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• Problem understanding treesort in Haskell

  - by Jerry

  I am trying to figure out how exactly does treesort from here work (I understand flatten, insert and foldr). I suppose what's being done in treesort is applying insert for each element on the list thus generating a tree and then flattening it. The only problem I can't overcome here is where the list (that is the argument of the function) is hiding (because it is not written anywhere as an argument except for the function type declaration). One more thing: since dot operator is function composition, why is it an error when I change: treesort = flatten . foldr insert Leaf to treesort = flatten( foldr insert Leaf )?

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• Matching of tuples

  - by Jack

  From what I understood I can use pattern-matching in a match ... with expression with tuples of values, so something like match b with ("<", val) -> if v < val then true else false | ("<=", val) -> if v <= val then true else false should be correct but it gives me a syntax error as if the parenthesis couldn't be used: File "ocaml.ml", line 41, characters 14-17: Error: Syntax error: ')' expected File "ocaml.ml", line 41, characters 8-9: Error: This '(' might be unmatched referring on first match clause.. Apart from that, can I avoid matching strings and applying comparisons using a sort of eval of the string? Or using directly the comparison operator as the first element of the tuple?

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• Pros and Cons of Different macro function / inline methods in C

  - by Robert S. Barnes

  According to the C FAQ, there are basically 3 practical methods for "inlining" code in C: #define MACRO(arg1, arg2) do { \ /* declarations */ \ stmt1; \ stmt2; \ /* ... */ \ } while(0) /* (no trailing ; ) */ or #define FUNC(arg1, arg2) (expr1, expr2, expr3) To clarify this one, the arguments are used in the expressions, and the comma operator returns the value of the last expression. or using the inline declaration which is supported as an extension to gcc and in the c99 standard. The do { ... } while (0) method is widely used in the Linux kernel, but I haven't encountered the other two methods very often if at all. I'm referring specifically to multi-statement "functions", not single statement ones like MAX or MIN. What are the pros and cons of each method, and why would you choose one over the other in various situations?

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• Boost::Container::Vector with Enum Template Argument - Not Legal Base Class

  - by CuppM

  Hi, I'm using Visual Studio 2008 with the Boost v1.42.0 library. If I use an enum as the template argument, I get a compile error when adding a value using push_back(). The compiler error is: 'T': is not a legal base class and the location of the error is move.hpp line 79. #include <boost/interprocess/containers/vector.hpp> class Test { public: enum Types { Unknown = 0, First = 1, Second = 2, Third = 3 }; typedef boost::container::vector<Types> TypesVector; }; int main() { Test::TypesVector o; o.push_back(Test::First); return 0; } If I use a std::vector instead it works. And if I resize the Boost version first and then set the values using the [] operator it also works. Is there some way to make this work using push_back()?

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• Why is floating point byte swapping different from integer byte swapping?

  - by CJ

  I have a binary file of doubles that I need to load using C++. However, my problem is that it was written in big-endian format but the fstream operator will then read the number wrong because my machine is little-endian. It seems like a simple problem to resolve for integers, but for doubles and floats the solutions I have found won't work. How can I (or should I) fix this? I read this as a reference for integer byte swapping: http://stackoverflow.com/questions/105252/how-do-i-convert-between-big-endian-and-little-endian-values-in-c

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• doubt in sizeof implementation

  - by aks

  Below is the program to find the size of a structure without using sizeof operator: struct MyStruct { int i; int j; }; int main() { struct MyStruct *p=0; int size = ((char*)(p+1))-((char*)p); printf("\nSIZE : [%d]\nSIZE : [%d]\n", size); return 0; } My doubt is: Why is typecasting to char * required? If I don't use the char* pointer, the output is 1 - WHY?

