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  • C++ design question on traversing binary trees

    - by user231536
    I have a binary tree T which I would like to copy to another tree. Suppose I have a visit method that gets evaluated at every node: struct visit { virtual void operator() (node* n)=0; }; and I have a visitor algorithm void visitor(node* t, visit& v) { //do a preorder traversal using stack or recursion if (!t) return; v(t); visitor(t->left, v); visitor(t->right, v); } I have 2 questions: I settled on using the functor based approach because I see that boost graph does this (vertex visitors). Also I tend to repeat the same code to traverse the tree and do different things at each node. Is this a good design to get rid of duplicated code? What other alternative designs are there? How do I use this to create a new binary tree from an existing one? I can keep a stack on the visit functor if I want, but it gets tied to the algorithm in visitor. How would I incorporate postorder traversals here ? Another functor class?

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  • Extracting bool from istream in a templated function

    - by Thomas Matthews
    I'm converting my fields class read functions into one template function. I have field classes for int, unsigned int, long, and unsigned long. These all use the same method for extracting a value from an istringstream (only the types change): template <typename Value_Type> Value_Type Extract_Value(const std::string& input_string) { std::istringstream m_string_stream; m_string_stream.str(input_string); m_string_stream.clear(); m_string_stream >> value; return; } The tricky part is with the bool (Boolean) type. There are many textual representations for Boolean: 0, 1, T, F, TRUE, FALSE, and all the case insensitive combinations Here's the questions: What does the C++ standard say are valid data to extract a bool, using the stream extraction operator? Since Boolean can be represented by text, does this involve locales? Is this platform dependent? I would like to simplify my code by not writing my own handler for bool input. I am using MS Visual Studio 2008 (version 9), C++, and Windows XP and Vista.

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  • Problems with Runt overlap? but *only* within Rails

    - by trisignia
    Martin Fowler's Runt library is really handy for date/time comparisons, and this code works great in an irb console: require 'runt' include Runt r_start = PDate.month(2010,12) r_end = PDate.month(2011,12) range = DateRange.new(r_start,r_end) o_start = PDate.month(2010,11) o_end = PDate.month(2012,2) o_range = DateRange.new(o_start,o_end) range.overlap?(o_range) but if I add the Runt gem to my Rails 2.3.5 app and try to run the same commands in script/console, I get this error: NoMethodError: undefined method `to_datetime' for Mon, 01 Nov 2010 00:00:00 +0000..Wed, 01 Feb 2012 00:00:00 +0000:Runt::DateRange from /Users/jacob/work/matchbook/vendor/gems/runt-0.7.6/lib/runt/sugar.rb:130:in method_missing' from /Users/jacob/work/matchbook/vendor/rails/activesupport/lib/active_support/core_ext/date_time/calculations.rb:120:in <=' from /Users/jacob/work/matchbook/vendor/gems/runt-0.7.6/lib/runt/pdate.rb:91:in <=>' from /Users/jacob/work/matchbook/vendor/gems/runt-0.7.6/lib/runt/daterange.rb:34:in member?' from /Users/jacob/work/matchbook/vendor/gems/runt-0.7.6/lib/runt/daterange.rb:34:in `overlap?' from (irb):10 Has anyone encountered this error before, or does anyone know how to begin debugging this? I've tried looking at the spaceship operator in the ActiveSupport calculations module, but I can't figure out how to pick apart the problem. Thanks very much for your help, Jacob

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  • Boost.MultiIndex: Are there way to share object between two processes?

    - by Arman
    Hello, I have a Boost.MultiIndex big array about 10Gb. In order to reduce the reading I thought there should be a way to keep the data in the memory and another client programs will be able to read and analyse it. What is the proper way to organize it? The array looks like: struct particleID { int ID;// real ID for particle from Gadget2 file "ID" block unsigned int IDf;// postition in the file particleID(int id,const unsigned int idf):ID(id),IDf(idf){} bool operator<(const particleID& p)const { return ID<p.ID;} unsigned int getByGID()const {return (ID&0x0FFF);}; }; struct ID{}; struct IDf{}; struct IDg{}; typedef multi_index_container< particleID, indexed_by< ordered_unique< tag<IDf>, BOOST_MULTI_INDEX_MEMBER(particleID,unsigned int,IDf)>, ordered_non_unique< tag<ID>,BOOST_MULTI_INDEX_MEMBER(particleID,int,ID)>, ordered_non_unique< tag<IDg>,BOOST_MULTI_INDEX_CONST_MEM_FUN(particleID,unsigned int,getByGID)> > > particlesID_set; Any ideas are welcome. kind regards Arman.

