Replace and ReplaceAll -- simple application
in [Mathematica]
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From: danl on 30 Dec 2009 04:15 > In article <hh9vfo$1rk$1(a)smc.vnet.net>, > Richard Fateman <fateman(a)cs.berkeley.edu> wrote: > >> Scot T. Martin wrote: >> > The problem is that "-1." is not "1". > >> And furthermore, this bug has been declared a feature. And defended, >> repeatedly. Just as I does not occur in -I. >> It can be fixed, should be fixed, and has been fixed in other CAS. > > > I'll defer to Richard Fateman (and/or other computer algebra experts) as > to whether this general behavior (!! especially with respect to -I !!) > is a bug that should be fixed, or a feature. > > But if it's gonna be considered a feature in Mathematica, I'd strongly > suggest that it should be strongly **featured** �� or, if you like, it > should be warned about!! �� in the Mathematica documentation, especially > at the more elementary levels of documentation, **so that ordinary users > can't miss being warned about this unexpected behavior** ^HH^H^H^H > sorry, so that they can't miss being warned about "this feature". > > For example, right after the first line of the Help for I, which says > > I represents the imaginary unit Sqrt[-1]. > > there might be a warning line which says > > WARNING: I is a number and not a symbol. Attempting to replace > or reverse the sign of I using a rule, such as expr/.{I->-I) may > produce unexpected results if the expr contains -I. In the Documentation Center search for I. Under Possible Issues one sees "Complex numbers are atomic objects and do not explicitly contain I:", along with FullForm representations that show this. I would not mind if an example along the lines you suggest, e.g. "3+4*I/.I->-I evaluates to 3+4*I" were added. Would having such an example there have helped you? > (By the way, note the wording above: "I represents", not "I is".) > > There are a very large number of other places in the Mathematica > documentation where emphasized WARNINGs like this would be helpful. It is difficult to anticipate all manner of misunderstandings of functionality. It is also the case that Mathematica is a complex language, hence has room for many such misunderstandings. I run into my share. The documentation mainly states what various functions do, not what they do not do. The misunderstanding you allude to above is fairly common, and perhaps deserves more attention in the ReplaceAll et al pages. I would not ascribe that level of importance to all such issues you have raised in this forum. > (I suppose there could even be optional warning messages that appear any > time a cell executes a rule containing a number on the LHS of a > ReplaceAll symbol. And, the documentation for ReplaceAll could have an > initial WARNING message that it is not just the kind of Global Search > and Replace command that many ordinary users, familiar with many other > word processing and software apps, might well think it is. ) I think the documetation is clear that it is a syntactic replacement and not a string replacement. Under "More Informnation: it states "ReplaceAll looks at each part of expr, tries all the rules on it, and then..." Having configurable warning messages of the sort you describe is on the wish list of many people. I think it is fair to call this a design weakness in the Mathematica language, that it is not easy to make such a message system. I do not think Mathematica is exceptional in this regard: the current message system is perhaps more advanced than one finds in other software of serious complexity. Daniel Lichtblau Wolfram Research
From: Andrzej Kozlowski on 30 Dec 2009 04:24 What I find kind of impressive is that there are people who find it amusing to keep posting essentially the same posts for about two decades and this despite the fact that they are being completely ignored by the developers (and there is no reason to think that anything will ever change in this respect). Masochism? I try to move with the times so what concerns me is that my Front End is crashing too often. Andrzej Kozlowski On 29 Dec 2009, at 15:18, DrMajorBob wrote: > True enough, I'd say. > > Bobby > > On Mon, 28 Dec 2009 03:57:38 -0600, Richard Fateman > <fateman(a)cs.berkeley.edu> wrote: > >> Scot T. Martin wrote: >>> The problem is that "-1." is not "1". >> >> And furthermore, this bug has been declared a feature. And defended, >> repeatedly. Just as I does not occur in -I. >> It can be fixed, should be fixed, and has been fixed in other CAS. >> > > > -- > DrMajorBob(a)yahoo.com >
