. For example, the method split translates every group of characters separated by a space into an element in a list:

In [23]:

# Split the string, default is by space

'microsoft azure 2019'.split()

Out[23]:

['microsoft', 'azure', '2019']

We can use the split function to separate strings on a specific character. We pass the character we would like to split on into the argument, which in this case is a comma. The result is a list, and each element corresponds to a set of characters that have been separated by a comma:

In [25]:

# Split the string by comma

'cloud computing,aws,azure,ibm'.split(',')

Out[25]:

['cloud computing', 'aws', 'azure', 'ibm']

Copy and Clone List:

When we set one variable B equal to A; both A and B are referencing the same list in memory:

In [26]:

# Copy (copy by reference) the list A

A = ["aws", "iot", "ai", "azure",2019,2020]

B = A

print('A:', A)

print('B:', B)

A: ['aws', 'iot', 'ai', 'azure', 2019, 2020]

B: ['aws', 'iot', 'ai', 'azure', 2019, 2020]

Initially, the value of the first element in B is set as hard rock. If we change the first element in A to banana, we get an unexpected side effect. As A and B are referencing the same list, if we change list A, then list B also changes. If we check the first element of B we get banana instead of hard rock:

In [27]:

# Examine the copy by reference

print('B[0]:', B[0])

A[0] = "banana"

print('B[0]:', B[0])

B[0]: aws

B[0]: banana

This is demonstrated in the following figure:

You can clone list A by using the following syntax:

In [28]:

# Clone (clone by value) the list A

B = A[:]

B

Out[28]:

['banana', 'iot', 'ai', 'azure', 2019, 2020]

Variable B references a new copy or clone of the original list; this is demonstrated in the following figure:

Now if you change A, B will not change:

In [29]:

print('B[0]:', B[0])

A[0] = "hard rock"

print('B[0]:', B[0])

B[0]: banana

B[0]: banana

Quiz on List

Q1. Create a list test_list, with the following elements 35, scientist, [60,70,80] and cloud.

In [ ]:

# Write your code and share the result in comment section.

Q2. Find the value stored at index 1 of a_list.

In [ ]:

# Write your code and share the result in comment section.

Q3. Retrieve the elements stored at index 1, 2 and 3 of test_list.

In [ ]:

# Write your code and share the result in comment section.

Q4. Concatenate the following lists A = [1, 'a'] and B = [2, 1, 'd']:

In [ ]:

# Write your code and share the result in comment section.

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