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  • Can't build pyxpcom on OS X 10.6

    - by Gj
    I've been following these instructions at https://developer.mozilla.org/en/Building_PyXPCOM but getting this: $ make make export make[2]: Nothing to be done for `export'. make[4]: Nothing to be done for `export'. make[4]: Nothing to be done for `export'. /opt/local/bin/python2.5 ../../../src/config/nsinstall.py -L /usr/local/pyxpcom/build/xpcom/src -m 644 ../../../src/xpcom/src/PyXPCOM.h ../../dist/include make[3]: Nothing to be done for `export'. /opt/local/bin/python2.5 ../../../../src/config/nsinstall.py -D ../../../dist/idl /opt/local/bin/python2.5 ../../../../src/config/nsinstall.py -D ../../../dist/idl make[4]: *** No rule to make target `_xpidlgen/py_test_component.h', needed by `export'. Stop. make[3]: *** [export] Error 2 make[2]: *** [export] Error 2 make[1]: *** [export] Error 2 make: *** [default] Error 2 Any ideas? An interesting anomaly is that despite me setting the PYTHON env variable to Python 2.6, the configure and make both seem to go after the 2.5... Thanks for any advice! PS here's the configure output: $ ../src/configure --with-libxul-sdk=/Users/me/xulrunner-sdk/ loading cache ./config.cache checking host system type... i386-apple-darwin10.3.0 checking target system type... i386-apple-darwin10.3.0 checking build system type... i386-apple-darwin10.3.0 checking for mawk... (cached) gawk checking for perl5... (cached) /opt/local/bin/perl5 checking for gcc... (cached) gcc checking whether the C compiler (gcc ) works... yes checking whether the C compiler (gcc ) is a cross-compiler... no checking whether we are using GNU C... (cached) yes checking whether gcc accepts -g... (cached) yes checking for c++... (cached) c++ checking whether the C++ compiler (c++ ) works... yes checking whether the C++ compiler (c++ ) is a cross-compiler... no checking whether we are using GNU C++... (cached) yes checking whether c++ accepts -g... (cached) yes checking for ranlib... (cached) ranlib checking for as... (cached) /usr/bin/as checking for ar... (cached) ar checking for ld... (cached) ld checking for strip... (cached) strip checking for windres... no checking whether gcc and cc understand -c and -o together... (cached) yes checking how to run the C preprocessor... (cached) gcc -E checking how to run the C++ preprocessor... (cached) c++ -E checking for a BSD compatible install... (cached) /usr/bin/install -c checking whether ln -s works... (cached) yes checking for minimum required perl version >= 5.006... 5.008009 checking for full perl installation... yes checking for /opt/local/bin/python... (cached) /opt/local/bin/python2.5 checking for doxygen... (cached) : checking for whoami... (cached) /usr/bin/whoami checking for autoconf... (cached) /opt/local/bin/autoconf checking for unzip... (cached) /usr/bin/unzip checking for zip... (cached) /usr/bin/zip checking for makedepend... (cached) /opt/local/bin/makedepend checking for xargs... (cached) /usr/bin/xargs checking for pbbuild... (cached) /usr/bin/xcodebuild checking for sdp... (cached) /usr/bin/sdp checking for gmake... (cached) /opt/local/bin/gmake checking for X... (cached) no checking whether the compiler supports -Wno-invalid-offsetof... yes checking whether ld has archive extraction flags... (cached) no checking that static assertion macros used in autoconf tests work... (cached) yes checking for 64-bit OS... yes checking for minimum required Python version >= 2.4... yes checking for -dead_strip option to ld... yes checking for ANSI C header files... (cached) yes checking for working const... (cached) yes checking for mode_t... (cached) yes checking for off_t... (cached) yes checking for pid_t... (cached) yes checking for size_t... (cached) yes checking for st_blksize in struct stat... (cached) yes checking for siginfo_t... (cached) yes checking for int16_t... (cached) yes checking for int32_t... (cached) yes checking for int64_t... (cached) yes checking for int64... (cached) no checking for uint... (cached) yes checking for uint_t... (cached) no checking for uint16_t... (cached) no checking for uname.domainname... (cached) no checking for uname.