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• lexical_cast int to string

  - by idimba

  Is it safe to ignore exception of boost::lexical_cast when converting int to std::string?

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• Unable to run OpenMPI across more than two machines

  - by rcollyer

  When attempting to run the first example in the boost::mpi tutorial, I was unable to run across more than two machines. Specifically, this seemed to run fine: mpirun -hostfile hostnames -np 4 boost1 with each hostname in hostnames as <node_name> slots=2 max_slots=2. But, when I increase the number of processes to 5, it just hangs. I have decreased the number of slots/max_slots to 1 with the same result when I exceed 2 machines. On the nodes, this shows up in the job list: <user> Ss orted --daemonize -mca ess env -mca orte_ess_jobid 388497408 \ -mca orte_ess_vpid 2 -mca orte_ess_num_procs 3 -hnp-uri \ 388497408.0;tcp://<node_ip>:48823 Additionally, when I kill it, I get this message: node2- daemon did not report back when launched node3- daemon did not report back when launched The cluster is set up with the mpi and boost libs accessible on an NFS mounted drive. Am I running into a deadlock with NFS? Or, is something else going on?

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• Cross platform unicode path handling

  - by Matt Joiner

  I'm using boost::filesystem for cross-platform path manipulation, but this breaks down when calls need to be made down into interfaces I don't control that won't accept UTF-8. For example when using the Windows API, I need to convert to UTF-16, and then call the wide-string version of whatever function I was about to call, and then convert any output back to UTF-8. While the wpath, and other w* forms of many of the boost::filesystem functions help keep sanity, are there any suggestions for how best to handle this conversion to wide-string forms where needed, while maintaining consistency in my own code?

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• Remove from a std::set<shared_ptr<T>> by T*

  - by Autopulated

  I have a set of shared pointers: std::set<boost::shared_ptr<T>> set; And a pointer: T* p; I would like to efficiently remove the element of set equal to p, but I can't do this with any of the members of set, or any of the standard algorithms, since T* is a completely different type to boost::shared_ptr<T>. A few approaches I can think of are: somehow constructing a new shared_ptr from the pointer that won't take ownership of the pointed to memory (ideal solution, but I can't see how to do this) wrapping / re-implementing shared_ptr so that I can do the above just doing my own binary search over the set Help!

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• C++: Best text accumulator

  - by MInner

  Text gets accumulates piecemeal before being sent to client. Now we use own class that allocates memory for each piece as char massive. (Anyway, works like char[][] + std::list<char*>). Then we build the whole string, convert it into std::sting and then create boost::asio::streambuf using it. That's slow enough, I assume. Correct me if I'm wrong. I know, in many cases simple FILE type from stdio.h is used. How does it works? Allocates memory at every write into it. So, is it faster and is there any way to read into boost::asio::streambuf from FILE?

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• List of functions references

  - by Ockonal

  Hello, I'm using boost::function for making references to the functions. Can I make a list of references? For example: boost::function<bool (Entity &handle)> behaviorRef; And I need in a list of such pointers. For example: std::vector<behaviorRef> listPointers; Of course it's wrong code due to behaviorRef isn't a type. So the question is: how can I store a list of pointers for the function?

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• LLVM Clang 5.0 explicit in copy-initialization error

  - by kevzettler

  I'm trying to compile an open source project on OSX that has only been tested on Linux. $: g++ -v Configured with: --prefix=/Applications/Xcode.app/Contents/Developer/usr --with-gxx-include-dir=/usr/include/c++/4.2.1 Apple LLVM version 5.0 (clang-500.2.79) (based on LLVM 3.3svn) Target: x86_64-apple-da I'm trying to compile with the following command line options g++ -MMD -Wall -std=c++0x -stdlib=libc++ -Wno-sign-compare -Wno-unused-variable -ftemplate-depth=1024 -I /usr/local/Cellar/boost/1.55.0/include/boost/ -g -O3 -c level.cpp -o obj-opt/level.o I am seeing several errors that look like this: ./square.h:39:70: error: chosen constructor is explicit in copy-initialization int strength = 0, double flamability = 0, map<SquareType, int> constructions = {}, bool ticking = false); The project states the following are requirements for the Linux setup. How can I confirm I'm making that? gcc-4.8.2 git libboost 1.5+ with libboost-serialize libsfml-dev 2+ (Ubuntu ppa that contains libsfml 2: ) freeglut-dev libglew-dev

