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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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• 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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• Python/Tkinter Audio Player

  - by Nicholas Quirk

  Hey everyone reading this, I've recently got into doing GUI development with Python. Tkinter seems like the easiest and most logical choice starting out. I did a little with wxPython but it was more sophisticated than what I needed. Anyway, I'm developing a media player. Right now it's a simple window with a button to load .wav files. The problem is I would like to implement a pause button now. But, when playing a audio file the GUI isn't accessible again (no buttons can be pushed) till the file is done playing. How can I make the GUI dynamic while an audio file is playing? I was thinking this maybe be because I'm using PyAudio, and their implementation doesn't allow this. Anyway, thanks for any advice before hand.

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• How does a programmer think?

  - by Gordon Potter

  This may be a hopelessly vague question. But I am interested to hear whatever logical thought processes people go through when learning a new concept or trying to get their brain around code they might not have ever seen before. Basically, what general steps does one take to to break down problems and what does it take to "get it"? If you were to diagram a flowchart of how your mental process works when you look at code or try to solve a problem what might it look like? What common references, tips, and mental assumptions do you find useful in problem solving? How is this different between different domains? For example in what ways is a web programmer's thought process similar or different from a traditional desktop app developer's process?

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• Difference between try-finally and try-catch

  - by Vijay Kotari

  What's the difference between try { fooBar(); } finally { barFoo(); } and try { fooBar(); } catch(Throwable throwable) { barFoo(throwable); // Does something with throwable, logs it, or handles it. } I like the second version better because it gives me access to the Throwable. Is there any logical difference or a preferred convention between the two variations? Also, is there a way to access the exception from the finally clause?

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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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• 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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• construct test environment for web application on PC - directory issues

  - by ernie

  I have a site that physically has this directory structure. -public_html --conf > contains file conf.php -SiteFiles -LiveSite > contains file ConfLive.php Directory public_html/conf/ contains a file called conf.php this file contains the following include include_once('/home/mydir/SiteFiles/LiveSite/conf/ConfLive.iphp'); I want to copy this application to test PC to test it. Test PC uses XAMPP Apache. "Root" directory on the test machine is: C:\xampp\htdocs\ My questions: 1. Where is logical path "/home/mydir/" defined? 2. What steps should I take to get this to work on my test machine preferably by server configuration and not changing application. Thanks. (PS maybe this question is better posed at Server Overflow site.)

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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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• 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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• 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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• 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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• 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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• 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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• 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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• Finding recurrence relations of an algorithm

  - by Roarke

  I'm reading my algorithms text book, and I'm reading about recurrence relations and finding the algorithms big O complexity. I run across this line "In the case of the merge-sort algorithm, we get the recurrence equation: t(n) = b if n < 2 = 2t(n/2) +bn if n >= 2 for b > 0 my response was "how the heck did we know that?!?!" So i'm wondering if there is a systematic approach, or just a logical way of getting these recurrence relations from the algorithms can some one explain where the b and the two 2's come from?

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• Optimal password salt length

  - by Juliusz Gonera

  I tried to find the answer to this question on Stack Overflow without any success. Let's say I store passwords using SHA-1 hash (so it's 160 bits) and let's assume that SHA-1 is enough for my application. How long should be the salt used to generated password's hash? The only answer I found was that there's no point in making it longer than the hash itself (160 bits in this case) which sounds logical, but should I make it that long? E.g. Ubuntu uses 8-byte salt with SHA-512 (I guess), so would 8 bytes be enough for SHA-1 too or maybe it would be too much?

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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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• 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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• 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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• Python threads all executing on a single core

  - by Rob Lourens

  I have a Python program that spawns many threads, runs 4 at a time, and each performs an expensive operation. Pseudocode: for object in list: t = Thread(target=process, args=(object)) # if fewer than 4 threads are currently running, t.start(). Otherwise, add t to queue But when the program is run, Activity Monitor in OS X shows that 1 of the 4 logical cores is at 100% and the others are at nearly 0. Obviously I can't force the OS to do anything but I've never had to pay attention to performance in multi-threaded code like this before so I was wondering if I'm just missing or misunderstanding something. Thanks.

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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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• Encrypt text using a number

  - by Adam Matan

  Project Euler I have recently begun to solve some of the Project Euler riddles. I found the discussion forum in the site a bit frustrating (most of the discussions are closed and poorly-threaded), So I have decided to publish my Python solutions on launchpad for discussion. The problem is that it seems quite unethical to publish these solutions, as it would let other people gain reputation without doing the programming work, which the site deeply discourages. My Encryption problem I want to encrypt my answers so that only those who have already solved the riddles can see my code. The logical key would be the answer to the riddle, which is always numeric. In order to prevent brute-force attacks on my answers, I want to find an encryption algorithm that takes a significantly long time (few seconds) to run. Do you know any such algorithm? I would fancy a Python package, which I can attach to the code, over an external program that might have portability issues. Thanks, Adam

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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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• 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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