__domainname... (cached) no checking for usable char16_t (2 bytes, unsigned)... (cached) no checking for usable wchar_t (2 bytes, unsigned)... (cached) no checking for compiler -fshort-wchar option... (cached) yes checking for visibility(hidden) attribute... (cached) yes checking for visibility(default) attribute... (cached) yes checking for visibility pragma support... (cached) yes checking For gcc visibility bug with class-level attributes (GCC bug 26905)... (cached) yes checking For x86_64 gcc visibility bug with builtins (GCC bug 20297)... (cached) no checking for dirent.h that defines DIR... (cached) yes checking for opendir in -ldir... (cached) no checking for sys/byteorder.h... (cached) no checking for compat.h... (cached) no checking for getopt.h... (cached) yes checking for sys/bitypes.h... (cached) no checking for memory.h... (cached) yes checking for unistd.h... (cached) yes checking for gnu/libc-version.h... (cached) no checking for nl_types.h... (cached) yes checking for malloc.h... (cached) no checking for X11/XKBlib.h... (cached) yes checking for io.h... (cached) no checking for sys/statvfs.h... (cached) yes checking for sys/statfs.h... (cached) no checking for sys/vfs.h... (cached) no checking for sys/mount.h... (cached) yes checking for sys/quota.h... (cached) yes checking for mmintrin.h... (cached) yes checking for new... (cached) yes checking for sys/cdefs.h... (cached) yes checking for gethostbyname_r in -lc_r... (cached) no checking for dladdr... (cached) yes checking for socket in -lsocket... (cached) no checking whether mmap() sees write()s... yes checking whether gcc needs -traditional... (cached) no checking for 8-bit clean memcmp... (cached) yes checking for random... (cached) yes checking for strerror... (cached) yes checking for lchown... (cached) yes checking for fchmod... (cached) yes checking for snprintf... (cached) yes checking for statvfs... (cached) yes checking for memmove... (cached) yes checking for rint... (cached) yes checking for stat64... (cached) yes checking for lstat64... (cached) yes checking for truncate64... (cached) no checking for statvfs64... (cached) no checking for setbuf... (cached) yes checking for isatty... (cached) yes checking for flockfile... (cached) yes checking for getpagesize... (cached) yes checking for localtime_r... (cached) yes checking for strtok_r... (cached) yes checking for wcrtomb... (cached) yes checking for mbrtowc... (cached) yes checking for res_ninit()... (cached) no checking for gnu_get_libc_version()... (cached) no ../src/configure: line 9881: AM_LANGINFO_CODESET: command not found checking for an implementation of va_copy()... (cached) yes checking for an implementation of __va_copy()... (cached) yes checking whether va_lists can be copied by value... (cached) no checking for C++ exceptions flag... (cached) -fno-exceptions checking for gcc 3.0 ABI... (cached) yes checking for C++ "explicit" keyword... (cached) yes checking for C++ "typename" keyword... (cached) yes checking for modern C++ template specialization syntax support... (cached) yes checking whether partial template specialization works... (cached) yes checking whether operators must be re-defined for templates derived from templates... (cached) no checking whether we need to cast a derived template to pass as its base class... (cached) no checking whether the compiler can resolve const ambiguities for templates... (cached) yes checking whether the C++ "using" keyword can change access... (cached) yes checking whether the C++ "using" keyword resolves ambiguity... (cached) yes checking for "std::" namespace... (cached) yes checking whether standard template operator!=() is ambiguous... (cached) unambiguous checking for C++ reinterpret_cast... (cached) yes checking for C++ dynamic_cast to void*... (cached) yes checking whether C++ requires implementation of unused virtual methods... (cached) yes checking for trouble comparing to zero near std::operator!=()... (cached) no checking for LC_MESSAGES... (cached) yes checking for tar archiver... checking for gnutar... (cached) gnutar gnutar checking for wget... checking for wget... (cached) wget wget checking for valid optimization flags... yes checking for gcc -pipe support... yes checking whether compiler supports -Wno-long-long... yes checking whether C compiler supports -fprofile-generate... yes checking for correct temporary object destruction order... yes checking for correct overload resolution with const and templates... no Building Python extensions using python-2.5 from /opt/local/Library/Frameworks/Python.framework/Versions/2.5 creating ./config.status creating config/autoconf.mk creating Makefile creating xpcom/Makefile creating xpcom/src/Makefile creating xpcom/src/loader/Makefile creating xpcom/src/module/Makefile creating xpcom/components/Makefile creating xpcom/test/Makefile creating xpcom/test/test_component/Makefile creating dom/Makefile creating dom/src/Makefile creating dom/test/Makefile creating dom/test/pyxultest/Makefile creating dom/nsdom/Makefile creating dom/nsdom/test/Makefile

