on goto
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From: Nathan on 6 May 2010 03:15 On May 6, 2:11 am, tonydee <tony_in_da...(a)yahoo.co.uk> wrote: > On May 6, 2:36 pm, Nathan <nathancba...(a)gmail.com> wrote: > > > > > On Apr 28, 1:16 am, Juha Nieminen <nos...(a)thanks.invalid> wrote: > > > Care to show an actual example of your "simpler, cleaner and easier to > > > follow" version of exiting a nested loop by meddling with the loop > > > conditions instead of using 'return'? For example, modify the following > > > code to conform to your specifications: > > > > Value_t* MyClass::findValue(const Value_t& value) > > > { > > > for(size_t xInd = 0; xInd < data.size(); ++xInd) > > > for(size_t yInd = 0; yInd < data[xInd].size(); ++yInd) > > > for(size_t zInd = 0; zInd < data[xInd][yInd].size(); ++zInd) > > > { > > > if(data[xInd][yInd][zInd] == value) > > > return &data[xInd][yInd][zInd]; > > > } > > > return 0; > > > } > > > Hi Juha, > > > I think that you either copied this from a poorly-written beginner C++ > > book or you failed to understand what the author was attempting to > > demonstrate with that kind of code and that you did not 'catch on' > > that it is not (in any way) demonstrative of how one walks a series of > > sequential cells in the real world. > > > Why do you insist on writing control structures that serve little > > actual purpose? For instance, it should be obvious that you only > > require ONE loop and a few conditionals to achieve your desired (or > > assumed) goal. E.G... > > > ,--- > > result = value > > xInd = 0 > > yInd = 0 > > zInd = 0 > > while ( data[xInd][yInd][zInd] != value && result != 0) > > { > > ++zInd; > > if (( zInd < data[xInd][yInd].size() ) != true ) { zInd = 0; + > > +yInd}; > > if (( yInd < data[xInd].size() ) != true ) { yInd = 0; ++xInd}; > > if (( xInd < data.size() ) != true ) { result = 0 };} > > > return result > > `--- > > Juha did ask for something "simpler, cleaner and easier to follow", > which your single loop is not. Well, 'y < x' must evaluate as 'true' in order for something to be classified as simpler. So, because *one loop* < *three loops*, my version certainly looks simpler. [e.g. less opcodes in the generated binary ] > Perhaps because of that, your code > needlessly compares yInd and xInd to their respective limits on every > iteration, rather than something like... > > while ( data[xInd][yInd][zInd] != value && result != 0) > { > if {++zInd == data[xInd][yInd].size()) { > zInd = 0; > if (++yInd == data[xInd].size()) { > yInd = 0; > if (++xInd == data.size()) > result = 0; > } > } > ... > > More importantly, it can dereference index [0] before checking size(), > so produces undefined behaviour. Anyway, IMHO it's far less clear > (=self-evidently correct & efficient as well as maintainable) than > Juha's code. > More maintainable by the original coder... or more maintainable by someone 'new' to the code??? If this function were many screens in length, and someone 'new' decides to have it perform an extra task 'just' before returning, wouldn't that person have a "devil of a time" debugging the program if he were not aware of the hidden alternative 'return' route tucked-away in that nest? > > Of course, this is a rather useless (maybe even retarded) function to > > begin with, because it only tells you IF the 'value' is located > > "somewhere" within that array -- it gives you absolutely no indication > > of "where" in that array you might be able to access the item which > > matches your search criteria. > > You missed that the original function was returning a pointer to the > matching cell, allowing a change to be made at that location, > considerably more useful than the search-term-else-0-sentinel version > you coded. > Ah, the '&' sign, I see it now. I should have "result = &data[xInd] [yInd][zInd]" in the third IF. > > Please pardon any butchering of C++ syntax in my pseudo snippet above > > -- I totally lack any C++ training. > > Fair enough... no worries. > > > But we alt.lang.asm folk *do* > > have an inkling of how to actually code our way out of a paper bag... > > I do believe. > > > Nathan. > > Your central point that a single loop can serve contributes an > interesting alternative, which I'm sure the readers here will > appreciate, Yes, rather than waste one's time bickering about the various esteemed implementation alternatives of a particular algo, why not consider an algo that renders those salient points mute? > if they bother to look past your smug attitude (which is > quite unwarranted given the serious errors in your implementation).... > This is Usenet, we do not charge for the entertainment value. :) Nathan.
