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• How to make Databinding type safe and support refactoring

  - by Ian Ringrose

  When I wish to bind a control to a property of my object, I have to provide the name of the property as a string. This is not very good because: If the property is removed or renamed, I don’t get a compiler warning. If a rename the property with a refactoring tool, it is likely the data binding will not be updated. I don’t get an error until runtime if the type of the property is wrong, e.g. binding an integer to a date chooser. Is there a design-paten that gets round this, but still has the ease of use of data-binding? (This is a problem in WinForm, Asp.net and WPF and most likely lots of other systems) I have now found "workarounds for nameof() operator in C#: typesafe databinding" that also has a good starting point for a solution.

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• R: ggplot2, how to add a number of layers to a plot at once to reduce code

  - by John

  library(ggplot2) This code produces a nice looking plot: qplot(cty, hwy, data = mpg, colour = displ) + scale_y_log2() + labs(x="x axis") + labs(y="y axis") + opts(title = "my title") But I want to setup variables to try and to reduce code repetition: log_scale <- scale_y_log2() xscale <- labs(x="x axis") yscale <- labs(y="y axis") title <- opts(title = "my title") my_scales <- c(log_scale, xscale, yscale, title) # make a variable to hold the scale info changes above So that I can do this and add a bunch of things at the same time: qplot(cty, hwy, data = mpg, colour = displ) + my_scales # add these to your plot. but I get this error: Error in object$class : $ operator is invalid for atomic vectors I realize that the things going into my_scales need to be layers / different types of objects, but I don't see what they should be.

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• Practical differences between classes and structs in .net (not conceptual)?

  - by Gulshan

  Whenever I tried to search about differences between classes and structs in C# or .net, I ended up with the conceptual overview of the two things like value type or the reference type, where the variables are allocated etc. But I need some practical differences. I have found some like different behavior of assignment operator, having constructors etc. Can anybody provide some more practical differences which will be directly useful while coding? Like the things works with one but not with other or same operation showing different behavior. And some common mistakes regarding these two. Also please suggest where to consider using a struct instead of a class. And where the structs should not be used.

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• Error while excuting a simple boost thread program

  - by Eternal Learner

  Hi All, Could you tell mw what is the problem with the below boost::thread program #include<iostream> #include<boost/thread/thread.hpp> boost::mutex mutex; class A { public: A() : a(0) {} void operator()() { boost::mutex::scoped_lock lock(mutex); } private: int a; }; int main() { boost::thread thr1(A()); boost::thread thr2(A()); thr1.join(); thr2.join(); } I get the error message: error: request for member 'join' in 'thr1', which is of non-class type 'boost::thread()(A ()())' BoostThread2.cpp:30: error: request for member 'join' in 'thr2', which is of non-class type 'boost::thread ()(A ()())'

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• varchar comparison in SQL Server

  - by Ram

  I am looking for some SQL varchar comparison function like C# string.compare (we can ignore case for now, should return zero when the character expression are same and a non zero expression when they are different) Basically I have some alphanumeric column in one table which needs to be verified in another table. I cannot do select A.col1 - B.col1 from (query) as "-" operator cannot be applied on character expressions I cannot cast my expression as int (and then do a difference/subtraction) as it fails select cast ('ASXT000R' as int) Conversion failed when converting varchar 'ASXT000R' to int Soundex would not do it as soundex is same for 2 similar strings Difference would not do it as select difference('abc','ABC') = 4 (as per msdn, difference is the difference in the soundex of 2 character expressions and difference =4 implies least different) Is there any other way of doing it ?

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• LINQ Quicksort is Unstable Except When Cascading

  - by Mystagogue

  On page 64 of "LINQ To Objects Using C# 4.0" (Tony Magennis) he states that LINQ's quicksort ordering algorithm is unstable... ...although this is simply solved by cascading the result into a ThenBy or ThenByDescending operator. Huh? Why would cascading an unstable sortation into another sortation fix the result? In fact, I'd say that isn't possible. The original order, once passed through an unstable sort, is simply lost. What am I missing here?