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• Loosely coupled implicit conversion

  - by ltjax

  Implicit conversion can be really useful when types are semantically equivalent. For example, imagine two libraries that implement a type identically, but in different namespaces. Or just a type that is mostly identical, except for some semantic-sugar here and there. Now you cannot pass one type into a function (in one of those libraries) that was designed to use the other, unless that function is a template. If it's not, you have to somehow convert one type into the other. This should be trivial (or otherwise the types are not so identical after-all!) but calling the conversion explicitly bloats your code with mostly meaningless function-calls. While such conversion functions might actually copy some values around, they essentially do nothing from a high-level "programmers" point-of-view. Implicit conversion constructors and operators could obviously help, but they introduce coupling, so that one of those types has to know about the other. Usually, at least when dealing with libraries, that is not the case, because the presence of one of those types makes the other one redundant. Also, you cannot always change libraries. Now I see two options on how to make implicit conversion work in user-code: The first would be to provide a proxy-type, that implements conversion-operators and conversion-constructors (and assignments) for all the involved types, and always use that. The second requires a minimal change to the libraries, but allows great flexibility: Add a conversion-constructor for each involved type that can be externally optionally enabled. For example, for a type A add a constructor: template <class T> A( const T& src, typename boost::enable_if<conversion_enabled<T,A>>::type* ignore=0 ) { *this = convert(src); } and a template template <class X, class Y> struct conversion_enabled : public boost::mpl::false_ {}; that disables the implicit conversion by default. Then to enable conversion between two types, specialize the template: template <> struct conversion_enabled<OtherA, A> : public boost::mpl::true_ {}; and implement a convert function that can be found through ADL. I would personally prefer to use the second variant, unless there are strong arguments against it. Now to the actual question(s): What's the preferred way to associate types for implicit conversion? Are my suggestions good ideas? Are there any downsides to either approach? Is allowing conversions like that dangerous? Should library implementers in-general supply the second method when it's likely that their type will be replicated in software they are most likely beeing used with (I'm thinking of 3d-rendering middle-ware here, where most of those packages implement a 3D vector).

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• How would I use for_each to delete every value in an STL map?

  - by stusmith

  Suppose I have a STL map where the values are pointers, and I want to delete them all. How would I represent the following code, but making use of std::for_each? I'm happy for solutions to use Boost. for( stdext::hash_map<int, Foo *>::iterator ir = myMap.begin(); ir != myMap.end(); ++ir ) { delete ir->second; // delete all the (Foo *) values. } (I've found Boost's checked_delete, but I'm not sure how to apply that to the pair<int, Foo *> that the iterator represents). (Also, for the purposes of this question, ignore the fact that storing raw pointers that need deleting in an STL container isn't very sensible).

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• A function where small changes in input always result in large changes in output

  - by snowlord

  I would like an algorithm for a function that takes n integers and returns one integer. For small changes in the input, the resulting integer should vary greatly. Even though I've taken a number of courses in math, I have not used that knowledge very much and now I need some help... An important property of this function should be that if it is used with coordinate pairs as input and the result is plotted (as a grayscale value for example) on an image, any repeating patterns should only be visible if the image is very big. I have experimented with various algorithms for pseudo-random numbers with little success and finally it struck me that md5 almost meets my criteria, except that it is not for numbers (at least not from what I know). That resulted in something like this Python prototype (for n = 2, it could easily be changed to take a list of integers of course): import hashlib def uniqnum(x, y): return int(hashlib.md5(str(x) + ',' + str(y)).hexdigest()[-6:], 16) But obviously it feels wrong to go over strings when both input and output are integers. What would be a good replacement for this implementation (in pseudo-code, python, or whatever language)?

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• How can I randomly iterate through a large Range?

