Talk:Idempotence

message: > Elevator call buttons are idempotent, though many people think they are not.
That cracked me up!
::who actually believes this daesotho 20:42, 21 Oct 2004 (UTC)
replies: text:  What are you talking about? Dysprosia 23:32, 21 Oct 2004 (UTC)

text: Obviously an old entry long ago deleted into history. Meaning that pressing the 3rd floor button several times does not cause a change to the elevator's destination. פשוט pashute ♫ (talk) 07:47, 2 August 2016 (UTC)
thread_title:  Humorous Example 

message: The article starts by claiming that there are two meanings, but clearly they are the same.  The "unary operation" definition is the same because the operation involved is function composition which is a binary operation.  If @ denotes function composition, then an idempotent function f is one satisfying f@f=f.  This is in fact suggested in the parenthetical comment "(or for a function, composed with)" near the start, but then the article continues as if functions are special. They aren't. McKay 10:52, 29 June 2006 (UTC)
thread_title:  There is only one meaning given here 

message: Primitive idempotents are important in quantum mechanics as they
are the pure states in density matrix or density operator theory.
The pure states can be reprsented by spinors, for example if |a>
is a spinor, then |a><a| is a primitive idempotent.  See the
"Density Matrix Formalism" portion of Frank Porter's quantum
mechanics class notes (Cal Tech):
http://www.cithep.caltech.edu/~fcp/physics/quantumMechanics/
Can we add
this to the discussion?  What other examples of idempotents are
important in physics and mathematics?  If this is something that
should be included, let me know or do it yourself.
CarlAB 02:57, 7 October 2006 (UTC)
thread_title:  Primitive Idempotents 

message: I don't agree with this sentence: If f is a unary operation, i.e. a map f from some set X into itself... An unary operation doesn't have to be from one set to itself. Even if we are talking about idempotence. For example, if X \subseteq Y, function f: X \rightarrow Y
might be idempotent. If I'm right, then the next few sentences would need corrections as well. -- Obradović Goran  (talk 20:22, 30 April 2007 (UTC)
thread_title:  Unary operation 

message: The first paragraph has embeded parens, and one is not closed. It is not clear how to fix this error.
: Fixed, I introduced this error when merging. Angus Lepper(T, C, D) 23:45, 9 June 2007 (UTC)
thread_title:  Typo 

message: But the probability distributions associated with eigenstates are idempotents.
How so?  The probability distribution is simply constant in time.  It's not equal to its square its composition with itself.  Sure, an idempotent function relates the distribution at one time to that at another, but that's because it's the identity function.  This doesn't belong in an article about idempotence, any more than anything else that's constant.  —Preceding unsigned comment added by 151.200.247.124 (talk) 01:36, 19 October 2007 (UTC)
replies: text: Since nobody has defended or changed this section, I've removed it. 72.75.97.3 (talk) 15:13, 8 December 2007 (UTC)
thread_title:  Physics 

message: I have heard a number of people use idempotence to describe web pages etc. A web request is idempotent if the same URL returns exactly the same page. POST actions are, by definition, not idempotent, but GET requests may be. So a Wikipedia page is idempotent only if nobody edits it. Many web pages fail to be idempotent because they include things like the current date and time. Thoughts? GhostInTheMachine (talk) 09:10, 30 April 2008 (UTC)
replies: text: Hmm. One way "idempotence" is defined in HTTP terms related to the effect on the server. The fact that you get pages with different times doesn't count. POST could return the same page, but order, as noted in the article, more than one car. Not being an expert, I believe that it should be in terms of server side effects, not client side effects. I first knew about it in terms of NFS, where reads or writes from/to a file are idempotent. Each includes the file offset. A file append operation is not idempotent, and NFS doesn't supply one.  (Networks can duplicate packets, so this is important.)  Also, it seem that POST could be idempotent, but is often used for cases that aren't. Gah4 (talk) 21:19, 14 October 2016 (UTC)
thread_title:  Add use for web 

message: I changed the classification :Category:Theoretical computer science to :Category:Computer science, since the meanings in computing described here, such as relating to databases and ESP, do in my opinion not belong to the realm of theoretical computer science. However, this change was reverted. What do others think, is this TCS or just CS?  --Lambiam 14:22, 7 August 2008 (UTC)
thread_title:  Theoretical computer science? 

message: Sometimes a unary operation is called idempotent if, whenever it is applied twice to any value, it gives the original value. For example, the complex conjugation of a number is considered an idempotent operation.
I've never heard of this. Could someone please point me to some references? (Because I haven't found any.) I think this is confusing (if not bogus), and should be removed. --Matt Kovacs (talk) 17:07, 24 January 2009 (UTC)
replies: text: In the context of operator algebras within functional analysis one finds the expanded usage of idempotence.Rgdboer (talk) 00:40, 25 January 2009 (UTC)
replies: text: Ah, okay, thanks. --Matt Kovacs (talk) 01:08, 25 January 2009 (UTC)
thread_title:  "Alternative definition", huh? 