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  • OleDbExeption Was unhandled in VB.Net

    - by ritch
    Syntax error (missing operator) in query expression '((ProductID = ?) AND ((? = 1 AND Product Name IS NULL) OR (Product Name = ?)) AND ((? = 1 AND Price IS NULL) OR (Price = ?)) AND ((? = 1 AND Quantity IS NULL) OR (Quantity = ?)))'. I need some help sorting this error out in Visual Basics.Net 2008. I am trying to update records in a MS Access Database 2008. I have it being able to update one table but the other table is just not having it. Private Sub Admin_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load 'Reads Users into the program from the text file (Located at Module.VB) ReadUsers() 'Connect To Access 2007 Database File con.ConnectionString = ("Provider=Microsoft.ACE.OLEDB.12.0;" & "Data Source=E:\Computing\Projects\Login\Login\bds.accdb;") con.Open() 'SQL connect 1 sql = "Select * From Clients" da = New OleDb.OleDbDataAdapter(sql, con) da.Fill(ds, "Clients") MaxRows = ds.Tables("Clients").Rows.Count intCounter = -1 'SQL connect 2 sql2 = "Select * From Products" da2 = New OleDb.OleDbDataAdapter(sql2, con) da2.Fill(ds, "Products") MaxRows2 = ds.Tables("Products").Rows.Count intCounter2 = -1 'Show Clients From Database in a ComboBox ComboBoxClients.DisplayMember = "ClientName" ComboBoxClients.ValueMember = "ClientID" ComboBoxClients.DataSource = ds.Tables("Clients") End Sub The button, the error appears on da2.update(ds, "Products") Private Sub Button4_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button4.Click Dim cb2 As New OleDb.OleDbCommandBuilder(da2) ds.Tables("Products").Rows(intCounter2).Item("Price") = ProductPriceBox.Text da2.Update(ds, "Products") 'Alerts the user that the Database has been updated MsgBox("Database Updated") End Sub However the code works on updating another table Private Sub UpdateButton_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles UpdateButton.Click 'Allows users to update records in the Database Dim cb As New OleDb.OleDbCommandBuilder(da) 'Changes the database contents with the content in the text fields ds.Tables("Clients").Rows(intCounter).Item("ClientName") = ClientNameBox.Text ds.Tables("Clients").Rows(intCounter).Item("ClientID") = ClientIDBox.Text ds.Tables("Clients").Rows(intCounter).Item("ClientAddress") = ClientAddressBox.Text ds.Tables("Clients").Rows(intCounter).Item("ClientTelephoneNumber") = ClientNumberBox.Text 'Updates the table withing the Database da.Update(ds, "Clients") 'Alerts the user that the Database has been updated MsgBox("Database Updated") End Sub

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  • Getting values from DataGridView back to XDocument (using LINQ-to-XML)

    - by Pretzel
    Learning LINQ has been a lot of fun so far, but despite reading a couple books and a bunch of online resources on the topic, I still feel like a total n00b. Recently, I just learned that if my query returns an Anonymous type, the DataGridView I'm populating will be ReadOnly (because, apparently Anonymous types are ReadOnly.) Right now, I'm trying to figure out the easiest way to: Get a subset of data from an XML file into a DataGridView, Allow the user to edit said data, Stick the changed data back into the XML file. So far I have Steps 1 and 2 figured out: public class Container { public string Id { get; set; } public string Barcode { get; set; } public float Quantity { get; set; } } // For use with the Distinct() operator public class ContainerComparer : IEqualityComparer<Container> { public bool Equals(Container x, Container y) { return x.Id == y.Id; } public int GetHashCode(Container obj) { return obj.Id.GetHashCode(); } } var barcodes = (from src in xmldoc.Descendants("Container") where src.Descendants().Count() > 0 select new Container { Id = (string)src.Element("Id"), Barcode = (string)src.Element("Barcode"), Quantity = float.Parse((string)src.Element("Quantity").Attribute("value")) }).Distinct(new ContainerComparer()); dataGridView1.DataSource = barcodes.ToList(); This works great at getting the data I want from the XML into the DataGridView so that the user has a way to manipulate the values. Upon doing a Step-thru trace of my code, I'm finding that the changes to the values made in DataGridView are not bound to the XDocument object and as such, do not propagate back. How do we take care of Step 3? (getting the data back to the XML) Is it possible to Bind the XML directly to the DataGridView? Or do I have to write another LINQ statement to get the data from the DGV back to the XDocument? Suggstions?

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  • What are the primitive Forth operators?

    - by Barry Brown
    I'm interested in implementing a Forth system, just so I can get some experience building a simple VM and runtime. When starting in Forth, one typically learns about the stack and its operators (DROP, DUP, SWAP, etc.) first, so it's natural to think of these as being among the primitive operators. But they're not. Each of them can be broken down into operators that directly manipulate memory and the stack pointers. Later one learns about store (!) and fetch (@) which can be used to implement DUP, SWAP, and so forth (ha!). So what are the primitive operators? Which ones must be implemented directly in the runtime environment from which all others can be built? I'm not interested in high-performance; I want something that I (and others) can learn from. Operator optimization can come later. (Yes, I'm aware that I can start with a Turing machine and go from there. That's a bit extreme.) Edit: What I'm aiming for is akin to bootstrapping an operating system or a new compiler. What do I need do implement, at minimum, so that I can construct the rest of the system out of those primitive building blocks? I won't implement this on bare hardware; as an educational exercise, I'd write my own minimal VM.

