Write a program to create a list and display the last two elements using negative indexing

Python lists are one of the most versatile data types that allow us to work with multiple elements at once. For example,

Inhaltsverzeichnis Show

Create Python Lists
Access List Elements
Negative indexing
List Slicing in Python
Add/Change List Elements
Delete List Elements
Python List Methods
List Comprehension: Elegant way to create Lists
Other List Operations in Python
List Membership Test
Iterating Through a List
Basic List Structure and Operations
What Are List Indexes?
Negative Indexes in Lists
Using Negative Indexes to Retrieve List Data
Reversing the List With Negative Index
Removing Elements With Negative Index
Advantages of Using Negative Indexing in Python List

# a list of programming languages ['Python', 'C++', 'JavaScript']

Create Python Lists

In Python, a list is created by placing elements inside square brackets [], separated by commas.

# list of integers my_list = [1, 2, 3]

A list can have any number of items and they may be of different types (integer, float, string, etc.).

# empty list my_list = [] # list with mixed data types my_list = [1, "Hello", 3.4]

A list can also have another list as an item. This is called a nested list.

# nested list my_list = ["mouse", [8, 4, 6], ['a']]

Access List Elements

There are various ways in which we can access the elements of a list.

List Index

We can use the index operator [] to access an item in a list. In Python, indices start at 0. So, a list having 5 elements will have an index from 0 to 4.

Trying to access indexes other than these will raise an IndexError. The index must be an integer. We can't use float or other types, this will result in TypeError.

Nested lists are accessed using nested indexing.

my_list = ['p', 'r', 'o', 'b', 'e'] # first item print(my_list[0]) # p # third item print(my_list[2]) # o # fifth item print(my_list[4]) # e # Nested List n_list = ["Happy", [2, 0, 1, 5]] # Nested indexing print(n_list[0][1]) print(n_list[1][3]) # Error! Only integer can be used for indexing print(my_list[4.0])

Output

p o e a 5 Traceback (most recent call last): File "<string>", line 21, in <module> TypeError: list indices must be integers or slices, not float

Negative indexing

Python allows negative indexing for its sequences. The index of -1 refers to the last item, -2 to the second last item and so on.

# Negative indexing in lists my_list = ['p','r','o','b','e'] # last item print(my_list[-1]) # fifth last item print(my_list[-5])

Output

e p

List indexing in Python

List Slicing in Python

We can access a range of items in a list by using the slicing operator :.

# List slicing in Python my_list = ['p','r','o','g','r','a','m','i','z'] # elements from index 2 to index 4 print(my_list[2:5]) # elements from index 5 to end print(my_list[5:]) # elements beginning to end print(my_list[:])

Output

['o', 'g', 'r'] ['a', 'm', 'i', 'z'] ['p', 'r', 'o', 'g', 'r', 'a', 'm', 'i', 'z']

Note: When we slice lists, the start index is inclusive but the end index is exclusive. For example, my_list[2: 5] returns a list with elements at index 2, 3 and 4, but not 5.

Add/Change List Elements

Lists are mutable, meaning their elements can be changed unlike string or tuple.

We can use the assignment operator = to change an item or a range of items.

# Correcting mistake values in a list odd = [2, 4, 6, 8] # change the 1st item odd[0] = 1 print(odd) # change 2nd to 4th items odd[1:4] = [3, 5, 7] print(odd)

Output

[1, 4, 6, 8] [1, 3, 5, 7]

We can add one item to a list using the append() method or add several items using the extend() method.

# Appending and Extending lists in Python odd = [1, 3, 5] odd.append(7) print(odd) odd.extend([9, 11, 13]) print(odd)

Output

[1, 3, 5, 7] [1, 3, 5, 7, 9, 11, 13]

We can also use + operator to combine two lists. This is also called concatenation.

The * operator repeats a list for the given number of times.

# Concatenating and repeating lists odd = [1, 3, 5] print(odd + [9, 7, 5]) print(["re"] * 3)

Output

[1, 3, 5, 9, 7, 5] ['re', 're', 're']

Furthermore, we can insert one item at a desired location by using the method insert() or insert multiple items by squeezing it into an empty slice of a list.

# Demonstration of list insert() method odd = [1, 9] odd.insert(1,3) print(odd) odd[2:2] = [5, 7] print(odd)

Output

[1, 3, 9] [1, 3, 5, 7, 9]

Delete List Elements

We can delete one or more items from a list using the Python del statement. It can even delete the list entirely.

# Deleting list items my_list = ['p', 'r', 'o', 'b', 'l', 'e', 'm'] # delete one item del my_list[2] print(my_list) # delete multiple items del my_list[1:5] print(my_list) # delete the entire list del my_list # Error: List not defined print(my_list)

Output

['p', 'r', 'b', 'l', 'e', 'm'] ['p', 'm'] Traceback (most recent call last): File "<string>", line 18, in <module> NameError: name 'my_list' is not defined

We can use remove() to remove the given item or pop() to remove an item at the given index.

The pop() method removes and returns the last item if the index is not provided. This helps us implement lists as stacks (first in, last out data structure).

And, if we have to empty the whole list, we can use the clear() method.

my_list = ['p','r','o','b','l','e','m'] my_list.remove('p') # Output: ['r', 'o', 'b', 'l', 'e', 'm'] print(my_list) # Output: 'o' print(my_list.pop(1)) # Output: ['r', 'b', 'l', 'e', 'm'] print(my_list) # Output: 'm' print(my_list.pop()) # Output: ['r', 'b', 'l', 'e'] print(my_list) my_list.clear() # Output: [] print(my_list)

Output

['r', 'o', 'b', 'l', 'e', 'm'] o ['r', 'b', 'l', 'e', 'm'] m ['r', 'b', 'l', 'e'] []

Finally, we can also delete items in a list by assigning an empty list to a slice of elements.

