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• What exactly is a variable in C++?

  - by FredOverflow

  The standard says A variable is introduced by the declaration of an object. The variable's name denotes the object. But what does this definition actually mean? Does a variable give a name to an object, i.e. are variables just a naming mechanism for otherwise anonymous objects? Or is a variable the name itself? Or is a variable a named object in the sense that every variable is also an object? Or is a variable just a "proxy" with a name that "delegates" all operations to the real object? To confuse things further, many C++ books seem to treat variables and objects as synonyms. What is your take on this?

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• Which NoSQL db to use with C?

  - by systemsfault

  Hello all, I'm working on an application that I'm going to write with C and i am considering to use a nosql db for storing timeseries data with at most 8 or 9 fields. But in every 5 minutes there will huge write operations such as 2-10 million rows and then there will be reads(but performance is not as crucial in read as in the write operation). I'm considering to use a NoSQL db here in order to store the data but couldn't decide on which one to use. Couchdb seems to have a stable driver called pillowtalk for C; but Mongo's driver doesn't look as promising as pillowtalk. I'm also open to other suggestions. What is your recommendation?

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• When is a program limited by the memory bandwidth?

  - by hanno

  I want to know if a program that I am using and which requires a lot of memory is limited by the memory bandwidth. When do you expect this to happen? Did it ever happen to you in a real life scenario? I found several articles discussing this issue, including http://www.cs.virginia.edu/~mccalpin/papers/bandwidth/node12.html http://www.cs.virginia.edu/~mccalpin/papers/bandwidth/node13.html http://ispass.org/ucas5/session2_3_ibm.pdf The first link is a bit old, but suggests that you need to perform less than about 1-40 floating point operations per floating point variable in order to see this effect (correct me if I'm wrong). How can I measure the memory bandwidth that a given program is using and how do I measure the (peak) bandwidth that my system can offer? I don't want to discuss any complicated cache issues here. I'm only interested in the communication between the CPU and the memory.

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• What types of conditions can be used for conditional compilation in C++?

  - by user1002288

  This is an exam question for C++: Which of the following statements accurately describe the condition that can be used for conditional compilation in C++? A. The condition can depend on the value of environment variables. B. The condition can depend on the value of any const variables. C. The condition can depend on the value of program variables. D. The condition can use the sizeof() operator to make decision about compiler-dependent operations based on the size of standard data type. E. The condition must evaluate to either a 0 or 1 during preprocessing. I think the answer is E. Is this correct?

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• Speed of interpolation algorithms, C# and C++ working together.

  - by Kaminari

  Hello. I need fast implementation of popular interpolation algorithms. I figured it out that C# in such simple algorithms will be much slower than C++ so i think of writing some native code and using it in my C# GUI. First of all i run some tests and few operations on 1024x1024x3 matrix took 32ms in C# and 4ms in C++ and that's what i basicly need. Interpolation however is not a good word because i need them only for downscaling. But the question is: Will it be faster than C# methods in Drawing2D Image outputImage = new Bitmap(destWidth, destHeight, PixelFormat.Format24bppRgb); Graphics grPhoto = Graphics.FromImage(outputImage); grPhoto.InterpolationMode = InterpolationMode.*; //all of them grPhoto.DrawImage(bmp, new Rectangle(0, 0, destWidth, destHeight), Rectangle(0, 0, sourceWidth, sourceHeight), GraphicsUnit.Pixel); grPhoto.Dispose(); Some of these method run in 20ms and some in 80. Is there a way to do it faster?

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• How to group rows into two groups in sql?

  - by user1055638

  Lets say I have such a table: id|time|operation 1 2 read 2 5 write 3 3 read 4 7 read 5 2 save 6 1 open and now I would like to do two things: Divide all these records into two groups: 1) all rows where operation equals to "read" 2) all other rows. Sum the time in each group. So that my query would result only into two rows. What I got so far is: select sum(time) as total_time, operation group by operation ; Although that gives me many groups, depending on the number of distinct operations. How I could group them only into two categories? Cheers!

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• Event(s) for opening the workbook AND worksheets

  - by KeyMs92

  I'm looking for an elegant solution to trigger an event for opening the workbook as well as opening different worksheets. I don't need seperate operations for each worksheet: they all trigger the same method. I know I can use the events Workbook_Activate / Workbook_Open and Workbook_SheetActivate at the same time but I don't know if this is 'the official way' to do it. Perhaps there's a way to do this with one event. I was also wondering if it is relevant in this matter where I put the code. I now have all the code inside "ThisWorkbook" and not in a "Module"...