  - by void

  I would like to randomly iterate through a range. Each value will be visited only once and all values will eventually be visited. For example: class Array def shuffle ret = dup j = length i = 0 while j > 1 r = i + rand(j) ret[i], ret[r] = ret[r], ret[i] i += 1 j -= 1 end ret end end (0..9).to_a.shuffle.each{|x| f(x)} where f(x) is some function that operates on each value. A Fisher-Yates shuffle is used to efficiently provide random ordering. My problem is that shuffle needs to operate on an array, which is not cool because I am working with astronomically large numbers. Ruby will quickly consume a large amount of RAM trying to create a monstrous array. Imagine replacing (0..9) with (0..99**99). This is also why the following code will not work: tried = {} # store previous attempts bigint = 99**99 bigint.times { x = rand(bigint) redo if tried[x] tried[x] = true f(x) # some function } This code is very naive and quickly runs out of memory as tried obtains more entries. What sort of algorithm can accomplish what I am trying to do? [Edit1]: Why do I want to do this? I'm trying to exhaust the search space of a hash algorithm for a N-length input string looking for partial collisions. Each number I generate is equivalent to a unique input string, entropy and all. Basically, I'm "counting" using a custom alphabet. [Edit2]: This means that f(x) in the above examples is a method that generates a hash and compares it to a constant, target hash for partial collisions. I do not need to store the value of x after I call f(x) so memory should remain constant over time. [Edit3/4/5/6]: Further clarification/fixes.

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• Best way to store list of numbers and to retrieve them

  - by bingoNumbers

  Hi. What is the best way to store a list of random numbers (like lotto/bingo numbers) and retrieve them? I'd like to store on a Database a number of rows, where each row contains 5-10 numbers ranging from 0 to 90. I will store a big number of those rows. What I'd like to be able is to retrieve the rows that have at least X number in common to a newly generated row. Example: [3,4,33,67,85,99] [55,56,77,89,98,99] [3,4,23,47,85,91] Those are on the DB I will generate this: [1,2,11,45,47,88] and now I want to get the rows that have at least 1 number in common with this one. The easiest (and dumbest?) way is to make 6 select and check for similar results. I thought to store numbers with a large binary string like 000000000000000000000100000000010010110000000000000000000000000 with 99 numbers where each number represent a number from 1 to 99, so if I have 1 at the 44th position, it means that I have 44 on that row. This method is probably shifting the difficult tasks to the Db but it's again not very smart. Any suggestion?

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• Python multiprocessing doesn't play nicely with uuid.uuid4().

  - by yig

  I'm trying to generate a uuid for a filename, and I'm also using the multiprocessing module. Unpleasantly, all of my uuids end up exactly the same. Here is a small example: import multiprocessing import uuid def get_uuid( a ): ## Doesn't help to cycle through a bunch. #for i in xrange(10): uuid.uuid4() ## Doesn't help to reload the module. #reload( uuid ) ## Doesn't help to load it at the last minute. ## (I simultaneously comment out the module-level import). #import uuid ## uuid1() does work, but it differs only in the first 8 characters and includes identifying information about the computer. #return uuid.uuid1() return uuid.uuid4() def main(): pool = multiprocessing.Pool( 20 ) uuids = pool.map( get_uuid, range( 20 ) ) for id in uuids: print id if __name__ == '__main__': main() I peeked into uuid.py's code, and it seems to depending-on-the-platform use some OS-level routines for randomness, so I'm stumped as to a python-level solution (to do something like reload the uuid module or choose a new random seed). I could use uuid.uuid1(), but only 8 digits differ and I think there are derived exclusively from the time, which seems dangerous especially given that I'm multiprocessing (so the code could be executing at exactly the same time). Is there some Wisdom out there about this issue?

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• Did I implement clock drift properly?

  - by David Titarenco

  I couldn't find any clock drift RNG code for Windows anywhere so I attempted to implement it myself. I haven't run the numbers through ent or DIEHARD yet, and I'm just wondering if this is even remotely correct... void QueryRDTSC(__int64* tick) { __asm { xor eax, eax cpuid rdtsc mov edi, dword ptr tick mov dword ptr [edi], eax mov dword ptr [edi+4], edx } } __int64 clockDriftRNG() { __int64 CPU_start, CPU_end, OS_start, OS_end; // get CPU ticks -- uses RDTSC on the Processor QueryRDTSC(&CPU_start); Sleep(1); QueryRDTSC(&CPU_end); // get OS ticks -- uses the Motherboard clock QueryPerformanceCounter((LARGE_INTEGER*)&OS_start); Sleep(1); QueryPerformanceCounter((LARGE_INTEGER*)&OS_end); // CPU clock is ~1000x faster than mobo clock // return raw return ((CPU_end - CPU_start)/(OS_end - OS_start)); // or // return a random number from 0 to 9 // return ((CPU_end - CPU_start)/(OS_end - OS_start)%10); } If you're wondering why I Sleep(1), it's because if I don't, OS_end - OS_start returns 0 consistently (because of the bad timer resolution, I presume). Basically, (CPU_end - CPU_start)/(OS_end - OS_start) always returns around 1000 with a slight variation based on the entropy of CPU load, maybe temperature, quartz crystal vibration imperfections, etc. Anyway, the numbers have a pretty decent distribution, but this could be totally wrong. I have no idea.