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  • How to make Stack.Pop threadsafe

    - by user260197
    I am using the BlockingQueue code posted in this question, but realized I needed to use a Stack instead of a Queue given how my program runs. I converted it to use a Stack and renamed the class as needed. For performance I removed locking in Push, since my producer code is single threaded. My problem is how can thread working on the (now) thread safe Stack know when it is empty. Even if I add another thread safe wrapper around Count that locks on the underlying collection like Push and Pop do, I still run into the race condition that access Count and then Pop are not atomic. Possible solutions as I see them (which is preferred and am I missing any that would work better?): Consumer threads catch the InvalidOperationException thrown by Pop(). Pop() return a nullptr when _stack-Count == 0, however C++-CLI does not have the default() operator ala C#. Pop() returns a boolean and uses an output parameter to return the popped element. Here is the code I am using right now: generic <typename T> public ref class ThreadSafeStack { public: ThreadSafeStack() { _stack = gcnew Collections::Generic::Stack<T>(); } public: void Push(T element) { _stack->Push(element); } T Pop(void) { System::Threading::Monitor::Enter(_stack); try { return _stack->Pop(); } finally { System::Threading::Monitor::Exit(_stack); } } public: property int Count { int get(void) { System::Threading::Monitor::Enter(_stack); try { return _stack->Count; } finally { System::Threading::Monitor::Exit(_stack); } } } private: Collections::Generic::Stack<T> ^_stack; };

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  • Databinding question: DataGridView <=> XDocument (using LINQ-to-XML)

    - by Pretzel
    Learning LINQ has been a lot of fun so far, but despite reading a couple books and a bunch of online resources on the topic, I still feel like a total n00b. Recently, I just learned that if my query returns an Anonymous type, the DataGridView I'm populating will be ReadOnly (because, apparently Anonymous types are ReadOnly.) Right now, I'm trying to figure out the easiest way to: Get a subset of data from an XML file into a DataGridView, Allow the user to edit said data, Stick the changed data back into the XML file. So far I have Steps 1 and 2 figured out: public class Container { public string Id { get; set; } public string Barcode { get; set; } public float Quantity { get; set; } } // For use with the Distinct() operator public class ContainerComparer : IEqualityComparer<Container> { public bool Equals(Container x, Container y) { return x.Id == y.Id; } public int GetHashCode(Container obj) { return obj.Id.GetHashCode(); } } var barcodes = (from src in xmldoc.Descendants("Container") where src.Descendants().Count() > 0 select new Container { Id = (string)src.Element("Id"), Barcode = (string)src.Element("Barcode"), Quantity = float.Parse((string)src.Element("Quantity").Attribute("value")) }).Distinct(new ContainerComparer()); dataGridView1.DataSource = barcodes.ToList(); This works great at getting the data I want from the XML into the DataGridView so that the user has a way to manipulate the values. Upon doing a Step-thru trace of my code, I'm finding that the changes to the values made in DataGridView are not bound to the XDocument object and as such, do not propagate back. How do we take care of Step 3? (getting the data back to the XML) Is it possible to Bind the XML directly to the DataGridView? Or do I have to write another LINQ statement to get the data from the DGV back to the XDocument? Suggstions?

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  • Regex with optional part doesn't create backreference

    - by padraigf
    I want to match an optional tag at the end of a line of text. Example input text: The quick brown fox jumps over the lazy dog {tag} I want to match the part in curly-braces and create a back-reference to it. My regex looks like this: ^.*(\{\w+\})? (somewhat simplified, I'm also matching parts before the tag): It matches the lines ok (with and without the tag) but doesn't create a back-reference to the tag. If I remove the '?' character, so regex is: ^.*(\{\w+\}) It creates a back-reference to the tag but then doesn't match lines without the tag. I understood from http://www.regular-expressions.info/refadv.html that the optional operator wouldn't affect the backreference: Round brackets group the regex between them. They capture the text matched by the regex inside them that can be reused in a backreference, and they allow you to apply regex operators to the entire grouped regex. but must've misunderstood something. How do I make the tag part optional and create a back-reference when it exists?

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  • how to implement intel's tbb::blocked_range2d in C++

    - by Hristo
    I'm trying to parallelize nested for loops with parellel_for() and the blocked_range2d from Intel's TBB using C++. The for loops look like this: for(int i = 0; i < N; ++i) { for(int j = 0; j < E[i]; ++j) { for(int k = 0; k < T; ++k) { score[k] += delta(i, trRating[k][i], exRating[j][i]); } } } ... and I am trying to do the following: class LoopBody { private: int *myscore; public: LoopBody(int *score) { myscore = score; } void operator()(const blocked_range2d<int> &r) const { for(int i = r.rows().begin(); i != r.rows().end(); ++i); for(int j = 0; j < E[i]; ++j) { for(int k = r.cols().begin(); k != r.cols().end(); ++k) { myscore[k] += foo(...); // uses i,j,k to look up indices in arrays } } } } }; void computeScores(int score[]) { parallel_for(blocked_range2d<int>(0, N, 0, T), LoopBody(score)); } ... but I am getting the following compile errors: error: identifier "i" is undefined for(int j = 0; j < E[i]; ++j) { ^ error: expected an identifier }; ^ I'm not really sure if I am doing this the right way, but any advice is appreciated. Also, this is my first time using Intel's TBB so I really don't know anything about it. Any ideas how make this work? Thanks, Hristo