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• Low-overhead way to access the memory space of a traced process?

  - by vovick

  Hello all. I'm looking for an efficient way to access(for both read and write operations) the memory space of my ptraced child process. The size of blocks being accessed may vary from several bytes up to several megabytes in size, so using the ptrace call with PTRACE_PEEKDATA and PTRACE_POKEDATA which read only one word at a time and switch context every time they're called seems like a pointless waste of resources. The only one alternative solution I could find, though, was the /proc/<pid>/mem file, but it has long since been made read only. Is there any other (relatively simple) way to do that job? The ideal solution would be to somehow share the address space of my child process with its parent and then use the simple memcpy call to copy data I need in both directions, but I have no clues how to do it and where to begin. Any ideas?

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• Is this a bad indexing strategy for a table?

  - by llamaoo7

  The table in question is part of a database that a vendor's software uses on our network. The table contains metadata about files. The schema of the table is as follows Metadata ResultID (PK, int, not null) MappedFieldname (char(50), not null) Fieldname (PK, char(50), not null) Fieldvalue (text, null) There is a clustered index on ResultID and Fieldname. This table typically contains millions of rows (in one case, it contains 500 million). The table is populated by 24 workers running 4 threads each when data is being "processed". This results in many non-sequential inserts. Later after processing, more data is inserted into this table by some of our in-house software. The fragmentation for a given table is at least 50%. In the case of the largest table, it is at 90%. We do not have a DBA. I am aware we desperately need a DB maintenance strategy. As far as my background, I'm a college student working part time at this company. My question is this, is a clustered index the best way to go about this? Should another index be considered? Are there any good references for this type and similar ad-hoc DBA tasks?

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• finding character in string C language

  - by iSight

  Hi, I am searching a character at first occurence in string using the following code. But, it is taking some time when the character is too long or the character that i am searching is at far extend, which makes delay in other operations. How could i tackle with this problem. The code is below here. Note: attrPtr is a char* which holds a reference to a string containing '"' character at far extend. int position = 0; char qolon = '"';//character to search while (*(attrPtr + position++) != qolon); char* attrValue = NULL; attrValue = (char*)malloc(position * sizeof(char)); strncpy(attrValue, attrPtr, position-1);

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• Special simple random number generator

  - by psihodelia

  How to create a function, which on every call generates a random integer number? This number must be most random as possible (according to uniform distribution). It is only allowed to use one static variable and at most 3 elementary steps, where each step consists of only one basic arithmetic operation of arity 1 or 2. Example: int myrandom(void){ static int x; x = some_step1; x = some_step2; x = some_step3; return x; } Basic arithmetic operations are +,-,%,and, not, xor, or, left shift, right shift, multiplication and division. Of course, no rand(), random() or similar staff is allowed.

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• How online-game clients are able to exchange data through internet so fast?

  - by Kirzilla

  Hello, Let's imagine really simple game... We have a labirinth and two players trying to find out exit in real time through internet. On every move game client should send player's coordinates to server and accept current coordinates of another client. How is it possible to make this exchange so fast (as all modern games do). Ok, we can use memcache or similar technology to reduce data mining operations on server side. We can also use fastest webserver etc., but we still will have problems with timings. So, the questions are... What protocol game clients are usually using for exchanging information with server? What server technologies are coming to solve this problem? What algorithms are applied for fighting with delays during game etc. PS: Sorry for my English and I hope that my question is clear. Thank you.

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• '<=' operator is not working in sql server 2000