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• constructing a recursive function returning an array

  - by Admiral Kunkka

  I'm developing a function that has a random chance to loop through itself and put it's results in one array for me to use later in my PHP class. Is there a better way to do this more organized, specifically case 5. The array becomes sloppy if it rolls 5, after 5, after 5 looking unpleasant. private function dice($sides) { return mt_rand(1, $sides); } private function randomLoot($dice) { switch($dice) { case 1: $array[] = "A fancy mug."; break; case 2: $array[] = "A buckler."; break; case 3: $array[] = "A sword."; break; case 4: $array[] = "A jewel."; break; case 5: $array[] = "A treasure chest with contents:"; $count = $this->dice(3); $i = 1; while($i <= $count) { $array[] = $this->randomLoot($this->dice(5)); $i++; } break; } return $array; }

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• Partitioning data set in r based on multiple classes of observations

  - by Danny

  I'm trying to partition a data set that I have in R, 2/3 for training and 1/3 for testing. I have one classification variable, and seven numerical variables. Each observation is classified as either A, B, C, or D. For simplicity's sake, let's say that the classification variable, cl, is A for the first 100 observations, B for observations 101 to 200, C till 300, and D till 400. I'm trying to get a partition that has 2/3 of the observations for each of A, B, C, and D (as opposed to simply getting 2/3 of the observations for the entire data set since it will likely not have equal amounts of each classification). When I try to sample from a subset of the data, such as sample(subset(data, cl=='A')), the columns are reordered instead of the rows. To summarize, my goal is to have 67 random observations from each of A, B, C, and D as my training data, and store the remaining 33 observations for each of A, B, C, and D as testing data. I have found a very similar question to mine, but it did not factor in multiple variables. I feel silly asking this question because it seems so simple, but I'm stumped. Also, this is my first question on this site, so I apologize in advance for any faux pas on my part.

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• Why does my mic boost automatically go to 100 on every boot?

  - by Ben

  When my computer turns on, it automatically sets the "mic boost" sound setting to 100. This causes a loud static sound in my speakers. I can manually go to alsamixer and turn the mic boost down manually, but I would prefer it if I didn't have to do this every time I turn the computer on. I've tried running sudo alsactl store after fixing the settings, and this does save them, but I have to run sudo alsactl restore to restore the settings. This means that I have to manually fix the sound every time I start the computer anyway, so it isn't really a fix. I tried putting sudo alsactl restore in my startup programs, but that didn't seem to fix anything. I'm running Ubuntu 12.04, but I started having this problem before upgrading from 11.10. I'm using a Sony Vaio laptop. I'm not really sure what made it start; it seemed like I just started having the problem randomly one day. Any help would be appreciated! Edit: here is the output from running amixer: http://paste.ubuntu.com/1060080/

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• Randomly sorting an array

  - by Cam

  Does there exist an algorithm which, given an ordered list of symbols {a1, a2, a3, ..., ak}, produces in O(n) time a new list of the same symbols in a random order without bias? "Without bias" means the probability that any symbol s will end up in some position p in the list is 1/k. Assume it is possible to generate a non-biased integer from 1-k inclusive in O(1) time. Also assume that O(1) element access/mutation is possible, and that it is possible to create a new list of size k in O(k) time. In particular, I would be interested in a 'generative' algorithm. That is, I would be interested in an algorithm that has O(1) initial overhead, and then produces a new element for each slot in the list, taking O(1) time per slot. If no solution exists to the problem as described, I would still like to know about solutions that do not meet my constraints in one or more of the following ways (and/or in other ways if necessary): the time complexity is worse than O(n). the algorithm is biased with regards to the final positions of the symbols. the algorithm is not generative. I should add that this problem appears to be the same as the problem of randomly sorting the integers from 1-k, since we can sort the list of integers from 1-k and then for each integer i in the new list, we can produce the symbol ai.

