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  • Math/numerical formula every computer programmer should know

    - by aaa
    This is a follow-up question to What should every programmer know and Is mathematics necessary. So the question is, as a computer programmer, what is the most important/useful mathematical or numerical formula that you use? By Formula I mean anything that involves less obvious manipulations, whenever binomial coefficients or bit hacks. I work with multidimensional arrays and various matrix representations. So for me most commonly used formulas are: A(i,j,k,..) = a[i + j*Dim0 + k*Dim0*Dim1 + ... to map indexes to one dimension ( which is basic address calculation which many people do not seem to know). And triangular number T(i) = (i*i + i)/2 which is related to binomial coefficients, used to calculate address in triangular matrixes and many other things. What is your workhorse formula that you think programmer should know?

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  • Help me understand this C code

    - by Benjamin
    INT GetTree (HWND hWnd, HTREEITEM hItem, HKEY *pRoot, TCHAR *pszKey, INT nMax) { TV_ITEM tvi; TCHAR szName[256]; HTREEITEM hParent; HWND hwndTV = GetDlgItem (hWnd, ID_TREEV); memset (&tvi, 0, sizeof (tvi)); hParent = TreeView_GetParent (hwndTV, hItem); if (hParent) { // Get the parent of the parent of the... GetTree (hWnd, hParent, pRoot, pszKey, nMax); // Get the name of the item. tvi.mask = TVIF_TEXT; tvi.hItem = hItem; tvi.pszText = szName; tvi.cchTextMax = dim(szName); TreeView_GetItem (hwndTV, &tvi); //send the TVM_GETITEM message? lstrcat (pszKey, TEXT ("\\")); lstrcat (pszKey, szName); } else { *pszKey = TEXT ('\0'); szName[0] = TEXT ('\0'); // Get the name of the item. tvi.mask = TVIF_TEXT | TVIF_PARAM; tvi.hItem = hItem; tvi.pszText = szName; tvi.cchTextMax = dim(szName); if (TreeView_GetItem (hwndTV, &tvi)) //*pRoot = (HTREEITEM)tvi.lParam; //original hItem = (HTREEITEM)tvi.lParam; else { INT rc = GetLastError(); } } return 0; } The block of code that begins with the comment "Get the name of the item" does not make sense to me. If you are getting the listview item why does the code set the parameters of the item being retrieved? If you already had the values there would be no need to retrieve them. Secondly near the comment "original" is the original line of code which will compile with a warning under embedded visual c++ 4.0, but if you copy the exact same code into visual studio 2008 it will not compile. Since I did not write any of this code, and am trying to learn, is it possible the original author made a mistake on this line? The *pRoot should point to HKEY type yet he is casting to an HTREEITEM type which should never work since the data types don't match?

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  • Sources for news about network security

    - by chris.nullptr
    I am a software developer that wants to stay up-to-date on network security news. What are some of the best sources online for not only keeping tabs on newly discovered security vulnerabilities that may affect projects I'm working on, but also best-practices when developing network software. Please keep in mind that I am looking for sources that cater to the software developer not IT administration.

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  • display records which exist in file2 but not in file1

    - by Phoenix
    log file1 contains records of customers(name,id,date) who visited yesterday log file2 contains records of customers(name,id,date) who visited today How would you display customers who visited yesterday but not today? Constraint is: Don't use auxiliary data structure because file contains millions of records. [So, no hashes] Is there a way to do this using Unix commands ??

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  • Searching for patterns to create a TCP Connection Pool for high performance messaging

    - by JoeGeeky
    I'm creating a new Client / Server application in C# and expect to have a fairly high rate of connections. That made me think of database connection pools which help mitigate the expense of creating and disposing connections between the client and database. I would like to create a similar capability for my application and haven't been able to find any good examples of how to apply this pattern. Do I really need to spin up an instance of a TcpClient every time I want to send a message to the server and receive a receipt message? Each connection is expected to transport between 1-5KB with each receiving a 1KB response message. I realize this question is somewhat vague, but I am starting from scratch so I am open to suggestions. Even if that means my suppositions are all wrong.