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  • Visual Studio expression containing a term named "by" cannot be evaluated in the watch window

    - by Andrei Pana
    Consider my C++ code below: int _tmain(int argc, _TCHAR* argv[]) { int by = 10; printf("%d\n", by); int bx = 20; printf("%d\n", (by + bx)); return 0; } which works fine. The funny thing is with the "by" variable. If I try to add a watch for a simple expression that contains by, the result will be CXX0030: Error: expression cannot be evaluated. For example, on a breakpoint on return 0, if I add the following watches I get the results mentioned: by : 10 bx : 20 by + 5 : CXX0030: Error: expression cannot be evaluated bx + 5 : 25 by + bx : CXX0030: Error: expression cannot be evaluated (by) + bx : 30 by + (bx) : CXX0030: Error: expression cannot be evaluated bx + (by) : CXX0014: Error: missing operrand This happens on VS2010, VS2008 on multiple computers. So, more out of curiosity, what is happening with "by"? Is it some kind of strange operator? Why doesn't bx get the same treatment? (I've tried google on this but it is quite difficult to get some relevant hits with terms like "by")

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  • (C#) Get index of current foreach iteration

    - by Graphain
    Hi, Is there some rare language construct I haven't encountered (like the few I've learned recently, some on Stack Overflow) in C# to get a value representing the current iteration of a foreach loop? For instance, I currently do something like this depending on the circumstances: int i=0; foreach (Object o in collection) { ... i++; } Answers: @bryansh: I am setting the class of an element in a view page based on the position in the list. I guess I could add a method that gets the CSSClass for the Objects I am iterating through but that almost feels like a violation of the interface of that class. @Brad Wilson: I really like that - I've often thought about something like that when using the ternary operator but never really given it enough thought. As a bit of food for thought it would be nice if you could do something similar to somehow add (generically to all IEnumerable objects) a handle on the enumerator to increment the value that an extension method returns i.e. inject a method into the IEnumerable interface that returns an iterationindex. Of course this would be blatant hacks and witchcraft... Cool though... @crucible: Awesome I totally forgot to check the LINQ methods. Hmm appears to be a terrible library implementation though. I don't see why people are downvoting you though. You'd expect the method to either use some sort of HashTable of indices or even another SQL call, not an O(N) iteration... (@Jonathan Holland yes you are right, expecting SQL was wrong) @Joseph Daigle: The difficulty is that I assume the foreach casting/retrieval is optimised more than my own code would be. @Jonathan Holland: Ah, cheers for explaining how it works and ha at firing someone for using it.

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  • how to get cartesian products between database and local sequences in linq?

    - by JD
    I saw this similar question here but can't figure out how to use Contains in Cartesian product desired result situation: http://stackoverflow.com/questions/1712105/linq-to-sql-exception-local-sequence-cannot-be-used-in-linq-to-sql-implementatio Let's say I have following: var a = new [] { 1, 4, 7 }; var b = new [] { 2, 5, 8 }; var test = from i in a from j in b select new { A = i, B = j, AB = string.Format("{0:00}a{1:00}b", i, j), }; foreach (var t in test) Console.Write("{0}, ", t.AB); This works great and I get a dump like so (note, I want the cartesian product): 01a02b, 01a05b, 01a08b, 04a02b, 04a05b, 04a08b, 07a02b, 07a05b, 07a08b, Now what I really want is to take this and cartesian product it again against an ID from a database table I have. But, as soon as I add in one more from clause that instead of referencing objects, references SQL table, I get an error. So, altering above to something like so where db is defined as a new DataContext (i.e., class deriving from System.Data.Linq.DataContext): var a = new [] { 1, 4, 7 }; var b = new [] { 2, 5, 8 }; var test = from symbol in db.Symbols from i in a from j in b select new { A = i, B = j, AB = string.Format("{0}{1:00}a{2:00}b", symbol.ID, i, j), }; foreach (var t in test) Console.Write("{0}, ", t.AB); The error I get is following: Local sequence cannot be used in LINQ to SQL implementations of query operators except the Contains operator Its related to not using Contains apparently but I'm unsure how Contains would be used when I don't really want to constrict the results - I want the Cartesian product for my situation. Any ideas of how to use Contains above and still yield the Cartesian product when joining database and local sequences?

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  • How can I get browser to prompt to save password?

