<< Chapter < Page Chapter >> Page >

(Hint: Remember that the plus sign when used with strings is the string concatenation operator.)

There are several ways that you can specify the index values that will produce an empty string. One of those ways is shown following the plus sign in Listing 6 .

Listing 6 . Print an empty string.
#print an empty string print("Empty: " + aStr[16:100])

In Listing 6 , both index values are outside the bounds of the index values of the characters in the string, which range from 0 through 15 inclusive.

Negative indices

Although it can be a little confusing, negative index values can be used to count from the right, as shown in Listing 7 .

Listing 7 . Negative indices.
#count from the right print(aStr[-5:-2]) #print tri

This fragment extracts and prints the characters tri from the word string , which is the last word in the string.

Eliminating confusion

Once you allow negative indices for slicing, things can become very confusing. The following explanation of how indices work with slicing is attributed to Guido van Rossum , the author of the Python programming language.

In this example, Mr. van Rossum is referring to a five-character string with a value of " HelpA ".

Eliminating confusion

The best way to remember how slices work is to think of the indices as pointing between characters, with the left edge of the firstcharacter numbered 0. Then the right edge of the last character of a string of n characters has index n, for example:

+---+---+---+---+---+| H | e | l | p | A | +---+---+---+---+---+0 1 2 3 4 5 -5 -4 -3 -2 -1

The first row of numbers gives the position of the indices 0...5 in the string; the second row gives the corresponding negative indices.

The slice from i to j consists of all characters between the edges labeled i and j, respectively.

For nonnegative indices, the length of a slice is the difference of the indices, if both are within bounds, e.g., the length of word[1:3]is 2.

Hopefully, this explanation will help you to understand and to remember how index values are used for the extraction of substrings from strings usingslicing.

Getting the length of a string

And finally, a built-in function named len() can be used to determine the number of characters actually contained in a string as shown in Listing 8 .

Listing 8 . Getting the length of a string.
#get the length of the string print(len(aStr))

For the example string used in this module, Listing 8 gets and prints the length of the string as 16.

If you count the characters in the string (beginning with 1) , you will conclude that there are 16characters in the string.

Note the difference between the number of characters and the maximum index value

For a string containing 16 characters, the valid index values range from 0 through 15 inclusive.

The complete output

This Python script file produces the output shown in the lower portion of Listing 9 .

A string is immutable

There is one more point that needs to be made here. Although you can use indexing and slicing to access the characters in a string, you cannot useindexing and slicing to assign new character values to those characters.

This is because a Python string is immutable . In other words, after it is created, it cannot be modified.


This section contains a variety of miscellaneous information.

Housekeeping material
  • Module name: Itse1359-1070-Strings Part 2
  • File: Itse1359-1070.htm
  • Published: 10/15/14
  • Revised: 01/31/16

Financial : Although the Connexions site makes it possible for you to download a PDF file for thismodule at no charge, and also makes it possible for you to purchase a pre-printed version of the PDF file, you should beaware that some of the HTML elements in this module may not translate well into PDF.

I also want you to know that, I receive no financial compensation from the Connexions website even if you purchase the PDF version of the module.

In the past, unknown individuals have copied my modules from cnx.org, converted them to Kindle books, and placed them for sale on Amazon.com showing me as the author. Ineither receive compensation for those sales nor do I know who does receive compensation. If you purchase such a book, please beaware that it is a copy of a module that is freely available on cnx.org and that it was made and published withoutmy prior knowledge.

Affiliation : I am a professor of Computer Information Technology at Austin Community College in Austin, TX.

Complete program listing

A complete listing of the program follows is shown in Listing 9 .

Listing 9 . The script named String01.py.
# File String01.py # Rev 2/4/00# Copyright 2000, R. G. Baldwin # Illustrates indexing and# slicing strings ##------------------------------- aStr = "This is a string"print(aStr[0]) #print Tprint(aStr[3]) #print sprint(aStr[0:4]) #print Thisprint(aStr[10:16]) #print stringprint(aStr[:4]) #print Thisprint(aStr[10:]) #print string#print the entire string print(aStr[:5]+ aStr[5:])print(aStr[:100])#print an empty string print("Empty: " + aStr[16:100]) #count from the rightprint(aStr[-5:-2]) #print tri#get the length of the string print(len(aStr))#========================================== #This script produces the following output,#which is not part of the script. Ts Thisstring Thisstring This is a stringThis is a string Empty:tri 16


Questions & Answers

Is there any normative that regulates the use of silver nanoparticles?
Damian Reply
what king of growth are you checking .?
What fields keep nano created devices from performing or assimulating ? Magnetic fields ? Are do they assimilate ?
Stoney Reply
why we need to study biomolecules, molecular biology in nanotechnology?
Adin Reply
yes I'm doing my masters in nanotechnology, we are being studying all these domains as well..
what school?
biomolecules are e building blocks of every organics and inorganic materials.
anyone know any internet site where one can find nanotechnology papers?
Damian Reply
sciencedirect big data base
Introduction about quantum dots in nanotechnology
Praveena Reply
what does nano mean?
Anassong Reply
nano basically means 10^(-9). nanometer is a unit to measure length.
do you think it's worthwhile in the long term to study the effects and possibilities of nanotechnology on viral treatment?
Damian Reply
absolutely yes
how to know photocatalytic properties of tio2 nanoparticles...what to do now
Akash Reply
it is a goid question and i want to know the answer as well
characteristics of micro business
for teaching engĺish at school how nano technology help us
Do somebody tell me a best nano engineering book for beginners?
s. Reply
there is no specific books for beginners but there is book called principle of nanotechnology
what is fullerene does it is used to make bukky balls
Devang Reply
are you nano engineer ?
fullerene is a bucky ball aka Carbon 60 molecule. It was name by the architect Fuller. He design the geodesic dome. it resembles a soccer ball.
what is the actual application of fullerenes nowadays?
That is a great question Damian. best way to answer that question is to Google it. there are hundreds of applications for buck minister fullerenes, from medical to aerospace. you can also find plenty of research papers that will give you great detail on the potential applications of fullerenes.
what is the Synthesis, properties,and applications of carbon nano chemistry
Abhijith Reply
Mostly, they use nano carbon for electronics and for materials to be strengthened.
is Bucky paper clear?
carbon nanotubes has various application in fuel cells membrane, current research on cancer drug,and in electronics MEMS and NEMS etc
so some one know about replacing silicon atom with phosphorous in semiconductors device?
s. Reply
Yeah, it is a pain to say the least. You basically have to heat the substarte up to around 1000 degrees celcius then pass phosphene gas over top of it, which is explosive and toxic by the way, under very low pressure.
Do you know which machine is used to that process?
how to fabricate graphene ink ?
for screen printed electrodes ?
What is lattice structure?
s. Reply
of graphene you mean?
or in general
in general
Graphene has a hexagonal structure
On having this app for quite a bit time, Haven't realised there's a chat room in it.
what is biological synthesis of nanoparticles
Sanket Reply
how did you get the value of 2000N.What calculations are needed to arrive at it
Smarajit Reply
Privacy Information Security Software Version 1.1a
Got questions? Join the online conversation and get instant answers!
Jobilize.com Reply

Get the best Algebra and trigonometry course in your pocket!

Source:  OpenStax, Itse 1359 introduction to scripting languages: python. OpenStax CNX. Jan 22, 2016 Download for free at https://legacy.cnx.org/content/col11713/1.32
Google Play and the Google Play logo are trademarks of Google Inc.

Notification Switch

Would you like to follow the 'Itse 1359 introduction to scripting languages: python' conversation and receive update notifications?