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• Haskell: What is the difference between $ (dollar) and $! (dollar exclamation point)

  - by Jelle Fresen

  Can anybody explain the difference in Haskell between the operators ($) and ($!) (dollar sign vs dollar sign exclamation point)? I haven't seen the use of $! anywhere so far, but while browsing through the Haskell reference on www.zvon.org, I noticed its existence and that it has the exact same definition as $. When trying some simple statements in a Haskell interpreter (ghci), I couldn't find any difference, nor could I find any reference to the operator in the top listed tutorials when googling for haskell tutorial. So, just out of curiosity, what is the difference, if at all?

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• Use case modelling for calculator

  - by kyrogue

  hi, i need help modelling a use case diagram from a topic, it will be in java GUI Design a Calculator that 1.Allow user to key in a legitimate arithmetic statement that involves number, operator +, - and bracket '(' and ')' ; 2.When user press “Calculate” button, display result; 3.Some legitimate statement would be ((3+2)-4+2) (equals 3) and (-2+3)-(3-1) (equals -1); 4.You should NOT use a pre-existing function that just take in the statement as a parameter and returns the result but you should write the logic of parsing every character in your code. 5.Store the last statement and answer so it is displayed when user press the “Last calculation” button. i have designed two use case diagrams using UML on netbeans 6.5.1, one of the use case i am not sure whether is it containing too much use cases etc, while the other is what i think could be too vague for the topic.i hope to get some feedback on whether the use case diagram are appropriate, thanks.

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• Get a list of documents from Mongo DB

  - by Andrei Neagu

  Hello, I want to do something like this: List<int> fff = new List<int>(); fff.Add(1); fff.Add(2); fff.Add(5); Mongo m = new Mongo(); m.Connect(); var dataBase = m.GetDatabase("database"); var collection = dataBase.GetCollection("coll"); IMongoQuery queryable = collection.AsQueryable(); MongoQueryProvider prov = new MongoQueryProvider(collection); var query = new MongoQuery(prov); var ffppp = from p221 in query where fff.Contains((int)p221["oid"]) select p221; This throws this error : The method 'Contains' could not be converted into a constant. I saw that mongo has an operator $in. Does any one know how can I use it from c#? (http://www.mongodb.org/display/DOCS/Advanced+Queries) Thanks

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• Is there alternative way to write this query?

  - by Kugel

  I have tables A, B, C, where A represents items which can have zero or more sub-items stored in C. B table only has 2 foreign keys to connect A and C. I have this sql query: select * from A where not exists (select * from B natural join C where B.id = A.id and C.value > 10); Which says: "Give me every item from table A where all sub-items have value less than 10. Is there a way to optimize this? And is there a way to write this not using exists operator?

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• Placement new in gcc

  - by Roman Prikhodchenko

  I need to find a workaround for a bug with placement new in g++. I now it was fixed in gcc-4.3 but I have to support versions 4.2 and 4.1. For example, following code compiles with an error "error: no matching function for call to 'operator new(long unsigned int, void*&)" template<class T, template<typename> class Alloc> inline void* type_ctor() { Alloc<T> a; void* p = a.allocate(1); new(p) T; return p; } ..... type_ctor<A, NewAllocator >();

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• how to get the size of a C global array into an assembly program written for the avr architecture co

  - by johannes

  I have a .c file with the following uint8_t buffer[32] I have a .S file where I want to do the following cpi r29, buffer+sizeof(buffer) The second argument for cpi muste be an imidiate value not a location. But unfortunetly sizeof() is a c operator. Both files, are getting compiled to seperate object files and linked afterwards. If I do avr-objdump -x file.c. Amongst other things, I get the size of the buffer. So it is already available in the object file. How do I access the size of the buffer in my assembly file at compile time?

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• How do you assign a variable with the result of a if..else block?

  - by Pierre Olivier Martel

  I had an argument with a colleague about the best way to assign a variable in an if..else block. His orignal code was : @products = if params[:category] Category.find(params[:category]).products else Product.all end I rewrote it this way : if params[:category] @products = Category.find(params[:category]).products else @products = Product.all end This could also be rewritten with a one-liner using a ternery operator (? :) but let's pretend that product assignment was longer than a 100 character and couldn't fit in one line. Which of the two is clearer to you? The first solution takes a little less space but I thought that declaring a variable and assigning it three lines after can be more error prone. I also like to see my if and else aligned, makes it easier for my brain to parse it!