From: DrMajorBob on 31 Dec 2009 03:12 OK... atomic object, then. I doubt whether any such list is complete in the documentation. Bobby On Wed, 30 Dec 2009 03:11:37 -0600, AES <siegman(a)stanford.edu> wrote: > In article <hhc79g$2np$1(a)smc.vnet.net>, > DrMajorBob <btreat1(a)austin.rr.com> wrote: > >> -1. is a Real, hence an Atom, so it has no subordinate parts. >> >> Bobby >> > > Thanks -- except, after digging out and reading the Basic Objects > tutorial (which was useful), I think you mean "hence an atomic object"? > > Is "Atom" a defined term in Mathematica? The tutorial doesn't seem to > use it. > > [And as an aside, is the shaded table of atomic objects in this tutorial > supposed to be a *complete* list of *all* the atomic objects in > Mathematica? I'm never clear whether these shaded lists are supposed to > contain all, or just some, of the objects they illustrate.]. > -- DrMajorBob(a)yahoo.com
From: AES on 31 Dec 2009 03:13 In article <hhf5kg$go6$1(a)smc.vnet.net>, Murray Eisenberg <murray(a)math.umass.edu> wrote: [Re documentation issues and I->-I] > Only gathering usage statistics, or having a focus group of users trying > stuff, might suffice to escalate some issues to the point of requiring > more prominent warnings. 1) Fully agree. My understanding is that many software vendors (and hardware equipment vendors, for that matter), at least the larger ones, do exactly this, systematically and extensively, on their products, and especially the interfaces to their products. I have no idea whether Wolfram does any of this or not. 2) On this point let's note that, to many users, the _interface_ to Mathematica -- what the user has to (learn to) type in, to get useful results out -- is the most important (and sometimes frustrating?) part of the product. What Mathematica does or can do -- it's "capabilities" as contrasted to its interface -- is of course also of primary importance; and Mathematica seems to rank very highly on this criterion. It's the user interface where many if not most of these problems arise. 3) And let's note the explicit assertions by Conrad Wolfram (in the screencasts/video gallery on the Wolfram web site), and by others, that Mathematica is intended to be a program that does *all* tasks, for *all* users, in a *single* application (with 'all' and 'single' taken very broadly). This means, necessarily: a) A *very* complex interface (with, in particular, a _huge_ vocabulary). b) And at the same time, a very broad and diverse set of users, with very different levels of education and knowledge and experience. And this may mean that this basic goal and approach of the Wolframs' for Mathematica may not be realistic or possible. The "focus groups" you suggest will have to be very diverse in makeup, corresponding to the huge diversity of the proposed users; and each different group of users will have different interface (and documentation) needs, and want very different things. If the Wolframs' are going to insist on following this path, then user documentation -- easily accessible, brilliantly designed documentation, readily available in different forms oriented to the needs of different users -- is the primary thing they have to focus on. Thus far, so far as I can see, Heikki Ruskeepaa may be the only person on the planet who recognizes this and does something about it. Mathematica's own documentation gets maybe a C- on this score. And simply expecting ordinary users to learn ever more arcane CAS concepts and terminology in order to use Mathematica effectively seems as unrealistic as it is absurd.
From: Richard Fateman on 31 Dec 2009 03:13
Andrzej Kozlowski wrote: > What I find kind of impressive is that there are people who find it > amusing to keep posting essentially the same posts for about two decades > and this despite the fact that they are being completely ignored by the > developers (and there is no reason to think that anything will ever > change in this respect). Masochism? > No, I think it is not masochism. It is an attempt to provide a useful to answer questions that come up from time to time from people who would otherwise dismiss a CAS as useless and stupid bedause they find unexplainable (to them) behavior. Some posts explains that a CAS really could do the mathematically obviously correct thing, even if it appears they do not. They may point out that a group of fans insists that the right thing for users to do is to discard their mathematically obvious understanding and study programming. Now sometimes this cuts the other way -- that is, what is "mathematically obvious" to some novice really can't be the default in a CAS because it contradicts what is "mathematically obvious" to most experts (or even some other novices). This is when mathematical ambiguities show up. But when the weight of all mathematical obviousness is on one side, and the developers could fix a bug but simply refuse to do so, then that is simply stubbornness. > I try to move with the times so what concerns me is that my Front End is > crashing too often. Maybe you should tie your shoelaces? > RJf |