  - by Lalit

  Hello, Scenario is, database is in the maintenance phase. this database is not developed by ours developer. it is an existing database developed by the 'xyz' company in sql server 2000. This is real time database, where i am working now. I wanted to write the stored procedure which will retrieve me the records From date1 to date 2.so query is : Select * from MyTableName Where colDate>= '3-May-2010' and colDate<= '5-Oct-2010' and colName='xyzName' whereas my understanding I must get data including upper bound date as well as lower bound date. but somehow I am getting records from '3-May-2010' (which is fine but) to '10-Oct-2010' As i observe in table design , for ColDate, developer had used 'varchar' to store the date. i know this is wrong remedy by them. so in my stored procedure I have also used varchar parameters as @FromDate1 and @ToDate to get inputs for SP. this is giving me result which i have explained. i tried to take the parameter type as 'Datetime' but it is showing error while saving/altering the stored procedure that "@FromDate1 has invalid datatype", same for "@ToDate". situation is that, I can not change the table design at all. what i have to do here ? i know we can use user defined table in sql server 2008 , but there is version sql server 2000. which does not support the same. Please guide me for this scenario. **Edited** I am trying to write like this SP: CREATE PROCEDURE USP_Data (@Location varchar(100), @FromDate DATETIME, @ToDate DATETIME) AS SELECT * FROM dbo.TableName Where CAST(Dt AS DATETIME) >=@fromDate and CAST(Dt AS DATETIME)<=@ToDate and Location=@Location GO but getting Error: Arithmetic overflow error converting expression to data type datetime. in sql server 2000 What should be that ? is i am wrong some where ? also (202 row(s) affected) is changes every time in circular manner means first time sayin (122 row(s) affected) run again saying (80 row(s) affected) if again (202 row(s) affected) if again (122 row(s) affected) I can not understand what is going on ?

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• Asp.net static object behaviour

  - by Circadian

  I have the following class as part of an asp.net application. public sealed class SomeClass { private static string appId = Guid.NewGuid().ToString(); public static ReadSomethingFromDb(){} public static WriteSomethingToDb(){} } There are more than one application instances in the same application pool, and they all access the same database. I want the operations performed by the above class to be uniquely tied to the instance that performed it, hence the use of the appId. So adding a record to the database would for example contain a name, address and appId. This has been simplified for discussion purposes. Assuming that I have two instances running at mysite.a and mysite.b the above class would generate two different guids. My problem is that mysite.a sometimes produces more than one guid, which is unexpected. Thank you in advance

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• Add 64 bit offset to a pointer

  - by Novox

  In F#, there's the NativePtr module, but it seems to only support 32 bit offsets for its’ add/get/set functions, just like System.IntPtr does. Is there a way to add a 64 bit offset to a native pointer (nativeptr<'a) in F#? Of course I could convert all addresses to 64 bit integers, do normal integer operations and then convert the result again to nativeptr<'a, but this would cost additional add and imul instructions. I really want the AGUs to perform the address calculations. For instance, using unsafe in C# you could do something like void* ptr = Marshal.AllocHGlobal(...).ToPointer(); int64 offset = ...; T* newAddr = (T*)ptr + offset; // T has to be an unmanaged type Well actually you can't, because there is no "unmanaged" constraint for type parameters, but at least you can do general pointer arithmetic in a non-generic way. In F# we finally got the unmanaged constraint; but how do I do the pointer arithmetic?

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• Common strategies to deal with rounding errors in currency-intensive soft?

  - by Max

  What is your advice on: compensation of accumulated error in bulk math operations on collections of Money objects. How is this implemented in your production code? (things like variable rounding, etc...) theory behind rounding in accountancy. any literature on topic. I currently read Fowler. He mentions Money type, but says nothing on strategies. Older posts on money-rounding (here, and here) do not provide a details and formality I need. Thanks for help.

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• Best neural network for certain type of pattern analysis?

  - by fred basset

  Hi All, I'm working on a system that will send telemetry data on machine operation back to a central server for analysis. One of the machine parameters we're measuring is motor current drawn vs time. After an operation is finished we plan to send back an array of currents vs time to the server. A successful operation would have a pattern like a trapezoid, problematic operations would have a pattern completely different, more like a large spike in values. Can anyone recommend a type of neural network that would be good at classifying these 1D vectors of current values into a pass/fail type output? Thanks, Fred

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• How often does memcache on Google AppEngine lose data?

  - by Freed

  Memcache in general and on AppEngine in specific is unreliable in the sense that my data may be deleted from the cache for whatever reason at any point in time. However, in some cases there might be cases where a small risk may be worth the added performance using memcache could give, such as updating some data in memcache that gets saved periodically to some other, more reliable storage. Are there any numbers from Google that could give me an indication of the actual probability that a memcache entry would be lost from the cache before its expiration time, given that I keep within my quotas? Are there any reasons other than hardware failure and administrative operations such as machines at the data centers being upgraded/moved/replaced that would cause entries to be removed from memcache prematurely?