From: Nathan on 6 May 2010 03:42 On May 6, 2:11 am, tonydee <tony_in_da...(a)yahoo.co.uk> wrote: > > Juha did ask for something "simpler, cleaner and easier to follow", > which your single loop is not. Perhaps because of that, your code > needlessly compares yInd and xInd to their respective limits on every > iteration, rather than something like... > > while ( data[xInd][yInd][zInd] != value && result != 0) > { > if {++zInd == data[xInd][yInd].size()) { > zInd = 0; > if (++yInd == data[xInd].size()) { > yInd = 0; > if (++xInd == data.size()) > result = 0; > } > } > ... > Isn't <something>.size() a method or function call? We'd also want to eliminate that needless activity from the loop by defining "zMax = data[xInd][yInd].size()", and etc., before the loop. Just because C++ gifts you with 'high-level' abstractions, that doesn't confer an automatic excuse to create inefficient, unmaintainable code.
From: Keith Thompson on 6 May 2010 03:50 Nathan <nathancbaker(a)gmail.com> writes: [...] > ,--- > result = value > xInd = 0 > yInd = 0 > zInd = 0 > while ( data[xInd][yInd][zInd] != value && result != 0) > { > ++zInd; > if (( zInd < data[xInd][yInd].size() ) != true ) { zInd = 0; + > +yInd}; > if (( yInd < data[xInd].size() ) != true ) { yInd = 0; ++xInd}; > if (( xInd < data.size() ) != true ) { result = 0 }; > } > return result > `--- [...] A style point: comparisons to true and false are almost always superfluous. Rather than if (( xInd < data.size() ) != true ) just write if ( xInd <= data.size() ) As for the overall structure of your proposed replacement, you've replaced three nested loops with one loop and three if statements. Furthermore, the original version only tests zInd on most iterations; your version tests zInd, yInd, and xInd on every iteration. The original problem is to traverse a 3-dimensional array. A triple nested loop is the most obvious way to do that. There might be some advantages in converting it to a single loop, but clarity isn't one of them, at least in this case. -- Keith Thompson (The_Other_Keith) kst-u(a)mib.org <http://www.ghoti.net/~kst> Nokia "We must do something. This is something. Therefore, we must do this." -- Antony Jay and Jonathan Lynn, "Yes Minister"
From: tonydee on 6 May 2010 04:22 On May 6, 4:42 pm, Nathan <nathancba...(a)gmail.com> wrote: > On May 6, 2:11 am, tonydee <tony_in_da...(a)yahoo.co.uk> wrote: > > while ( data[xInd][yInd][zInd] != value && result != 0) > > { > > if {++zInd == data[xInd][yInd].size()) { > > zInd = 0; > > if (++yInd == data[xInd].size()) { > > yInd = 0; > > if (++xInd == data.size()) > > result = 0; > > } > > } > > ... > > Isn't <something>.size() a method or function call? Yes. > We'd also want to > eliminate that needless activity from the loop by defining "zMax = > data[xInd][yInd].size()", and etc., before the loop. Possibly - some people do exactly that. Still, using trivial inline member function calls to access data member is such an important part of encapsulation in OO programming that all modern C++ optimisers definitely produce equally efficient code despite the repeated calls to std::vector<>::size(). That's true for most containers, but not all. The concern has more validity if the container type is unknown (e.g. a template parameter), or there's concern it might change during maintenance. Counter-balancing that is the simple, idiomatic usage and concision as presented. Delocalising the arguments in the comparisons by putting the values in local variables on other lines, which by default remains in scope much longer than you're likely to need it (possibly with a value that is invalidated by subsequent code), tends to have a net detrimental effect on code clarity and correctness, and for no performance benefit in this case. > Just because C++ gifts you with 'high-level' abstractions, that > doesn't confer an automatic excuse to create inefficient, > unmaintainable code. A good C++ programmer understands what the optimiser can be expected to do, and creates clean, concise code when possible, and more explicitly assists the compiler/optimiser when actually beneficial. Cheers, Tony
From: Nathan Baker on 6 May 2010 04:51
"Keith Thompson" <kst-u(a)mib.org> wrote in message news:ln4oilxyd9.fsf(a)nuthaus.mib.org... > > A style point: comparisons to true and false are almost always > superfluous. Rather than > if (( xInd < data.size() ) != true ) > just write > if ( xInd <= data.size() ) > A true point. But I guess, for comparison sake, it is superfluous. :) > The original problem is to traverse a 3-dimensional array. A triple > nested loop is the most obvious way to do that. There might be > some advantages in converting it to a single loop, but clarity > isn't one of them, at least in this case. > I guess, the dragon that I am pointing at, is that CircuitCity does not yet sell 3-dinensional RAM. So, why not just do... ,--- xyzMax = data[xInd][yInd].size() + data[xInd].size() + data.size(); result = 0; i = -1; while ( i < xyzMax ) { ++i; if ( data[i] = value ); { result = &data[i]; i = xyzMax; } } return result; `--- ....a linear solution to a linear problem??? Nathan. |