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• Custom Memory Allocator for STL map

  - by Prasoon Tiwari

  This question is about construction of instances of custom allocator during insertion into a std::map. Here is a custom allocator for std::map<int,int> along with a small program that uses it: #include <stddef.h> #include <stdio.h> #include <map> #include <typeinfo> class MyPool { public: void * GetNext() { return malloc(24); } void Free(void *ptr) { free(ptr); } }; template<typename T> class MyPoolAlloc { public: static MyPool *pMyPool; typedef size_t size_type; typedef ptrdiff_t difference_type; typedef T* pointer; typedef const T* const_pointer; typedef T& reference; typedef const T& const_reference; typedef T value_type; template<typename X> struct rebind { typedef MyPoolAlloc<X> other; }; MyPoolAlloc() throw() { printf("-------Alloc--CONSTRUCTOR--------%08x %32s\n", this, typeid(T).name()); } MyPoolAlloc(const MyPoolAlloc&) throw() { printf(" Copy Constructor ---------------%08x %32s\n", this, typeid(T).name()); } template<typename X> MyPoolAlloc(const MyPoolAlloc<X>&) throw() { printf(" Construct T Alloc from X Alloc--%08x %32s %32s\n", this, typeid(T).name(), typeid(X).name()); } ~MyPoolAlloc() throw() { printf(" Destructor ---------------------%08x %32s\n", this, typeid(T).name()); }; pointer address(reference __x) const { return &__x; } const_pointer address(const_reference __x) const { return &__x; } pointer allocate(size_type __n, const void * hint = 0) { if (__n != 1) perror("MyPoolAlloc::allocate: __n is not 1.\n"); if (NULL == pMyPool) { pMyPool = new MyPool(); printf("======>Creating a new pool object.\n"); } return reinterpret_cast<T*>(pMyPool->GetNext()); } //__p is not permitted to be a null pointer void deallocate(pointer __p, size_type __n) { pMyPool->Free(reinterpret_cast<void *>(__p)); } size_type max_size() const throw() { return size_t(-1) / sizeof(T); } void construct(pointer __p, const T& __val) { printf("+++++++ %08x %s.\n", __p, typeid(T).name()); ::new(__p) T(__val); } void destroy(pointer __p) { printf("-+-+-+- %08x.\n", __p); __p->~T(); } }; template<typename T> inline bool operator==(const MyPoolAlloc<T>&, const MyPoolAlloc<T>&) { return true; } template<typename T> inline bool operator!=(const MyPoolAlloc<T>&, const MyPoolAlloc<T>&) { return false; } template<typename T> MyPool* MyPoolAlloc<T>::pMyPool = NULL; int main(int argc, char *argv[]) { std::map<int, int, std::less<int>, MyPoolAlloc<std::pair<const int,int> > > m; //random insertions in the map m.insert(std::pair<int,int>(1,2)); m[5] = 7; m[8] = 11; printf("======>End of map insertions.\n"); return 0; } Here is the output of this program: -------Alloc--CONSTRUCTOR--------bffcdaa6 St4pairIKiiE Construct T Alloc from X Alloc--bffcda77 St13_Rb_tree_nodeISt4pairIKiiEE St4pairIKiiE Copy Constructor ---------------bffcdad8 St13_Rb_tree_nodeISt4pairIKiiEE Destructor ---------------------bffcda77 St13_Rb_tree_nodeISt4pairIKiiEE Destructor ---------------------bffcdaa6 St4pairIKiiE ======Creating a new pool object. Construct T Alloc from X Alloc--bffcd9df St4pairIKiiE St13_Rb_tree_nodeISt4pairIKiiEE +++++++ 0985d028 St4pairIKiiE. Destructor ---------------------bffcd9df St4pairIKiiE Construct T Alloc from X Alloc--bffcd95f St4pairIKiiE St13_Rb_tree_nodeISt4pairIKiiEE +++++++ 0985d048 St4pairIKiiE. Destructor ---------------------bffcd95f St4pairIKiiE Construct T Alloc from X Alloc--bffcd95f St4pairIKiiE St13_Rb_tree_nodeISt4pairIKiiEE +++++++ 0985d068 St4pairIKiiE. Destructor ---------------------bffcd95f St4pairIKiiE ======End of map insertions. Construct T Alloc from X Alloc--bffcda23 St4pairIKiiE St13_Rb_tree_nodeISt4pairIKiiEE -+-+-+- 0985d068. Destructor ---------------------bffcda23 St4pairIKiiE Construct T Alloc from X Alloc--bffcda43 St4pairIKiiE St13_Rb_tree_nodeISt4pairIKiiEE -+-+-+- 0985d048. Destructor ---------------------bffcda43 St4pairIKiiE Construct T Alloc from X Alloc--bffcda43 St4pairIKiiE St13_Rb_tree_nodeISt4pairIKiiEE -+-+-+- 0985d028. Destructor ---------------------bffcda43 St4pairIKiiE Destructor ---------------------bffcdad8 St13_Rb_tree_nodeISt4pairIKiiEE Last two columns of the output show that an allocator for std::pair<const int, int> is constructed everytime there is a insertion into the map. Why is this necessary? Is there a way to suppress this? Thanks! Edit: This code tested on x86 machine with g++ version 4.1.2. If you wish to run it on a 64-bit machine, you'll have to change at least the line return malloc(24). Changing to return malloc(48) should work.