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• C# implicit conversions

  - by Chris Boden

  Hello, I'm currently working on an application where I need to load data from an SQL database and then assign the retrieved values into the properties of an object. I'm doing this by using reflection since the property names and column names are the same. However, many of the properties are using a custom struct type that is basically a currency wrapper for the decimal type. I have defined an implicit conversion in my struct: public static implicit operator Currency(decimal d) { return new Currency(d); } This works fine when I use it in code. However, when I have this: foreach (PropertyInfo p in props) { p.SetValue(this, table.Rows[0][p.Name], null); } It throws an ArgumentException stating that it cannot convert from System.Decimal to Currency. I'm confused since it works fine in any other circumstance.

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• matlab noninteger step indexing

  - by rlbond

  So, I have a vector: k = 1:100; And I want to take 19 elements from it, which are roughly equally-spaced. So I write this: m = k(1:(99/18):end); This works great, except for a tiny problem: Warning: Integer operands are required for colon operator when used as index m = 1 7 12 18 23 29 34 40 45 51 56 62 67 73 78 84 89 95 100 Now, I understand why this comes up, but I'd like to get rid of that warning. Is there a "right" way to do this without a warning?

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• Postgres column casting...

  - by Simon

  I have a query SELECT assetid, type_code, version, name, short_name, status, languages, charset, force_secure, created, created_userid, updated, updated_userid, published, published_userid, status_changed, status_changed_userid FROM sq_ast WHERE assetid = 7 which doesn't work and throws ERROR: operator does not exist: character varying = integer LINE 4: FROM sq_ast WHERE assetid = 7 I can get it to work by doing SELECT assetid, type_code, version, name, short_name, status, languages, charset, force_secure, created, created_userid, updated, updated_userid, published, published_userid, status_changed, status_changed_userid FROM sq_ast WHERE assetid = '7' Please note the quoting of the 7 in the WHERE clause... I am deploying an huge application and I cannot rewrite the core... similarly I don't want to risk changing the type of the column... I'm no Postgres expert... please help... Is there an option for strict casting of columns???

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• How does sizeof calculate the size of structures

  - by Gearoid Murphy

  I know that a char and an int are calculated as being 8 bytes on 32 bit architectures due to alignment, but I recently came across a situation where a structure with 3 shorts was reported as being 6 bytes by the sizeof operator. Code is as follows: #include <iostream> using namespace std ; struct IntAndChar { int a ; unsigned char b ; }; struct ThreeShorts { unsigned short a ; unsigned short b ; unsigned short c ; }; int main() { cout<<sizeof(IntAndChar)<<endl; // outputs '8' cout<<sizeof(ThreeShorts)<<endl; // outputs '6', I expected this to be '8' return 0 ; } Compiler : g++ (Debian 4.3.2-1.1) 4.3.2. This really puzzles me, why isn't alignment enforced for the structure containing 3 shorts?

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• How to read from a method that returns a filehandle in perl?

  - by Ryan Thompson

  I have an object with a method that returns a filehandle, and I want to read from that handle. The following doesn't work, because the right angle bracket of the method call is interpreted as the closing angle bracket of the input reader: my $input = <$object->get_handle()>; That gets parsed as: my $input = ( < $object- > ) get_handle() >; which is obviously a syntax error. Is there any way I can perform a method call within an angle operator, or do I need to break it into two steps like this? my $handle = $object->get_handle(); my $input = <$handle>;

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• Does std::multiset guarntee insertion order?

  - by Naveen

  I have a std::multiset which stores elements of class A. I have provided my own implementation of operator< for this class. My question is if I insert two equivalent objects into this multiset is their order guaranteed? For example, first I insert a object a1 into the set and then I insert an equivalent object a2 into this set. Can I expect the a1 to come before a2 when I iterate through the set? If no, is there any way to achieve this using multiset?

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• Completion block not being called. How to check validity?

  - by HCHogan

  I have this method which takes a block, but that block isn't always called. See the method: - (void)updateWithCompletion:(void (^)(void))completion { [MYObject myMethodWithCompletion:^(NSArray *array, NSError *error) { if (error) { NSLog(@"%s, ERROR not nil", __FUNCTION__); completion(); return; } NSLog(@"%s, calling completion %d", __FUNCTION__, &completion); completion(); NSLog(@"%s, finished completion", __FUNCTION__); }]; } I have some more NSLogs inside completion. Sometimes this program counter just blows right past the call to completion() in the code above. I don't see why this would be as the calling code always passes a literal block of code as input. If you're curious of the output of the line containing the addressof operator, it's always something different, but never 0 or nil. What would cause completion not to be executed?