    - by Eric
    Hey, I'm working on a web app that has a login dialog that works like this: User clicks "login" Login form HTML is loaded with AJAX and displayed in DIV on page User enters user/pass in fields and clicks submit. It's NOT a <form> -- user/pass are submitted via AJAX If user/pass are okay, page reloads with user logged in. If user/pass are bad, page does NOT reload but error message appears in DIV and user gets to try again. Here's the problem: the browser never offers the usual "Save this password? Yes / Never / Not Now" prompt that it does for other sites. I tried wrapping the <div> in <form> tags with "autocomplete='on'" but that made no difference. Is it possible to get the browser to offer to store the password without a major rework of my login flow? thanks Eric p.s. to add to my question, I'm definitely working with browers that store passwords, and I've never clicked "never for this site" ...this is a technical issue with the browser not detecting that it's a login form, not operator error :-)

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  • How to force c# binary int division to return a double?

    - by Wayne
    How to force double x = 3 / 2; to return 1.5 in x without the D suffix or casting? Is there any kind of operator overload that can be done? Or some compiler option? Amazingly, it's not so simple to add the casting or suffix for the following reason: Business users need to write and debug their own formulas. Presently C# is getting used like a DSL (domain specific language) in that these users aren't computer science engineers. So all they know is how to edit and create a few types of classes to hold their "business rules" which are generally just math formulas. But they always assume that double x = 3 / 2; will return x = 1.5 however in C# that returns 1. A. they always forget this, waste time debugging, call me for support and we fix it. B. they think it's very ugly and hurts the readability of their business rules. As you know, DSL's need to be more like natural language. Yes. We are planning to move to Boo and build a DSL based on it but that's down the road. Is there a simple solution to make double x = 3 / 2; return 1.5 by something external to the class so it's invisible to the users? Thanks! Wayne

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  • How to force c# binary int division to return a double?

    - by Wayne
    How to force double x = 3 / 2; to return 1.5 in x without the D suffix or casting? Is there any kind of operator overload that can be done? Or some compiler option? Amazingly, it's not so simple to add the casting or suffix for the following reason: Business users need to write and debug their own formulas. Presently C# is getting used like a DSL (domain specific language) in that these users aren't computer science engineers. So all they know is how to edit and create a few types of classes to hold their "business rules" which are generally just math formulas. But they always assume that double x = 3 / 2; will return x = 1.5 however in C# that returns 1. A. they always forget this, waste time debugging, call me for support and we fix it. B. they think it's very ugly and hurts the readability of their business rules. As you know, DSL's need to be more like natural language. Yes. We are planning to move to Boo and build a DSL based on it but that's down the road. Is there a simple solution to make double x = 3 / 2; return 1.5 by something external to the class so it's invisible to the users? Thanks! Wayne

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  • Good hash function for a 2d index

    - by rlbond
    I have a struct called Point. Point is pretty simple: struct Point { Row row; Column column; // some other code for addition and subtraction of points is there too } Row and Column are basically glorified ints, but I got sick of accidentally transposing the input arguments to functions and gave them each a wrapper class. Right now I use a set of points, but repeated lookups are really slowing things down. I want to switch to an unordered_set. So, I want to have an unordered_set of Points. Typically this set might contain, for example, every point on a 80x24 terminal = 1920 points. I need a good hash function. I just came up with the following: struct PointHash : public std::unary_function<Point, std::size_t> { result_type operator()(const argument_type& val) const { return val.row.value() * 1000 + val.col.value(); } }; However, I'm not sure that this is really a good hash function. I wanted something fast, since I need to do many lookups very quickly. Is there a better hash function I can use, or is this OK?

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  • Singleton with inheritance, Derived class is not able to get instantiated in parent?

    - by yesraaj
    Below code instantiates a derived singleton object based on environment variable. The compiler errors saying error C2512: 'Dotted' : no appropriate default constructor. I don't understand what the compiler is complaining about. #include <stdlib.h> #include <iostream> #include <string> using namespace std; class Dotted; class Singleton{ public: static Singleton instant(){ if (!instance_) { char * style = getenv("STYLE"); if (!style){ if (strcmp(style,"dotted")==0) { instance_ = new Dotted(); return *instance_; } } else{ instance_ = new Singleton(); return *instance_; } } return *instance_; } void print(){cout<<"Singleton";} ~Singleton(){}; protected: Singleton(){}; private: static Singleton * instance_; Singleton(const Singleton & ); void operator=(const Singleton & ); }; class Dotted:public Singleton{ public: void print(){cout<<"Dotted";} protected: Dotted(); }; Dotted::Dotted():Singleton(){} int main(){ Singleton::instant().print(); cin.get(); }

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  • NAMESPACE_SEPARATOR constant