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• java.io in debian

  - by Stig

  Hello, i try to compile a java program but in the import section of the code fails: import java.net.; import java.io.; import java.util.; import java.text.; import java.awt.; //import java.awt.image.; import java.awt.event.; //import java.awt.image.renderable.; import javax.swing.; import javax.swing.border.; //import javax.swing.border.EtchedBorder; //import javax.media.jai.; //import javax.media.jai.operator.; //import com.sun.media.jai.codec.; //import java.lang.reflect.; how can i fix the problem in a linux debian machine?. Thanks

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• How can I search for numbers in a varchar column

  - by dave

  I've got a simple nvarchar(25) column in an SQL database table. Most of the time, this field should contain alphanumeric text. However, due to operator error, there are many instances where it contains only a number. Can I do a simple search in SQL to identify these cases? That is, determine which rows in the table contain only digits in this column. As an extension, could I also search for those column values which contain only digits and a space and/or slash. In other languages (eg. Perl, Java) a regular expression would resolve this quickly and easily. But I haven't been able to find the equivalent in SQL.

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• File::GlobMapper doesnt seem to be exporting globmap

  - by justintime

  The following is pretty well copied from the documentation. use File::GlobMapper qw( globmap ); for my $pair (globmap '<*.tar.gz>' => '<#1.tgz>' ) { } And it gives String found where operator expected at globmapper_test1.pl line 4, near "globmap '<*.tar.gz>'" (Do you need to predeclare globmap?) (Using ActivePerl 5.10.0 on Windows) Side questions - if GlobMapper only exports one function, why is it set so you have to export it explicitly?

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• Excel VBA to check autofilter for data

  - by cav719

  I need help checking for autofiltered rows not including the header. I want it to give a message box "No records found." then exit sub or continue with copy paste if there are rows beyond the header row. I know I need an If/Else entry after the filter to check for data but I'm having trouble figuring how to check. This code is being done from a UserForm I created. Here is my script: Private Sub Searchbycompanyfield_Click() If CompanyComboBox1.Value = "" Then MsgBox "Please enter a Company to begin search." Exit Sub End If ActiveSheet.Range("$A:$H").AutoFilter Field:=1, Criteria1:=EQDataEntry.CompanyComboBox1.Value, Operator:=xlOr Cells.Select Selection.Copy Sheets("Sheet2").Select Range("A5").Select ActiveSheet.Paste Call MessageBoxYesOrNoMsgBox End Sub Any help would be greatly appreciated.

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• Apply CSS Style on all elements except with a SPECIFIC ID

  - by Rajesh Paul

  CSS Code(what I need) <style> div[id!='div1']// I actually needed an inequality operator for NOT EQUAL TO { font-size:40px; } </style> HTML code <body> <div>abc</div> <div>def</div> <div id='div1'>ghi</div> </body> The CSS didn't work as I intended. I actually wanted to define the style for all <div>-elements except the one with id='div1'. How can I do that?

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• Expression.OrElse, dynamically creating a condition.