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• EXCEL workbook, intermitently, takes 30 seconds to load

  - by Julio Nobre

  I am trying to figure out why a simple .XLS EXCEL workbook is taking, randomly, 30 seconds to open. Before answering: Please, bear mind the following: Problem symptoms Hanging is intermitent and it takes exactly 30 seconds; During hanging there is no cpu or disk activity; It only happens during workbook load. Every runs smooth after that; Windows Explorer.exe hangs on folder, but all other folders, system and applications are still responsive; There are no consecutive hangings. I have to wait for while to reproduce this behaviour; All workbooks where located on a local drive (C:\BPI); The workbook has no macros and no addins; Office 2003 is being used for several years; The computer is running Windows XP; Computer has several network mapped drives, all addressed to main file server; Recently, main fileserver was replaced by Windows 2011 SBS Standard Edition What I have done so far I have traced machine Explorer.exe, using Process Monitor, added Duration column, and filtered by Duration 1. That's is how I found that hanging was taking exactly 30 seconds. For further information, please refer to Oliver Salzburg tutorial. Using Process Monitor, I have also figured out than five operations were taking most of sample collecting duration. Looking at sample image below, column Operation below you will notice that one single operation was taking 29 seconds; I have tried different workbooks (all of them smaller than 30 KB); I have, temporarily, removed all shortcuts on User Document's folder that were pointing to network drives or shares; I have runned CCleaner to fix registry issues; I made sure that there were no external links on tested workbooks; I have reproduced this behaviour for hours; I have extensivelly researched for hours on the web; Process Monitor's collected and filtered data

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• Selecting an element based on class and id in jquery - similar to an SQL query

  - by Ankur

  I want to select an element based on it's class and id, I have seen many similar questions on SO. Most of the users ask why would you ever want to do this - which is not very helpful, trust me, this is the simplest way of solving my UI problem, and it's a very unique and intereting UI related to making generic triple stores easily browsable. At the moment the jQUery looks like this: $(document).ready( function() { $(".predicates").hide(); $(".objects").hide(); $("#subject").click( function() { $("#predId, .predicates").toggle(); // this line } ); $("#predId").click( function() { $("#objId, .objects").toggle(); } ); }); The line I am really interested in is this $("#predId, .predicates").toggle(); how do I get the select to ensure that both #predId and .predicates are true, at the moment it seems that only one or the other has to be true. Is there something like an && operator?

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• std::map default value for build-in type

  - by Qifa Zhao

  Recently, I was confused by the std::map operator[] function. In the MSDN library, it says: "If the argument key value is not found, then it is inserted along with the default value of the data type." I tryed to search much more exactly explanation for this issue. For example here: std::map default value In this page, Michael Anderson said that "the default value is constructed by the default constructor(zero parameter constructor)". Now my quest comes to this:"what the default value for the build-in type?". Was it compiler related? Or is there a standard for this issue by the c++ stardard committee? I did a test on visual studio 2008 for the "int" type, and found the "int" type is construted with the value 0.

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• g++ How to get warning on ignoring function return value

  - by ArunSaha

  lint produces some warning like: foo.c XXX Warning 534: Ignoring return value of function bar() From the lint manual 534 Ignoring return value of function 'Symbol' (compare with Location) A function that returns a value is called just for side effects as, for example, in a statement by itself or the left-hand side of a comma operator. Try: (void) function(); to call a function and ignore its return value. See also the fvr, fvo and fdr flags in §5.5 "Flag Options". I want to get this warning, if there exists any, during compilation. Is there any option in gcc/g++ to achieve this? I had turned on -Wall but that apparently did not detect this.

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• Weird behaviour with vector::erase and std::remove_if with end range different from vector.end()

  - by Edison Gustavo Muenz

  Hi, I need to remove elements from the middle of a std::vector. So I tried: struct IsEven { bool operator()(int ele) { return ele % 2 == 0; } }; int elements[] = {1, 2, 3, 4, 5, 6}; std::vector<int> ints(elements, elements+6); std::vector<int>::iterator it = std::remove_if(ints.begin() + 2, ints.begin() + 4, IsEven()); ints.erase(it, ints.end()); After this I would expect that the ints vector have: [1, 2, 3, 5, 6]. In the debugger of Visual studio 2008, after the std::remove_if line, the elements of ints are modified, I'm guessing I'm into some sort of undefined behaviour here. So, how do I remove elements from a Range of a vector?

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• How can I match at the beginning of any line, including the first, with a Perl regex?