>>> my_list = ['p','r','o','b','l','e','m'] >>> my_list[2:3] = [] >>> my_list ['p', 'r', 'b', 'l', 'e', 'm'] >>> my_list[2:5] = [] >>> my_list ['p', 'r', 'm']

Python List Methods

Python has many useful list methods that makes it really easy to work with lists. Here are some of the commonly used list methods.

Methods	Descriptions
append()	adds an element to the end of the list
extend()	adds all elements of a list to another list
insert()	inserts an item at the defined index
remove()	removes an item from the list
pop()	returns and removes an element at the given index
clear()	removes all items from the list
index()	returns the index of the first matched item
count()	returns the count of the number of items passed as an argument
sort()	sort items in a list in ascending order
reverse()	reverse the order of items in the list
copy()	returns a shallow copy of the list

# Example on Python list methods my_list = [3, 8, 1, 6, 8, 8, 4] # Add 'a' to the end my_list.append('a') # Output: [3, 8, 1, 6, 8, 8, 4, 'a'] print(my_list) # Index of first occurrence of 8 print(my_list.index(8)) # Output: 1 # Count of 8 in the list print(my_list.count(8)) # Output: 3

List Comprehension: Elegant way to create Lists

List comprehension is an elegant and concise way to create a new list from an existing list in Python.

A list comprehension consists of an expression followed by for statement inside square brackets.

Here is an example to make a list with each item being increasing power of 2.

pow2 = [2 ** x for x in range(10)] print(pow2)

Output

[1, 2, 4, 8, 16, 32, 64, 128, 256, 512]

This code is equivalent to:

pow2 = [] for x in range(10): pow2.append(2 ** x)

A list comprehension can optionally contain more for or if statements. An optional if statement can filter out items for the new list. Here are some examples.

>>> pow2 = [2 ** x for x in range(10) if x > 5] >>> pow2 [64, 128, 256, 512] >>> odd = [x for x in range(20) if x % 2 == 1] >>> odd [1, 3, 5, 7, 9, 11, 13, 15, 17, 19] >>> [x+y for x in ['Python ','C '] for y in ['Language','Programming']] ['Python Language', 'Python Programming', 'C Language', 'C Programming']

Visit Python list comprehension to learn more.

Other List Operations in Python

List Membership Test

We can test if an item exists in a list or not, using the keyword in.

my_list = ['p', 'r', 'o', 'b', 'l', 'e', 'm'] # Output: True print('p' in my_list) # Output: False print('a' in my_list) # Output: True print('c' not in my_list)

Output

True False True

Iterating Through a List

Using a for loop we can iterate through each item in a list.

for fruit in ['apple','banana','mango']: print("I like",fruit)

Output

I like apple I like banana I like mango

If you are using Quantopian, it is advisable that you become familiar with numpy and pandas. For example:

>>> import numpy as np >>> -1*np.arange(20) array([ 0, -1, -2, -3, -4, -5, -6, -7, -8, -9, -10, -11, -12, -13, -14, -15, -16, -17, -18, -19])

Then you will have a[1]==-1, a[5]==-5, etc.

Python lists can be indexed in a variety of ways, including positive and negative numbers. This article will discuss how to use the “-” minus parameter to access specific elements in a list.

Basic List Structure and Operations

A list is defined using square brackets and elements within them. Each element is separated with the help of a comma.

Syntax:

list_name = [element_!, element_2, ..., element_n]

Code:

int_list= [3, 4, 5, 5, 6] # a list containing all integers char_list = ['a', 'b', 'c', 'd'] # al list of characters str_list = ["apple", 'banana', 'lemon', 'orange'] # a list of strings

What Are List Indexes?

Indexing is the fundamental unit of any linear data structure in any programming language. For every computer, the default index started from 0 and goes on till n-1. Here n denotes the total number of elements in the respective data structure. Types are:

Positive indexing: Starts from 0 and increases
Negative indexing: starts from the last element and movement is from tail to head with each traversal.

These help us to access the individual elements of this data structure. Let us see the operations in the next section.

Negative Indexes in Lists

This is the main point of focus on our topic. The “-1” parameter in lists is to denote the Python interpreter that operations are from the last element. First, we shall see how we can access the elements using the index numbers.

Indexing In Lists

Using Negative Indexes to Retrieve List Data

my_list = [45, 5, 33, 1, -9, 8, 76] print(my_list[-1]) print(my_list[-2]) print(my_list[-3])

Output:

Reversing the List With Negative Index

Using built-in function “reverse()”:

my_list = [45, 5, 33, 1, -9, 8, 76] my_list.reverse()

Output:

[76, 8, -9, 1, 33, 5, 45]

Reversing the list using -1

my_list = [45, 5, 33, 1, -9, 8, 76] my_list[::-1]

Output:

[76, 8, -9, 1, 33, 5, 45] # reversed list

Removing Elements With Negative Index

Using the pop() function and giving -1 as parameter inside it we can remove the last element of that list and we get a new list.

my_list = [45, 5, 33, 1, -9, 8, 76] my_list.pop(-1) my_list

Output:

76 # popped out element [45, 5, 33, 1, -9, 8] # new list returned

Advantages of Using Negative Indexing in Python List

Reduces the lines of code and makes reverse in one line.
Makes complex operations easier.
Takes minimum time to operate providing less complexity.

Conclusion

This concludes our tutorial on Python negative indexing. We hope you found it helpful! If you have any questions or comments, please feel free to share them below. Thanks for reading!