  - by Jim

  Hi, I am attempting to create a dynamic where clause using the standard expression API. var query = ( from p in Parties orderby p.PartyId orderby p.FullName select p ).AsQueryable(); Expression<Func<Party, bool>> @fn = (p) => SqlMethods.Like(p.FullName, "%smith%") || SqlMethods.Like(p.Person.FirstName, "%smith%"); Expression<Func<Party, bool>> @sn = (p) => SqlMethods.Like(p.Person.FirstName, words[0]); ParameterExpression pe = Expression.Parameter(typeof(Party), "p"); Expression orelse = Expression.OrElse( Expression.Lambda(@fn, pe), Expression.Lambda(@sn, pe) ); The expressions above will ultimately be added to a where clause. I need to add a bunch of 'likes'. How do I do this? I get InvalidOperationException on the operator OrElse I have also tried Expression.Or Thanks Regards Craig.

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• Problem of using cin twice.

  - by gc

  Here is the code: string str; cinstr; cout<<"first input:"<<str<<endl; getline(cin, str); cout<<"line input:"<<str<<endl; The result is that getline never pauses for user input, therefore the second output is always empty. After spending some time on it, I realized after the first call "cinstr", it seems '\n' is still stored in cin (using cin.peek() to check), which ends getline immediately. The solution will be adding one more line between the first usage and the second one: cin.ignore(numeric_limits::max(), '\n'); However, I still don't understand, why is '\n' left there after the first call? What does istream& operator really do?

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• How to preserve data integrity while minimizing the transmission size

  - by user1500578

  we have sensors in the wild that send their data to a server every day via TCP/IP, either through 3G or through satellite for the physical layer. The sensors can automatically switch from one to the other depending on their location and the quality of the signal with the local 3G operator. Given that the 3G and satellite communications are very expensive, we want to minimize the amount of data to send. But also, we want to protect ourselves from lost data. What would be the best strategy to ensure with reasonable certainty that the integrity of our data is preserved, while minimizing the amount of redundancy, i.e the amount of data transmitted ? I've read about the zfec codec, but I'm not sure if we need to transmit all the chunks, or if we need to send a hash code along each chunk.

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• When does ref($variable) return 'IO'?

  - by Zaid

  Here's the relevant excerpt from the documentation of the ref function: The value returned depends on the type of thing the reference is a reference to. Builtin types include: SCALAR ARRAY HASH CODE REF GLOB LVALUE FORMAT IO VSTRING Regexp Based on this, I imagined that calling ref on a filehandle would return 'IO'. Surprisingly, it doesn't: use strict; use warnings; open my $fileHandle, '<', 'aValidFile'; close $fileHandle; print ref $fileHandle; # prints 'GLOB', not 'IO' perlref tries to explain why: It isn't possible to create a true reference to an IO handle (filehandle or dirhandle) using the backslash operator. The most you can get is a reference to a typeglob, which is actually a complete symbol table entry [...] However, you can still use type globs and globrefs as though they were IO handles. In what circumstances would ref return 'IO' then?

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• PHP - Loop thru recordset and fire event each n rows

  - by Luciano

  I'm looking for the right logic to loop thru a recordset and fire an event each n times. Searching on Google i've found some discussion on similar situations, but it seems that solutions don't fits my needs. Let's say i have a recordset of 22 rows. I want to loop thru each row and launch a function on the 4th, the 8th, the 12th and so on... Using the modulus operator as shown in this answer, if($i % 4 == 0), i get the event fired each 4 rows, but 22 its not a multiple of 4 so the event is fired till the 20th row and then nothing. Maybe i need to make a division counting rows in 'excess'? Since the recordset will be between 50 and 200 rows i think its not necessary run multiple query of 4 rows, am I wrong? Thanks in advance!

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• Having an @ inside of a string - PHP

  - by Msencenb

  So I know that you use a backslash to escape most things in php however the @ symbol is an operator that suppresses error messages. I'm trying to put an email string like this inside of an array "[email protected]", however php is throwing an error. How do you escape the @ sign? EDIT: Here is the code example that was throwing the error: $arr = array(3=> "[email protected]",4=> "[email protected]"); However replacing the double quotes with single quotes fixes the error as answered below.... why is that true?

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• Need a regular expression for an Irish phone number

  - by Eoghan O'Brien

  I need to validate an Irish phone number but I don't want to make it too user unfriendly, many people are used to writing there phone number with brackets wrapping their area code followed by 5 to 7 digits for their number, some add spaces between the area code or mobile operator. The format of Irish landline numbers is an area code of between 1 and 4 digits and a number of between 5 to 8 digits. e.g. (021) 9876543 (01)9876543 01 9876543 (0402)39385 I'm looking for a regular expression for Javascript/PHP. Thanks.