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• static arrays defined with unspecified size, empty brackets?

  - by ahmadabdolkader

  For the C++ code fragment below: class Foo { int a[]; // no error }; int a[]; // error: storage size of 'a' isn't known void bar() { int a[]; // error: storage size of 'a' isn't known } why isn't the member variable causing an error too? and what is the meaning of this member variable? I'm using gcc version 3.4.5 (mingw-vista special) through CodeBlocks 8.02. On Visual Studio Express 2008 - Microsoft(R) C/C++ Optimizing Compiler 15.00.30729.01 for 80x86, I got the following messages: class Foo { int a[]; // warning C4200: nonstandard extension used : zero-sized array in struct/union - Cannot generate copy-ctor or copy-assignment operator when UDT contains a zero-sized array }; int a[]; void bar() { int a[]; // error C2133: 'a' : unknown size } Now, this needs some explaination too.

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• Dynamically allocated structure and casting.

  - by Simone Margaritelli

  Let's say I have a first structure like this: typedef struct { int ivalue; char cvalue; } Foo; And a second one: typedef struct { int ivalue; char cvalue; unsigned char some_data_block[0xFF]; } Bar; Now let's say I do the following: Foo *pfoo; Bar *pbar; pbar = new Bar; pfoo = (Foo *)pbar; delete pfoo; Now, when I call the delete operator, how much memory does it free? sizeof(int) + sizeof(char) Or sizeof(int) + sizeof(char) + sizeof(char) * 0xFF ? And if it's the first case due to the casting, is there any way to prevent this memory leak from happening? Note: please don't answer "use C++ polymorphism" or similar, I am using this method for a reason.

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• What does the symbol :=: mean

  - by Dan Maguire

  I've found the symbol :=: in some Clarion code and I can't seem to figure out exactly what it does. The code was written by a previous developer many years ago, so I can't ask him. I also have not been able to find any results for "colon equals colon" in Google. Here is an example of the code, where bufSlcdpaDtl is a file object: lCCRecord Like(bufSlcdpaDtl),Pre(lCCRecord) ! ...other stuff... lCCRecord :=: bufSlcdpaDtl I'm wondering if it's something similar to ::= in Python or possibly the assignment operator :=.

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• Ruby RegEx not matching valid expression

  - by Matthew Carriere

  I have the following expression: ^\w(\s(+|-|\/|*)\s\w)*$ This simply looks to match a mathematical expression, where a user is prompted for terms separated by basic operators (ex: price + tax) The user may enter more than just 2 terms and one operator (ex: price + tax + moretax) I tested this expression in Rubular http://rubular.com/ With the terms: a + a (MATCH) a + a + a (MATCH) a + a + a + a a a + a a Everything works, but when I use it in Ruby it does not work! expression =~ /^\w(\s(+|-|\/|*)\s\w)*$/ I started picking the expression apart and noticed that if I remove the start of line caret it finds matches but isn't correct. a + a (MATCH) a a (MATCH) <-- this is not correct Why is this expression not working in Ruby code? (I am using Ruby 1.8.7 p174)

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• How does this C# asp.net random password code work?

  - by quakkels

  Hello all, I'm new to .NET and C# and I'm trying to figure out how this code works: public static string CreateRandomPassword(int PasswordLength) { String _allowedChars = "abcdefghijkmnopqrstuvwxyzABCDEFGHJKLMNOPQRSTUVWXYZ23456789"; Byte[] randomBytes = new Byte[PasswordLength]; RNGCryptoServiceProvider rng = new RNGCryptoServiceProvider(); rng.GetBytes(randomBytes); char[] chars = new char[PasswordLength]; int allowedCharCount = _allowedChars.Length; for(int i = 0;i<PasswordLength;i++) { /// /// I don't understand how this line works: /// chars[i] = _allowedChars[(int)randomBytes[i] % allowedCharCount]; } return new string(chars); } I think I've got a pretty good handle on most of this. I haven't been able to understand the following line: chars[i] = _allowedChars[(int)randomBytes[i] % allowedCharCount]; I understand that the code generates random binary numbers and uses those random numbers in the for loop to select a character from the _allowedChars string. What I don't get is why this code uses the modulous operator (%) to get the _allowedChars index value. Thanks for any help

