<< Chapter < Page Chapter >> Page >

Then the code in the function creates a new list and attempts to use the second incoming parameter to replace the original list pointed to by that parameter withthe new list. The last line of text in Figure 1 confirms that this was not successful . The copy of the incoming parameter points to the new list but this does not cause the original reference to point to the new list.

A non-technical explanation

This same situation occurs in the Java courses that I teach. I often attempt to explain it this way. Assume that I write my home address on a piece of paperand make a copy of that piece of paper. Then I lose the copy. If a burglar finds the copy, he has a pointer to my house. He could use that pointer in an attemptto break into my house. However, even if he were to scratch out the address on the copy and write a new address on the copy, that would not change the addresswritten on the original. The original would still point to the same old address.

This is essentially what happens in Listing 1 when the code creates a new list and assigns its value to the copy of the original reference. The copy points tothe new list inside the function but the original reference outside the function continues to point to the original list.

In effect, the copies that are received as parameters by a function become local variables within the function. Those variables, and in this case the newlist that is pointed to by one of those variables, all cease to exist when the function terminates unless returned by the function.

Visualization of the code in Listing 1

Figure 2 shows a visualization of the code in Listing 1 after all of the code has been executed.

Figure 2. Visualization of the code in Listing 1.

Visualization of the code in Listing 1

I recommend that you create a visualization for the code in Listing 1 and step through the program one instruction at a time. As you do that, pay attention tothe movements of the red and green arrows on the left, the diagram on the right, and the printed material at the bottom. That should help you to betterunderstand the concept of "passing by value."

A modified approach to the same objective

Listing 2 shows a modification to the code from Listing 1 that is designed to accomplish what Listing 1 attempted to do but was unable to do.

Listing 2 . Another list-modifier function.
# Illustrates pass by value or reference #---------------------------------------------------def listModifier(listA,listB): """Illustrates pass by value or reference"""print("In listModifier") print("Use incoming parameter to append to listA")listA.append(3.14159) print("New listA = " + str(listA))print("Assign a new list to listB") listB = ["A","new","list"]print("New listB = " + str(listB)) return listB#End function definition #Call the functionprint("Create two lists") aList = ["ab","cd","ef"]bList = ["The","old","list"] print("aList = " + str(aList))print("bList = " + str(bList)) print("Call listModifier")bList = listModifier(aList,bList) print("Back from listModifier")print("aList = " + str(aList)) print("bList = " + str(bList))

Questions & Answers

where we get a research paper on Nano chemistry....?
Maira Reply
nanopartical of organic/inorganic / physical chemistry , pdf / thesis / review
Ali
what are the products of Nano chemistry?
Maira Reply
There are lots of products of nano chemistry... Like nano coatings.....carbon fiber.. And lots of others..
learn
Even nanotechnology is pretty much all about chemistry... Its the chemistry on quantum or atomic level
learn
Google
da
no nanotechnology is also a part of physics and maths it requires angle formulas and some pressure regarding concepts
Bhagvanji
hey
Giriraj
Preparation and Applications of Nanomaterial for Drug Delivery
Hafiz Reply
revolt
da
Application of nanotechnology in medicine
what is variations in raman spectra for nanomaterials
Jyoti Reply
ya I also want to know the raman spectra
Bhagvanji
I only see partial conversation and what's the question here!
Crow Reply
what about nanotechnology for water purification
RAW Reply
please someone correct me if I'm wrong but I think one can use nanoparticles, specially silver nanoparticles for water treatment.
Damian
yes that's correct
Professor
I think
Professor
Nasa has use it in the 60's, copper as water purification in the moon travel.
Alexandre
nanocopper obvius
Alexandre
what is the stm
Brian Reply
is there industrial application of fullrenes. What is the method to prepare fullrene on large scale.?
Rafiq
industrial application...? mmm I think on the medical side as drug carrier, but you should go deeper on your research, I may be wrong
Damian
How we are making nano material?
LITNING Reply
what is a peer
LITNING Reply
What is meant by 'nano scale'?
LITNING Reply
What is STMs full form?
LITNING
scanning tunneling microscope
Sahil
how nano science is used for hydrophobicity
Santosh
Do u think that Graphene and Fullrene fiber can be used to make Air Plane body structure the lightest and strongest. Rafiq
Rafiq
what is differents between GO and RGO?
Mahi
what is simplest way to understand the applications of nano robots used to detect the cancer affected cell of human body.? How this robot is carried to required site of body cell.? what will be the carrier material and how can be detected that correct delivery of drug is done Rafiq
Rafiq
if virus is killing to make ARTIFICIAL DNA OF GRAPHENE FOR KILLED THE VIRUS .THIS IS OUR ASSUMPTION
Anam
analytical skills graphene is prepared to kill any type viruses .
Anam
Any one who tell me about Preparation and application of Nanomaterial for drug Delivery
Hafiz
what is Nano technology ?
Bob Reply
write examples of Nano molecule?
Bob
The nanotechnology is as new science, to scale nanometric
brayan
nanotechnology is the study, desing, synthesis, manipulation and application of materials and functional systems through control of matter at nanoscale
Damian
Is there any normative that regulates the use of silver nanoparticles?
Damian Reply
what king of growth are you checking .?
Renato
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
?
Kyle
yes I'm doing my masters in nanotechnology, we are being studying all these domains as well..
Adin
why?
Adin
what school?
Kyle
biomolecules are e building blocks of every organics and inorganic materials.
Joe
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
Good
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?

Ask