message: The External Link to SSW is broken ...  —Preceding unsigned comment added by 77.12.219.245 (talk) 21:19, 16 January 2010 (UTC)
thread_title:  Broken Link 

message: Dunno if it's just me, but this reads as an endorsement for "Cfengine", and is at best, off-topic
It was inserted here: http://en.wikipedia.org/w/index.php?title=Idempotence&diff=prev&oldid=342290536  —Preceding unsigned comment added by Jonschreiber (talk • contribs) 03:27, 11 May 2010 (UTC)
thread_title:  Advertising Paragraph 

message: This part of the computer science definition is not correct:
In computer science, the term idempotent is used to describe methods or subroutine calls that can safely be called multiple times, as invoking the procedure a single time or multiple times has the same result; i.e., after any number of method calls all variables have the same value as they did after the first call. Any method or subroutine that has no side effects is also idempotent.
According to this description, the following function would be idempotent, since it has no side effects:
def f(x): return x+1
But it's not.  It's a pure function, or side effects-free if you prefer, but certainly not idempotent.  f . f = f must hold in computing too, for a given function or method to be called idempotent, and in the case above f(f(x)) != f(x). —Preceding unsigned comment added by Gniemeyer (talk • contribs) 21:01, 31 August 2010 (UTC)
replies: text: I don't think it's completely wrong, but the present explanation in the article is definitely unclear. In computer programming, a piece of code usually is reentrant if running it multiple times in sequence has the same effect as running it just once. This is the case for your function.  The reason reentrant is sometimes called idempotent is that in imperative programming, the effect of a piece of code is thought of as what modifications it causes to be made to the program state, and that effect can be modeled as a function on the space of potential program states - reentrance then is idempotence of that function. Rp (talk) 10:04, 14 September 2012 (UTC)
replies: text: I see nothing wrong or unclear, except that it is perhaps not clear that the procedure is not intended to be interpreted as a function (i.e. acting on its own output). An idempotent procedure is one that, when called more than once in sequence, has no additional side effects. This has nothing to do with reentrancy, which refers to a procedure that may be halted at any (interruptable) point in its execution, another call to it executed (e.g. in an interrupt), and the first then completed with the same effect as if the calls had executed sequentially. — Quondum 14:46, 14 September 2012 (UTC)
replies: text: Well, it is often implied that a reentrant procedure is idempotent, but it certainly isn't a requirement, so what I wrote is wrong - thanks for the correction.  I maintain that it would be useful to explain how imperative code is interpreted as a function when it's called idempotent. Rp (talk) 09:38, 17 September 2012 (UTC)
replies: text: Seems to me that this is mixing the mathematical and CS definitions.  And since CS uses math, that could easily happen. I first knew idempotence from descriptions of NFS, where it is important when using UDP for the transport protocol. Networks can duplicate packets.  I suspect it is not unusual for math and CS to use the same term with different meanings, such that you have to say which one you are using.  Also, for NFS, the server could crash while doing an operation. The client will then repeat the request, without knowing if the previous request was satisfied. There is nothing related to f . f = f in that case. Gah4 (talk) 21:27, 14 October 2016 (UTC)
thread_title:  Definition for computer science is wrong 

message: Both terms are used on this page, as nouns. Is one preferable for Wikipedia? Do the terms differ in connotation (or denotation)? What's the relation between them? A footnote is given for Benjamin Pierce having introduced the concept of "idempotence", but did he actually use that term or just "idempotent"?
148.87.23.9 (talk) 19:07, 5 March 2019 (UTC)
thread_title:  Idempotence or Idempotency? 

message: The photo of the Polish calculator seems rather odd. Since the fact it's Polish is irrelevant, perhaps an English one would fit better in an article in the English Wikipedia. At first sight I thought it had something to do with Reverse Polish Notation but, of course, that isn't the case. Also, it isn't clear why the ON button is singled out in the caption as an example of idempotence, when the OFF button also has the characteristic. So, taken together it seems to be a rather weak example. The pedestrian crossing example seems much better. 87.75.117.183 (talk) 02:06, 23 September 2019 (UTC)
replies: text: When I tried to add a photo some time ago, the desk calculator was the least odd one I could find at commons. If you have a better one at hand, please replace the current one by yours. - Jochen Burghardt (talk) 08:38, 23 September 2019 (UTC)
replies: text: Either way, please do not single out one button. Both are idempotent. At least change the text accordingly. Tessus (talk) 08:34, 13 January 2020 (UTC)
thread_title:  Polish calculator 

message: Other admins should feel free to adjust or undo the protection without asking me. Tom Harrison Talk 14:40, 19 May 2022 (UTC)
replies: text: As for the trigger of the protection, note that the walk button example is present in section Idempotence#Applied_examples, even with a source. Despite 49.184.196.30's wordy edit comments, it is my impression that s/he just wanted to make a constructive edit, not to vandalize. However, I'd prefer to omit the inserted sentence, not because it is WP:OR, but because we already have the everyday example in the image, and the aforesaid section. - Jochen Burghardt (talk) 13:13, 20 May 2022 (UTC)
thread_title:  Page protected