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• n++ vs n=n+1. Which one is faster

  - by piemesons

  Somebody asked me Is n++ faster than n=n+1? My answer:-- ++ is a unary operator in C which(n++) takes only one machine instruction to execute while n=n+1 takes more than one machine instructions to execute. Anyone correct me if I am wrong, but in Assembler it take something like this: n++: inc n n = n + 1; mov ax n add ax 1 mov n ax its not exactli this, but it's near it.but in most cases a good compiler will change n = n + 1 to ++n.So A good compiler will generate same code for both and hence the same time to execute.

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• Implement functionality in PHP?

  - by Rachel

  How can we Implement Bisect Python functionality in PHP Implement function bisect_left($arr, $item); as a pure-PHP routine to do a binary-bisection search for the position at which to insert $item into $list, maintaining the sort order therein. Assumptions: Assume that $arr is already sorted by whatever comparisons would be yielded by the stock PHP < operator, and that it's indexed on ints. The function should return an int, representing the index within the array at which $item would be inserted to maintain the order of the array. The returned index should be below any elements in $arr equal to $item, i.e., the insertion index should be "to the left" of anything equal to $item. Search routine should not be linear! That is, it should honor the name, and should attempt to find it by iteratively bisecting the list and comparing only around the midpoint.

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• Help using Horner's rule and Hash Functions in JAVA?

  - by Matt

  I am trying to use Horner's rule to convert words to integers. I understand how it works and how if the word is long, it may cause an overflow. My ultimate goal is to use the converted integer in a hash function h(x)=x mod tableSize. My book suggests, because of the overflow, you could "apply the mod operator after computing each parenthesized expression in Horner's rule." I don't exactly understand what they mean by this. Say the expression looks like this: ((14*32+15)*32+20)*32+5 Do I take the mod tableSize after each parenthesized expression and add them together? What would it look like with this hash function and this example of Horner's rule?

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• Optimal method to create a large string containing several variables?

  - by Runcible

  I want to create a string that contains many variables: std::string name1 = "Frank"; std::string name2 = "Joe"; std::string name3 = "Nancy"; std::string name4 = "Sherlock"; std::string sentence; sentence = name1 + " and " + name2 + " sat down with " + name3; sentence += " to play cards, while " + name4 + " played the violin."; This should produce a sentence that reads Frank and Joe sat down with Nancy to play cards, while Sherlock played the violin. My question is: What is the optimal way to accomplish this? I am concerned that constantly using the + operator is ineffecient. Is there a better way?

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• Creating a Rails query from a hash of user input

  - by Jamie

  I'm attempting to create a fairly complex search engine for a project using a variable number of search criteria. The user input is sorted into an array of hashes. The hashes contain the following information: { :column => "", :value => "", :operator => "", # Such as: =, !=, <, >, etc. :and_or => "", # Two possible values: "and" and "or" } How can I loop through this array and use the information in these hashes to make an ActiveRecord WHERE query?

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• How to model a social news feed on Google App Engine

  - by PEZ

  We want to implement a "News feed" where a user can see messages broadcasted by her friends, sorted with newest message first. But the feed should reflect changes in her friends list. (If she adds new friends, messages from those should be included in the feed, and if she removes friends their messages should not be included.) If we use the pubsub-test example and attach a recipient list to each message this means a lot of manipulation of the message recipients lists when users connect and disconnect friends. We first modeled publish-subscribe "fan out" using conventional RDBMS thinking. It seemed to work at first, but then, since the IN operator works the way it does, we quickly realized we couldn't continue on that path. We found Brett Slatkin's presentation from last years Google I/O and we have now watched it a few times but it isn't clear to us how to do it with "dynamic" recipient lists. What we need are some hints on how to "think" when modeling this.