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• Generating different randoms valid for a day on different independent devices?

  - by Pentium10

  Let me describe the system. There are several mobile devices, each independent from each other, and they are generating content for the same record id. I want to avoid generating the same content for the same record on different devices, for this I though I would use a random and make it so too cluster the content pool based on these randoms. Suppose you have choices from 1 to 100. Day 1 Device#1 will choose for the record#33 between 1-10 Device#2 will choose for the record#33 between 40-50 Device#3 will choose for the record#33 between 50-60 Device#1 will choose for the record#55 between 40-50 Device#2 will choose for the record#55 between 1-10 Device#3 will choose for the record#55 between 10-20 Device#1 will choose for the record#11 between 1-10 Device#2 will choose for the record#22 between 1-10 Device#3 will choose for the record#99 between 1-10 Day 2 Device#1 will choose for the record#33 between 90-100 Device#2 will choose for the record#33 between 1-10 Device#3 will choose for the record#33 between 50-60 They don't have access to a central server. Data available for each of them: IMEI (unique per mobile) Date of today (same on all devices) Record id (same on all devices) What do you think, how is it possible? ps. tags can be edited

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• Python coin-toss

  - by Andy

  i am new to Python, and i can't wrap my head around this. I have following function defined: def FlipCoins(num_flips): heads_rounds_won = 0 for i in range(10000): heads = 0 tails = 0 for j in range(num_flips): dice = random.randint(0,1) if dice==1: heads += 1 else: tails += 1 if heads > tails: heads_rounds_won += 1 return heads_rounds_won Here is what it should do (but apparently doesn't): flip a coin num_flip times, count heads and tails, and see if there are more heads than tails. If yes, increment head_rounds_won by 1. Repeat 10000 times. I would assume that head_rounds_won will approximate 5000 (50%). And it does that for odd numbers as input. For example, 3, 5 or 7 will produce about 50%. However, even numbers will produce much lower results, more like 34%. Small numbers especially, with higher even numbers, like for example 800, the difference to 50% is much narrower. Why is this the case? Shouldn't any input produce about 50% heads/tails?

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• Classifying captured data in unknown format?

  - by monch1962

  I've got a large set of captured data (potentially hundreds of thousands of records), and I need to be able to break it down so I can both classify it and also produce "typical" data myself. Let me explain further... If I have the following strings of data: 132T339G1P112S 164T897F5A498S 144T989B9B223T 155T928X9Z554T ... you might start to infer the following: possibly all strings are 14 characters long the 4th, 8th, 10th and 14th characters may always be alphas, while the rest are numeric the first character may always be a '1' the 4th character may always be the letter 'T' the 14th character may be limited to only being 'S' or 'T' and so on... As you get more and more samples of real data, some of these "rules" might disappear; if you see a 15 character long string, then you have evidence that the 1st "rule" is incorrect. However, given a sufficiently large sample of strings that are exactly 14 characters long, you can start to assume that "all strings are 14 characters long" and assign a numeric figure to your degree of confidence (with an appropriate set of assumptions around the fact that you're seeing a suitably random set of all possible captured data). As you can probably tell, a human can do a lot of this classification by eye, but I'm not aware of libraries or algorithms that would allow a computer to do it. Given a set of captured data (significantly more complex than the above...), are there libraries that I can apply in my code to do this sort of classification for me, that will identify "rules" with a given degree of confidence? As a next step, I need to be able to take those rules, and use them to create my own data that conforms to these rules. I assume this is a significantly easier step than the classification, but I've never had to perform a task like this before so I'm really not sure how complex it is. At a guess, Python or Java (or possibly Perl or R) are possibly the "common" languages most likely to have these sorts of libraries, and maybe some of the bioinformatic libraries do this sort of thing. I really don't care which language I have to use; I need to solve the problem in whatever way I can. Any sort of pointer to information would be very useful. As you can probably tell, I'm struggling to describe this problem clearly, and there may be a set of appropriate keywords I can plug into Google that will point me towards the solution. Thanks in advance

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• Template problems: No matching function for call

