Re: Mathematica daily WTF

*To*: mathgroup at smc.vnet.net*Subject*: [mg115352] Re: Mathematica daily WTF*From*: DrMajorBob <btreat1 at austin.rr.com>*Date*: Sat, 8 Jan 2011 03:40:29 -0500 (EST)*References*: <201101070911.EAA14093@smc.vnet.net>*Reply-to*: drmajorbob at yahoo.com

I never use echo on anything but symbols, so the problem doesn't come up. But you're right in general, of course. Bobby On Fri, 07 Jan 2011 04:51:12 -0600, Leonid Shifrin <lshifr at gmail.com> wrote: > Bobby, > > the problem with this approach is that if <s> contains side effects > (think > i++, for instance), then > echo[] will introduce one additional evaluation of <s>, in addition to > the > actual place where <s> > is in code. This may break things. Ingolf's version is free from this > problem, because he uses > pure function which evaluates the argument just once at the > parameter-passing stage. There was > a discussion on a similar topic some while ago, where I posted a solution > similar to both yours > and Ingolf's (modulo debug mode), which is not ideal either but free from > this problem: > > http://groups.google.com/group/comp.soft-sys.math.mathematica/browse_thread/thread/bfd67e9122b1fdec > > My function will fail in cases like the following: > > Clear[a]; > ShowIt[a] = 2 > > (which are presumably pathological uses anyway), and generally when used > to > display held arguments. > There are workarounds for my version for such cases but they are more > complex. The debug mode > can be easily added to it as well. > > Regards, > Leonid > > > > On Fri, Jan 7, 2011 at 12:11 PM, DrMajorBob <btreat1 at austin.rr.com> > wrote: > >> I'd try something like: >> >> Clear[echo] >> Attributes[echo] = {HoldAll}; >> echo[s_, place_] /; debug := >> Print[ToString[Unevaluated[s]], " = ", s, " at ", place] >> >> With a module like this: >> >> debug = True; >> bookSave[num_]:=Module[{book,dir}, >> >> (* some code *) >> echo[dir, 1]; >> echo[book, 2]; >> echo[num, 3]; >> >> (* other code *) >> >> ] >> >> The printed output was: >> >> dir$3287 = /Users/keleen/TS/Wings/ at 1 >> >> book$3287 = Book1 at 2 >> >> 1 = 1 at 3 >> >> It's annoying that "num" prints as its value, rather than its name. >> >> Unevaluated is still a mystery. >> >> Bobby >> >> On Thu, 06 Jan 2011 01:01:37 -0600, Ingolf Dahl <ingolf.dahl at telia.com> >> wrote: >> >> > Andrzej, >> > Since you ask for "single examples", I would like to hear your opinion >> > about >> > my "Debugging code snippet" >> > >> > ((If[debug, Print["Place1 ", #]]; #) &)@ >> > >> > I have started with the functional identity operator # &@, and then >> added >> > the global variable debug, which if set to True prints out the value >> of >> > the >> > variable as side effect. The code snippet can be used to monitor the >> > values >> > of sub expressions during debugging, without disturbing the flow of >> the >> > program, and it does not necessarily need to be removed afterwards. >> > Best regards >> > >> > Ingolf Dahl >> > ingolf.dahl at telia.com >> > >> > This code snippet can be inserted into d >> >> -----Original Message----- >> >> From: Andrzej Kozlowski [mailto:akoz at mimuw.edu.pl] >> >> Sent: den 3 januari 2011 09:58 >> >> To: mathgroup at smc.vnet.net >> >> Subject: [mg115174] Re: Mathematica daily WTF >> >> >> >> I don't want to get involved in what is likely to turn out a >> >> "linguistic" >> > dispute, but I think >> >> your ideas about what constitutes "functional" and "procedural" are >> > misconceived and do >> >> not correspond to what other's mean by these terms. While there is >> some >> > difference between >> >> the meaning of "function" in mathematics and in programming, both >> >> concepts >> > originate from >> >> the same source. To quote Thompson's "Haskel. The craft of functional >> > programming": "A >> >> function is something that we can picture as a box with some inputs >> and >> >> an >> > output:..." >> >> followed by a picture which is exactly the same that I used to draw >> in >> >> my >> > lectures on >> >> introductory set theory and analysis for many years before I heard of >> > functional >> >> programming. In this sense functions are ubiquitous in mathematics >> and >> > science. "Procedural >> >> programming", on the other hand, is programming by "change of state" >> or >> > "side-effects", and >> >> as the latter expression suggests, is less natural for the human mind >> >> even >> > if it could be >> >> claimed to c! >> >> orrespond more closely to what goes on at "machine level". In any >> case, >> >> I >> > cannot think of >> >> any scientific or mathematical problems that can be more naturally >> > formulated in terms of >> >> "side-effects" than in terms of "functions". Perhaps they exist and >> I my >> > bias is due to several >> >> decades of doing mathematics but I seriously can't think of a single >> > example. Can you >> >> provide one? >> >> >> >> Andrzej Kozlowski >> >> >> >> >> >> On 2 Jan 2011, at 10:55, AES wrote: >> >> >> >> > In article <ifmrvv$pim$1 at smc.vnet.net>, >> >> > Andrzej Kozlowski <akoz at mimuw.edu.pl> wrote: >> >> > >> >> >> >> >> >> anyway, it does not matter as far as the point I was making is >> > concerned, >> >> >> which is that the C-like structure of Mathematica procedural >> programs >> > is >> >> >> helpful to people (somewhat) familiar with C or Fortran. >> >> >> >> >> > >> >> > I'd argue it is also extremely helpful to people who _think_ >> >> physically, >> >> > or if you like procedurally, and who are primarily focused on >> solving >> >> > problems that have an inherently procedural character. >> >> > >> >> > The successive steps (lines, cells, expressions) in a procedural >> >> program >> >> > will very often state or mimic or reproduce what happens as a >> function >> >> > of time in a dynamic system, or as a function of distance as a wave >> >> > propagates, or mimic the flow of control in a complex system, or . >> . . >> >> > >> >> > As such, they simplify the process of _coding_ these programs; they >> >> > _document_ and make readable what the program is doing, step by >> step; >> >> > they make it easy to _insert later refinements_ inside the >> procedure >> >> > (e.g., tests for current values or for exceptional cases at points >> >> > within the procedure). >> >> > >> >> > All of these things are much more valuable to some of us in our >> use of >> >> > Mathematica than the speed at which the code executes, or the >> brevity >> >> > with which it can be typed. And none of this is to argue that many >> >> > basic functions within the language (things like Fourier >> transforms, >> >> > finding matrix eigensolutions, many others) should not be provided >> and >> >> > used as pre-coded non-procedural routines within larger programs. >> >> > >> >> > I make a lot of use of self-programmed Modules[] in my own >> >> programming. >> >> > The active or working part of the completed program, where >> numerical >> >> > results get asked fror and results displayed, can be quite briefly >> >> > written, mostly just setting input variables, then calling these >> >> > modules. But these modules themselves are heavily procedurally >> coded >> >> > internally, and I think that makes a lot of sense. >> >> > >> > >> > >> >> >> -- >> DrMajorBob at yahoo.com >> >> -- DrMajorBob at yahoo.com

**References**:**Re: Mathematica daily WTF***From:*DrMajorBob <btreat1@austin.rr.com>