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• Calculating co-ordinate of a point on a path given a distance

  - by Alex

  I'm working on a project that surveys the condition of a road or highway using a calibrated trip computer connected to a rugged-PC. An operator keys in defect codes as they travel along a pre-defined route. I need to show an indicator on the map screen that shows the vehicles current position, taking into account the distance data from the trip computer. I know the exact lat lon co-ordinates at the starting point of each section of road, and the road is made up of a series of points. The question is: how can I calculate the lat lon co-ordinates of the vehicle assuming that it has continued on the route and traveled a certain distance (e.g. 1.4km). The co-ordinates would be 'locked onto' the road line, as shown in blue on the diagram below. Thanks, Alex

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• Looking for a better way to integrate a static list into a set of classes

  - by EvilTeach

  I'm trying to expand my sons interest from Warcraft 3 programming into C++ to broaden his horizons to a degree. We are planning on porting a little game that he wrote. The context goes something like this. There are Ships and Missiles, for which Ships will use Missiles and interact with them A Container exists which will hold 'a list' of ships. A Container exists which will hold 'a list' of planets. One can apply a function over all elements in the Container (for_each) Ships and Missles can be created/destroyed at any time New objects automatically insert themselves into the proper container. I cobbled a small example together to do that job, so we can talk about topics (list, templates etc) but I am not pleased with the results. #include <iostream> #include <list> using namespace std; /* Base class to hold static list in common with various object groups */ template<class T> class ObjectManager { public : ObjectManager ( void ) { cout << "Construct ObjectManager at " << this << endl; objectList.push_back(this); } virtual ~ObjectManager ( void ) { cout << "Destroy ObjectManager at " << this << endl; } void for_each ( void (*function)(T *) ) { for (objectListIter = objectList.begin(); objectListIter != objectList.end(); ++objectListIter) { (*function)((T *) *objectListIter); } } list<ObjectManager<T> *>::iterator objectListIter; static list<ObjectManager<T> *> objectList; }; /* initializer for static list */ template<class T> list<ObjectManager<T> *> ObjectManager<T>::objectList; /* A simple ship for testing */ class Ship : public ObjectManager<Ship> { public : Ship ( void ) : ObjectManager<Ship>() { cout << "Construct Ship at " << this << endl; } ~Ship ( void ) { cout << "Destroy Ship at " << this << endl; } friend ostream &operator<< ( ostream &out, const Ship &that ) { out << "I am a ship"; return out; } }; /* A simple missile for testing */ class Missile : public ObjectManager<Missile> { public : Missile ( void ) : ObjectManager<Missile>() { cout << "Construct Missile at " << this << endl; } ~Missile ( void ) { cout << "Destroy Missile at " << this << endl; } friend ostream &operator<< ( ostream &out, const Missile &that ) { out << "I am a missile"; return out; } }; /* A function suitable for the for_each function */ template <class T> void show ( T *it ) { cout << "Show: " << *it << " at " << it << endl; } int main ( void ) { /* Create dummy planets for testing */ Missile p1; Missile p2; /* Demonstrate Iterator */ p1.for_each(show); /* Create dummy ships for testing */ Ship s1; Ship s2; Ship s3; /* Demonstrate Iterator */ s1.for_each(show); return 0; } Specifically, The list is effectively embedded in each ship though the inheritance mechanism. One must have a ship, in order to access the list of ships. One must have a missile in order to be able to access the list of missiles. That feels awkward. My question boils down to "Is there a better way to do this?" Automatic object container creation Automatic object insertion Container access without requiring an object in the list to access it. I am looking for better ideas. All helpful entries get an upvote. Thanks Evil.