  - by Nick Sweet

  I'm trying to create a template class, and when I define a non-member template function, I get the "No matching function for call to randvec()" error. I have a template class defined as: template <class T> class Vector { T x, y, z; public: //constructors Vector(); Vector(const T& x, const T& y, const T& z); Vector(const Vector& u); //accessors T getx() const; T gety() const; T getz() const; //mutators void setx(const T& x); void sety(const T& y); void setz(const T& z); //operations void operator-(); Vector plus(const Vector& v); Vector minus(const Vector& v); Vector cross(const Vector& v); T dot(const Vector& v); void times(const T& s); T length() const; //Vector<T>& randvec(); //operators Vector& operator=(const Vector& rhs); friend std::ostream& operator<< <T>(std::ostream&, const Vector<T>&); }; and the function in question, which I've defined after all those functions above, is: //random Vector template <class T> Vector<double>& randvec() { const int min=-10, max=10; Vector<double>* r = new Vector<double>; int randx, randy, randz, temp; const int bucket_size = RAND_MAX/(max-min +1); temp = rand(); //voodoo hackery do randx = (rand()/bucket_size)+min; while (randx < min || randx > max); r->setx(randx); do randy = (rand()/bucket_size)+min; while (randy < min || randy > max); r->sety(randy); do randz = (rand()/bucket_size)+min; while (randz < min || randz > max); r->setz(randz); return *r; } Yet, every time I call it in my main function using a line like: Vector<double> a(randvec()); I get that error. However, if I remove the template and define it using 'double' instead of 'T', the call to randvec() works perfectly. Why doesn't it recognize randvec()? P.S. Don't mind the bit labeled voodoo hackery - this is just a cheap hack so that I can get around another problem I encountered.

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• Is it possible to shuffle a 2D matrix while preserving row AND column frequencies?

  - by j_random_hacker

  Suppose I have a 2D array like the following: GACTG AGATA TCCGA Each array element is taken from a small finite set (in my case, DNA nucleotides -- {A, C, G, T}). I would like to randomly shuffle this array somehow while preserving both row and column nucleotide frequencies. Is this possible? Can it be done efficiently? [EDIT]: By this I mean I want to produce a new matrix where each row has the same number of As, Cs, Gs and Ts as the corresponding row of the original matrix, and where each column has the same number of As, Cs, Gs and Ts as the corresponding column of the original matrix. Permuting the rows or columns of the original matrix will not achieve this in general. (E.g. for the example above, the top row has 2 Gs, and 1 each of A, C and T; if this row was swapped with row 2, the top row in the resulting matrix would have 3 As, 1 G and 1 T.) It's simple enough to preserve just column frequencies by shuffling a column at a time, and likewise for rows. But doing this will in general alter the frequencies of the other kind. My thoughts so far: If it's possible to pick 2 rows and 2 columns so that the 4 elements at the corners of this rectangle have the pattern XY YX for some pair of distinct elements X and Y, then replacing these 4 elements with YX XY will maintain both row and column frequencies. In the example at the top, this can be done for (at least) rows 1 and 2 and columns 2 and 5 (whose corners give the 2x2 matrix AG;GA), and for rows 1 and 3 and columns 1 and 4 (whose corners give GT;TG). Clearly this could be repeated a number of times to produce some level of randomisation. Generalising a bit, any "subrectangle" induced by a subset of rows and a subset of columns, in which the frequencies of all rows are the same and the frequencies of all columns are the same, can have both its rows and columns permuted to produce a valid complete rectangle. (Of these, only those subrectangles in which at least 1 element is changed are actually interesting.) Big questions: Are all valid complete matrices reachable by a series of such "subrectangle rearrangements"? I suspect the answer is yes. Are all valid subrectangle rearrangements decomposable into a series of 2x2 swaps? I suspect the answer is no, but I hope it's yes, since that would seem to make it easier to come up with an efficient algorithm. Can some or all of the valid rearrangements be computed efficiently? This question addresses a special case in which the set of possible elements is {0, 1}. The solutions people have come up with there are similar to what I have come up with myself, and are probably usable, but not ideal as they require an arbitrary amount of backtracking to work correctly. Also I'm concerned that only 2x2 swaps are considered. Finally, I would ideally like a solution that can be proven to select a matrix uniformly at random from the set of all matrices having identical row frequencies and column frequencies to the original. I know, I'm asking for a lot :)

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• Qt, MSVC, and /Zc:wchar_t- == I want to blow up the world