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• SQL Operators as text in where clause

  - by suggy1982

  I have the following table, which is used for storing bandings. The table is maintained via a web frontend. CREATE TABLE [dbo].[Banding]( [BandingID] [int] IDENTITY(1,1) NOT NULL, [ValueLowerLimitOperator] [varchar](10) NULL, [ValueLowerLimit] [decimal](9, 2) NULL, [ValueUpperLimitOperator] [varchar](10) NULL, [ValueUpperLimit] [decimal](9, 2) NULL, [VolumeLowerLimitOperator] [varchar](10) NULL The operator fields store values such as < = <=. I want to get to a position where I can use the operators values stored in the table in a case statement in a where clause. Like this. SELECT * FROM table WHERE CASE ValueLowerLimitOperator WHEN '<' THEN VALUE < X WHEN '>' THEN VALUE > X END rather than having to write mutiple case or if statements for each permutation. Does anyone have any suggestions how I can decode the operators values stored in the table as part of my query and then use them in a case/where statement?

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• Template type deduction with a non-copyable class

  - by Evan Teran

  Suppose I have an autolocker class which looks something like this: template <T> class autolocker { public: autolocker(T *l) : lock(l) { lock->lock(); } ~autolocker() { lock->unlock(); } private: autolocker(const autolocker&); autolocker& operator=(const autolocker&); private: T *lock; } Obviously the goal is to be able to use this autolocker with anything that has a lock/unlock method without resorting to virtual functions. Currently, it's simple enough to use like this: autolocker<some_lock_t> lock(&my_lock); // my_lock is of type "some_lock_t" but it is illegal to do: autolocker lock(&my_lock); // this would be ideal Is there anyway to get template type deduction to play nice with this (keep in my autolocker is non-copyable). Or is it just easiest to just specify the type?

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• C# : Number Conversion Problem

  - by Sayem Ahmed

  Today I faced a strange problem in C#. I have an ASP.NET page where user can enter certain price, quantity etc. I get the price value, convert it to double, then multiply it with 100 and then typecast it to an integer. When the price is "33.30", after converting it to double it remains 33.3 (obviously...), but after multiplying it with 100, it becomes 3329.9999999999995, and when I cast it to integer by applying simple cast operator "(int) (price * 100) ", it becomes 3329. Right now I have no idea why this is happening. So I thought may be you guys can help :) .

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• DataSet with many OR clauses

  - by Silvan

  Hello :) I've got a little problem with a query which I create in the Visual Studio Designer. I need a query with a lot of 'OR'-clauses for the same column. I found the operator 'IN', but I don't know how to use that in the Visual Studio Designer: Example IN: SELECT EmployeeID, FirstName, LastName, HireDate, City FROM Employees WHERE City IN ('Seattle', 'Tacoma', 'Redmond') I tried to do it in this way: SELECT [MachineryId], [ConstructionSiteId], [DateTime], [Latitude], [Longitude], [HoursCounter] FROM [PositionData] WHERE [MachineryID] IN @MachineryIDs But this doesn't work. Is there another way to handle a lot of OR clauses? Thank you very much for your help.

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• Pointing to vectors

  - by Matt Munson

  #include <iostream> #include <vector> using namespace std; int main () { vector <int> qwerty; qwerty.push_back(5); vector <int>* p = &qwerty; cout << p[0]; //error: no match for 'operator<<' in 'std::cout << * p' } I'm generally unclear on how to use pointers with vectors, so I'm pretty mystified as to why this is not working. To my mind, this should print 5 to screen.

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• rails - Redirecting console output to a file

  - by egarcia

  On a bash console, if I do this: cd mydir ls -l > mydir.txt The operator captures the standard input and redirects it to a file; so I get the listing of files in mydir.txt instead of in the standard output. Is there any way to do something similar on the rails console? I've got a ruby statement that generates lots of prints (~8k lines) and I'd like to be able to see it completely, but the console only "remembers" the last 1024 lines or so. So I thought about redirecting to a file - If anyone knows a better option, I'm all ears.

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