    - by ts
    Hello, Having namespaces in PHP is great. Having '\' as namespace separator is a little bit ... awkward (but if there is someone who thinks that is cool & sexy, I am adding tag "rant" to this post. ;) . So, here goes the question: Are you using in your code NAMESPACE_SEPARATOR constant? As in code below: <?php if (!\defined('NAMESPACE_SEPARATOR') { \define('NAMESPACE_SEPARATOR', '\\'); } // if Pros: consistent with DIRECTORY_SEPARATOR (which all of us are using ;) no mess with escaping (think of '\Foo\Bar' but '\\' . Foo' . '\\' . 'Bar') more readable (IMHO) which gives us in effect an opportunity to write good, namespace-aware autoloaders can resist another change if something scary happens (as with ol'good '::' from PHP 6 alpha) can hide uniquess of '\' as namespace operator in programming language land from strangers ;) Cons: "The reason for DIRECTORY_SEPARATOR is that the value is platform dependent, the namespace separator isn't." (as stated in http://bugs.php.net/bug.php?id=43046) 19 characters instead of 1 ( \ ) or 4 ('\\') There are places where you can't use this (full qualified class names as default class variables) ie: class A { protected $sDefaultReporterClass = '\My\Namespace\DefaultReporter'; } So, what are you thinking ?

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  • Why does std::cout convert volatile pointers to bool?

    - by Joseph Garvin
    If you try to cout a volatile pointer, even a volatile char pointer where you would normally expect cout to print the string, you will instead simply get '1' (assuming the pointer is not null I think). I assume output stream operator<< is template specialized for volatile pointers, but my question is, why? What use case motivates this behavior? Example code: #include <iostream> #include <cstring> int main() { char x[500]; std::strcpy(x, "Hello world"); int y; int *z = &y; std::cout << x << std::endl; std::cout << (char volatile*)x << std::endl; std::cout << z << std::endl; std::cout << (int volatile*)z << std::endl; return 0; } Output: Hello world 1 0x8046b6c 1

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  • Are there some cases where Python threads can safely manipulate shared state?

    - by erikg
    Some discussion in another question has encouraged me to to better understand cases where locking is required in multithreaded Python programs. Per this article on threading in Python, I have several solid, testable examples of pitfalls that can occur when multiple threads access shared state. The example race condition provided on this page involves races between threads reading and manipulating a shared variable stored in a dictionary. I think the case for a race here is very obvious, and fortunately is eminently testable. However, I have been unable to evoke a race condition with atomic operations such as list appends or variable increments. This test exhaustively attempts to demonstrate such a race: from threading import Thread, Lock import operator def contains_all_ints(l, n): l.sort() for i in xrange(0, n): if l[i] != i: return False return True def test(ntests): results = [] threads = [] def lockless_append(i): results.append(i) for i in xrange(0, ntests): threads.append(Thread(target=lockless_append, args=(i,))) threads[i].start() for i in xrange(0, ntests): threads[i].join() if len(results) != ntests or not contains_all_ints(results, ntests): return False else: return True for i in range(0,100): if test(100000): print "OK", i else: print "appending to a list without locks *is* unsafe" exit() I have run the test above without failure (100x 100k multithreaded appends). Can anyone get it to fail? Is there another class of object which can be made to misbehave via atomic, incremental, modification by threads? Do these implicitly 'atomic' semantics apply to other operations in Python? Is this directly related to the GIL?

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  • How do I pass a const reference in C#?

    - by Maciek
    In C++, passing const references is a common practice - for instance : #include <iostream> using namespace std; class X { public : X() {m_x = 0; } X(const int & x) {m_x = x; } X(const X & other) { *this = other; } X & operator = (const X & other) { m_x = other.m_x; return *this; } void print() { cout << m_x << endl; } private : int m_x; }; void main() { X x1(5); X x2(4); X x3(x2); x2 = x1; x1.print(); x2.print(); x3.print(); } This very simple example illustrates how it's done - pretty much. However I've noticed that in C# this doesn't seem to be the case. Do I have to pass const references in C# ? what do I need the "ref" keyword for? Please note that I know and understand what C# reference and value types are.

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  • Using boost::random as the RNG for std::random_shuffle

    - by Greg Rogers
    I have a program that uses the mt19937 random number generator from boost::random. I need to do a random_shuffle and want the random numbers generated for this to be from this shared state so that they can be deterministic with respect to the mersenne twister's previously generated numbers. I tried something like this: void foo(std::vector<unsigned> &vec, boost::mt19937 &state) { struct bar { boost::mt19937 &_state; unsigned operator()(unsigned i) { boost::uniform_int<> rng(0, i - 1); return rng(_state); } bar(boost::mt19937 &state) : _state(state) {} } rand(state); std::random_shuffle(vec.begin(), vec.end(), rand); } But i get a template error calling random_shuffle with rand. However this works: unsigned bar(unsigned i) { boost::mt19937 no_state; boost::uniform_int<> rng(0, i - 1); return rng(no_state); } void foo(std::vector<unsigned> &vec, boost::mt19937 &state) { std::random_shuffle(vec.begin(), vec.end(), bar); } Probably because it is an actual function call. But obviously this doesn't keep the state from the original mersenne twister. What gives? Is there any way to do what I'm trying to do without global variables?

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