A basic question about list in Python

Question

0.00/5 (No votes)

See more:

L=[1,2,3,4,5]

print L[4:1] #prints an empty list. Can not traverse in reverse direction.

How does L[::-1] can traverse in reverse direction?

What I have tried:

Trying to understand list traversal in python by different indices/slicing.

Posted 15-Oct-18 20:50pm

Member 13508014

Updated 15-Oct-18 21:55pm

Add a Solution

1 solution

Add a Solution

Add your solution here

Treat my content as plain text, not as HTML

Preview 0

…

Existing Members

Sign in to your account

...or Join us

Download, Vote, Comment, Publish.

Your Email
Password
Forgot your password?

Your Email
This email is in use. Do you need your password?
Optional Password

I have read and agree to the Terms of Service and Privacy Policy
Please subscribe me to the CodeProject newsletters

When answering a question please:

Read the question carefully.
Understand that English isn't everyone's first language so be lenient of bad spelling and grammar.
If a question is poorly phrased then either ask for clarification, ignore it, or edit the question and fix the problem. Insults are not welcome.
Don't tell someone to read the manual. Chances are they have and don't get it. Provide an answer or move on to the next question.

Let's work to help developers, not make them feel stupid.

This content, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

Richard MacCutchan · Accepted Answer · 2018-10-15T21:56:00

Solution 1

See 3. An Informal Introduction to Python — Lists[^].

[edit]
The missing start and stop values indicate that it should use the entire list. Setting a step value of -1 causes the values to be reversed, so it treats it as

Python

list[end:start:step]

However, this only works for types that have a __reversed__() method. It is just a shorthand way of calling list.reverse();.
[/edit]

Posted 15-Oct-18 21:56pm

Richard MacCutchan

Updated 16-Oct-18 1:49am

v2

Comments

Member 13508014 16-Oct-18 4:38am

I read the article. I still have the same doubt.

Richard MacCutchan 16-Oct-18 4:56am

Look at the preceding sections where it explains how slices are addressed. In fact, I would suggest you work through the entire tutorial, you will learn far more than posting questions here.

Member 13508014 16-Oct-18 5:13am

I understood the examples given there. All examples given there makes it to understand as python maintains the list as single linked list. That is why L[4:1] gives empty list, because a singly linked list can not be traversed in reverse way. But if that is the case how does L[start:end:-1] can traverse in reverse direction is what I did not understand. This is not explained in the article.

Richard MacCutchan 16-Oct-18 10:37am

See updated solution.

Member 13508014 16-Oct-18 12:09pm

Excellent. It clarified my doubt.