  - by Noah Roberts

  So Qt is compiled with /Zc:wchar_t- on windows. What this means is that instead of wchar_t being a typedef for some internal type (__wchar_t I think) it becomes a typedef for unsigned short. The really cool thing about this is that the default for MSVC is the opposite, which of course means that the libraries you're using are likely compiled with wchar_t being a different type than Qt's wchar_t. This doesn't become an issue of course until you try to use something like std::wstring in your code; especially when one or more libraries have functions that accept it as parameters. What effectively happens is that your code happily compiles but then fails to link because it's looking for definitions using std::wstring<unsigned short...> but they only contain definitions expecting std::wstring<__wchar_t...> (or whatever). So I did some web searching and ran into this link: http://bugreports.qt.nokia.com/browse/QTBUG-6345 Based on the statement by Thiago Macieira, "Sorry, we will not support building Qt like this," I've been worried that fixing Qt to work like everything else might cause some problem and have been trying to avoid it. We recompiled all of our support libraries with the /Zc:wchar_t- flag and have been fairly content with that until a couple days ago when we started trying to port over (we're in the process of switching from Wx to Qt) some serialization code. Because of how win32 works, and because Wx just wraps win32, we've been using std::wstring to represent string data with the intent of making our product as i18n ready as possible. We did some testing and Wx did not work with multibyte characters when trying to print special stuff (even not so special stuff like the degree symbol was an issue). I'm not so sure that Qt has this problem since QString isn't just a wrapper to the underlying _TCHAR type but is a Unicode monster of some sort. At any rate, the serialization library in boost has compiled parts. We've attempted to recompile boost with /Zc:wchar_t- but so far our attempts to tell bjam to do this have gone unheeded. We're at an impasse. From where I'm sitting I have three options: Recompile Qt and hope it works with /Zc:wchar_t. There's some evidence around the web that others have done this but I have no way of predicting what will happen. All attempts to ask Qt people on forums and such have gone unanswered. Hell, even in that very bug report someone asks why and it just sat there for a year. Keep fighting with bjam until it listens. Right now I've got someone under me doing that and I have more experience fighting with things to get what I want but I do have to admit to getting rather tired of it. I'm also concerned that I'll KEEP running into this issue just because Qt wants to be a c**t. Stop using wchar_t for anything. Unfortunately my i18n experience is pretty much 0 but it seems to me that I just need to find the right to/from function in QString (it has a BUNCH) to encode the Unicode into 8-bytes and visa-versa. UTF8 functions look promising but I really want to be sure that no data will be lost if someone from Zimbabfuckegypt starts writing in their own language and the documentation in QString frightens me a little into thinking that could happen. Of course, I could always run into some library that insists I use wchar_t and then I'm back to 1 or 2 but I rather doubt that would happen. So, what's my question... Which of these options is my best bet? Is Qt going to eventually cause me to gouge out my own eyes because I decided to compile it with /Zc:wchar_t anyway? What's the magic incantation to get boost to build with /Zc:wchar_t- and will THAT cause permanent mental damage? Can I get away with just using the standard 8-bit (well, 'common' anyway) character classes and be i18n compliant/ready? How do other Qt developers deal with this mess?

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• Can first descendent be selected directly?

  - by Ben Huh

  I am currently using find() and first() method to select the first descendent element from each of the <div> elements that contains the parent class. But I find this quite cumbersome since find() method would produce a set of matched elements before the first element is being picked. The following is the skeleton of my code: HTML <div class=parent> <ul> <li>random characters</li> <li>random characters</li> <li>random characters</li> <li>random characters</li> </ul> </div> <div class=parent> <ul> <li>random characters</li> <li>random characters</li> <li>random characters</li> <li>random characters</li> </ul> </div> <div class=non-parent> <ul> <li>random characters</li> <li>random characters</li> <li>random characters</li> <li>random characters</li> </ul> </div> <div class=parent> <ul> <li>random characters</li> <li>random characters</li> <li>random characters</li> <li>random characters</li> </ul> </div> // .....the list continues Javascript $('.parent').each(function() { $(this).find('ul li').first().css('color', 'red'); // do other stuff within each loop }); I have seen people using $(".parent li:first") selector. But, because I am doing it in a loop, I am not sure how or whether if this could be done and would like some advice. Thanks.

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