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• Working with sets of rows in (My)SQL and comparing values

  - by Pep.

  Hello, I am trying to figure out the SQL for doing some relatively simple operations on sets of records in a table but I am stuck. Consider a table with multiple rows per item, all identified by a common key. For example: serial model color XX1 A blue XX2 A blue XX3 A green XX5 B red XX6 B blue XX1 B blue What I would for example want to do is: Assuming that all model A rows must have the same color, find the rows which dont. (for example, XX3 is green). Assuming that a given serial number can only point to a single type of model, find out the rows which that does not occur (for example XX1 points both to A and B) These are all simple logically things to do. To abstract it, I want to know how to group things by using a single key (or combination of keys) and then compare the values of those records. Should I use a join on the same table? should i use some sort of array or similar? thanks for your help

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• fastest way to perform string search in general and in python

  - by Rkz

  My task is to search for a string or a pattern in a list of documents that are very short (say 200 characters long). However, say there are 1 million documents of such time. What is the most efficient way to perform this search?. I was thinking of tokenizing each document and putting the words in hashtable with words as key and document number as value, there by creating a bag of words. Then perform the word search and retrieve the list of documents that contained this word. From what I can see is this operation will take O(n) operations. Is there any other way? may be without using hash-tables?. Also, is there a python library or third party package that can perform efficient searches?

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• name of class that manipulates the entities

  - by cyberguest

  hi, i have a general question regarding naming convention. if I separate the data and operations into two separate classes. one has the data elements (entity), the other class manipulates the entity class. what do we usually call that class that manipulates the entity class? (the entity I am referring to has nothing to do with any kind of entity framework) manager? controller? operator? manipulator? thanks in advance

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• MYSQL how to sum rows with same key, then delete the duplicate rows

  - by Bhante-S

  What I have: key data 1      22 1       5 2       6 3       1 3      -3 What I want: key data 1      27 2       6 3      -2 I don’t mind doing this with two or more queries, esp. if they are simple--makes for easier maintenance. Also the tables are fairly small (<2,000 records). The ‘key’ field is indexed and allows duplicates. Muchas Gracias

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• Java Insert Help...???

  - by El Classico

  I am trying to do an Insert, Update and Delete on a table in MS Access. Everything works fine for a SELECT statement. But when doing the other three operations, I don't seem to get any errors, but the actions are not reflected on to the DB. Please help... THe INSERT statement is as follows: PreparedStatement ps = con.prepareStatement("INSERT INTO Student VALUES (?, ?, ?, ?, ?, ?, ?, ?)"); ps.setInt(1,1); ps.setString(2,"ish"); ps.setInt(3,100); ps.setInt(4,100); ps.setInt(5,100); ps.setInt(6,300); ps.setInt(7,100); ps.setString(8,"A"); ps.executeUpdate(); Also may I know why PreparedStatement is used except for SELECT statement...

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• Clarifying... So Background Jobs don't Tie Up Application Resources (in Rails)?

  - by viatropos

  I'm trying to get a better grasp of the inner workings of background jobs and how they improve performance. I understand that the goal is to have the application return a response to the user as fast as it can, so you don't want to, say, parse a huge feed that would take 10 seconds because it would prevent the application from being able to process any other requests. So it's recommended to put any operations that take more than say 500ms to execute, into a queued background job. What I don't understand is, doesn't that just delay the same problem? I know the user who invoked that background job will get an immediate response, but what if another user comes right when that background job starts (and it takes 10 seconds to finish), wont that user have to wait? Or is the main issue that, requests are the only thing that can happen one-at-a-time, while on the other hand a request can start while one+ background jobs are in the middle of running? Is that correct?

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• Storing SQL Tables for use in visual studio

  - by Raven Dreamer

  Greetings. I'm trying to create a windows form application that manipulates data from several tables stored on a SQL server. 1) What's the best way to store the data locally, while the application is running? I had a previous program that only modified one table, and that was set up to use a datagridview. However, as I don't necessarily want to view all the tables, I am looking for another way to store the data retrieved by the SELECT * FROM ... query. 2) Is it better to load the tables, make changes within the C# application, and then update the modified tables at the end, or simply perform all operations on the database, remotely (retrieving the tables each time they are needed)? Thank you.

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