  - by JoelFan

  According the the Perl documentation on regexes: By default, the "^" character is guaranteed to match only the beginning of the string ... Embedded newlines will not be matched by "^" ... You may, however, wish to treat a string as a multi-line buffer, such that the "^" will match after any newline within the string ... you can do this by using the /m modifier on the pattern match operator. The "after any newline" part means that it will only match at the beginning of the 2nd and subsequent lines. What if I want to match at the beginning of any line (1st, 2nd, etc.)? EDIT: OK, it seems that the file has BOM information (3 chars) at the beginning and that's what's messing me up. Any way to get ^ to match anyway? EDIT: So in the end it works (as long as there's no BOM), but now it seems that the Perl documentation is wrong, since it says "after any newline"

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• Storing multiple checkbox values in database

  - by Madjokr

  Hi, I want to store multiple column values in table.Lets take a example .. What are your favourite colors? the choices can be red,blue,green, orange. So lets assume, the user selects atleast 2 values. Is there any way to store the multiple values in table. I have implemented by concatinating choices of users in a column in the table. I later found that it is a bad practise. Currently i can think of using Bitwise operator and habtm. What are the different ways for storing multiple choices values in table? If I am implementing in rails, What is the best way to implement this with OOP concepts? Is there any builtin options in rails?

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• Intermittent extrememly long response times when downloading documents

  - by pap

  I have a Java web application running om Tomcat 7 with an Apache httpd 2.2 fronting with mod_jk/AJP. One part of the application is serving files (up to 4mb size). Now, normally this all runs very smooth with stable, low response-times. However, in rare instances (<0.1% of downloads), the downloadtime will go beyond 1 minute. After activating the ThreadStuckValve in Tomcat, I can see that the long responses seem to be stuck at org.apache.tomcat.jni.Socket.sendbb(Native method) i.e network I/O. At most, these long-running downloads take 5 minutes, which I strongly suspect is because of the default 300 second timout in Apache 2.2 (http://httpd.apache.org/docs/2.2/mod/core.html, "TimeOut directive"). To me, this looks like network problems. The Apache timeout (if that is what is kicking in at the 5 minute mark) indicates that ACK packets are not being transmitted correctly. My questions are what could be causing this? Closed browser at receiving end but socket not signaled as closed properly? Packet loss or some other network failure in transit? Where would I start troubleshooting this? We're running Tomcat and Apache on Windows server 2008-R2 in a vmware virtualized server.

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• Why will this code compile using ifort compiler and not when using gfortran compiler? Help!

  - by CuriousCompiler

  I'm rewriting some code to make a program compile with the gfortran compiler as opposed to ifort compiler I usually use. The code follows: _Subroutine SlideBits (WORD, BITS, ADDR) Implicit None Integer(4) WORD Integer(4) BITS Integer(4) ADDR Integer(4) ADDR1 ADDR1 = 32 - ADDR WORD = (WORD .And. (.Not.ISHFT(1,ADDR1))) .Or. ISHFT(BITS,ADDR1) End_ When I compile the above code using the gfortran compiler, I recieve this error: WORD = (WORD .And. (.Not.ISHFT(1,ADDR1))) .Or. ISHFT(BITS,ADDR1) Error: Operand of .NOT. operator at (1) is INTEGER(4) All three of the variables coming into the subroutine are integers. I've looked around a bit and the gfortran wiki states that the gfortran compiler should be able to handle logical statments being applied to integer values. Several other sites I've visited either quote from the gnu wiki or agree with it. This is the first time I've seen this error as the Intel Fortran compiler (ifort) I normally use compiles cleanly.

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• Is this call to a function object inlined?

  - by dehmann

  In the following code, Foo::add calls a function via a function object: struct Plus { inline int operator()(int x, int y) const { return x + y; } }; template<class Fct> struct Foo { Fct fct; Foo(Fct f) : fct(f) {} inline int add(int x, int y) { return fct(x,y); // same efficiency adding directly? } }; Is this the same efficiency as calling x+y directly in Foo::add? In other words, does the compiler typically directly replace fct(x,y) with the actual call, inlining the code, when compiling with optimizations enabled?

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• Is there a better way than #if DebugMode for logging

  - by Daniel

  I'm making a c++ library thats going to be P/Invoked from c#, so i am unable to breakpoint/debug the c++ side of things. So i decided to add logging so i can see if anything goes wrong and where it happens. I add a #define DebugMode 1 in order to determine if i am to log or not. First of all i'm not very good at c++ but i know enough to get around. So my questions are: Is there a better way than wrapping #if DebugMode #endifs around every Log call? I could simply do that inside the method and just return if logging isn't disabled but won't that mean then all those logging strings will be in the assembly? How can i emulate what printf does with its "..." operator enabling me to pass something like Log("Variable x is {0}", x); Thanks!

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