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• How to delete characters and append strings?

  - by devin250

  i am adding a new record to xml file im first quering all existing items and storing the count in an int int number = query.count() and then incrementing number by 1; number = number +1; now i want to format this value in a string having "N00000000" format and the number will ocuppy the last positions Pseudo code: //declare the format string sting format = "N00000000" //calculate the length of number string int length =number.ToString().Length(); // delete as many characters from right to left as the length of number string ??? // finally concatenate both strings with + operator ??? help please

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• Run time error '3075' in Access 2007

  - by Thys

  I am getting the following error when I try to open a report in Access 2007. The code works fine in Access 2003. run time error '3075' Syntax error (missing operator) in query expression '[COUNTRY_ID]=' here is the code giving the error... How could I fix this? Private Sub List25_Click() Combo20.SetFocus 'DoCmd.FindRecord List25.ItemData(List25.ListIndex) Forms![Country Rate Administration].Filter = "[COUNTRY_ID]=" & List25.ItemData(List25.ListIndex) Forms![Country Rate Administration].FilterOn = True End Sub Thansk in advance for your help!

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• What programs should I write to truly experience this fancy new language ?

  - by privatehuff

  Tried Scheme at one point, just built up half of a "math" and "string" library before getting bored... Similar experience with Java, but stopped early because I was appalled at the lack of operator overloading. When you try out a new language, is there a program/game/function/exercise/problem that you use to get into the hot meaty center and really EXPERIENCE the language? I've been wanted to try Python, Ruby, some lisps, etc but can't seem to find any meaningful work to do with them, or any reason to use them for anything over languages I already know. Sorry this is a discussion, but you are EXACTLY the people I want to get input from on this

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• Cross-reference between delphi records

  - by Paul-Jan

  Let's say I have a record TQuaternion and a record TVector. Quaternions have some methods with TVector parameters. On the other hand, TVector supports some operations that have TQuaternion parameters. Knowing that Delphi (Win32) does not allow for forward record declarations, how do I solve this elegantly? Using classes is not really an option here, because I really want to use operator overloading for this rare case where it actually makes good sense. For now I simply moved these particular methods out of the records and into separate functions, the good old-fashioned way. Better suggestions are most welcome.

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• Copy constructor demo (crashing... case 2)

  - by AKN

  Please have a glance at this program: class CopyCon { public: char *name; CopyCon() { name = new char[20]; name = "Hai";//_tcscpy(name,"Hai"); } CopyCon(const CopyCon &objCopyCon) { name = new char[_tcslen(objCopyCon.name)+1]; _tcscpy(name,objCopyCon.name); } ~CopyCon() { if( name != NULL ) { delete[] name; name = NULL; } } }; int main() { CopyCon obj1; CopyCon obj2(obj1); cout<<obj1.name<<endl; cout<<obj2.name<<endl; } This program crashes on execution. Error: "Expression: _BLOCK_TYPE_IS_VALID(pHead-nBlockUse)" If I assign "Hai" to name using aasignment operator, its crashing. Where as when I use string func _tcscpy to assign "Hai" to name, its working perfectly. Can some one explain why so?

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• Logic differences in C and Java

  - by paragjain16

  Compile and run this code in C #include <stdio.h> int main() { int a[] = {10, 20, 30, 40, 50}; int index = 2; int i; a[index++] = index = index + 2; for(i = 0; i <= 4; i++) printf("%d\n", a[i]); } Output : 10 20 4 40 50 Now for the same logic in Java class Check { public static void main(String[] ar) { int a[] = {10, 20, 30, 40, 50}; int index = 2; a[index++] = index = index + 2; for(int i = 0; i <= 4; i++) System.out.println(a[i]); } } Output : 10 20 5 40 50 Why is there output difference in both languages, output is understandable for Java but I cannot understand output in C One more thing, if we apply the prefix ++ operator, we get the same result in both languages, why?

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