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  • Discovering a functional algorithm from a mutable one

    - by Garrett Rowe
    This isn't necessarily a Scala question, it's a design question that has to do with avoiding mutable state, functional thinking and that sort. It just happens that I'm using Scala. Given this set of requirements: Input comes from an essentially infinite stream of random numbers between 1 and 10 Final output is either SUCCEED or FAIL There can be multiple objects 'listening' to the stream at any particular time, and they can begin listening at different times so they all may have a different concept of the 'first' number; therefore listeners to the stream need to be decoupled from the stream itself. Pseudocode: if (first number == 1) SUCCEED else if (first number >= 9) FAIL else { first = first number rest = rest of stream for each (n in rest) { if (n == 1) FAIL else if (n == first) SUCCEED else continue } } Here is a possible mutable implementation: sealed trait Result case object Fail extends Result case object Succeed extends Result case object NoResult extends Result class StreamListener { private var target: Option[Int] = None def evaluate(n: Int): Result = target match { case None => if (n == 1) Succeed else if (n >= 9) Fail else { target = Some(n) NoResult } case Some(t) => if (n == t) Succeed else if (n == 1) Fail else NoResult } } This will work but smells to me. StreamListener.evaluate is not referentially transparent. And the use of the NoResult token just doesn't feel right. It does have the advantage though of being clear and easy to use/code. Besides there has to be a functional solution to this right? I've come up with 2 other possible options: Having evaluate return a (possibly new) StreamListener, but this means I would have to make Result a subtype of StreamListener which doesn't feel right. Letting evaluate take a Stream[Int] as a parameter and letting the StreamListener be in charge of consuming as much of the Stream as it needs to determine failure or success. The problem I see with this approach is that the class that registers the listeners should query each listener after each number is generated and take appropriate action immediately upon failure or success. With this approach, I don't see how that could happen since each listener is forcing evaluation of the Stream until it completes evaluation. There is no concept here of a single number generation. Is there any standard scala/fp idiom I'm overlooking here?

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  • Closures and universal quantification

    - by Apocalisp
    I've been trying to work out how to implement Church-encoded data types in Scala. It seems that it requires rank-n types since you would need a first-class const function of type forAll a. a -> (forAll b. b -> b). However, I was able to encode pairs thusly: import scalaz._ trait Compose[F[_],G[_]] { type Apply = F[G[A]] } trait Closure[F[_],G[_]] { def apply[B](f: F[B]): G[B] } def pair[A,B](a: A, b: B) = new Closure[Compose[PartialApply1Of2[Function1,A]#Apply, PartialApply1Of2[Function1,B]#Apply]#Apply, Identity] { def apply[C](f: A => B => C) = f(a)(b) } For lists, I was able to get encode cons: def cons[A](x: A) = { type T[B] = B => (A => B => B) => B new Closure[T,T] { def apply[B](xs: T[B]) = (b: B) => (f: A => B => B) => f(x)(xs(b)(f)) } } However, the empty list is more problematic and I've not been able to get the Scala compiler to unify the types. Can you define nil, so that, given the definition above, the following compiles? cons(1)(cons(2)(cons(3)(nil)))

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  • (For what) Are Fortran, Cobol and Co. used today?

    - by lamas
    I'm a relatively young programmer and so I don't really know much about languages like Fortran or Cobol that have their origins in the beginning of modern informatics. I'm a bit confused because it seems like there are many people out there saying that these two languages are still very alive and being used all over the world whereas others say the opposite. In addition, it seems like there are only very few questions tagged Fortran or Cobol here on stackoverflow. Can someone "demystify" the situation for me? Who uses these senior languages these days and are they even used anymore? Do you have any experiences with one of the languages or do you know something about their latest developments?

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  • How to sort a list in Scala by two fields?

    - by Twistleton
    how to sort a list in Scala by two fields, in this example I will sort by lastName and firstName? case class Row(var firstName: String, var lastName: String, var city: String) var rows = List(new Row("Oscar", "Wilde", "London"), new Row("Otto", "Swift", "Berlin"), new Row("Carl", "Swift", "Paris"), new Row("Hans", "Swift", "Dublin"), new Row("Hugo", "Swift", "Sligo")) rows.sortBy(_.lastName) I try things like this rows.sortBy(_.lastName + _.firstName) but it doesn't work. So I be curious for a good and easy solution. Thanks in advance! Pongo

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  • C# property definition

    - by Sunny
    For C# properties, I can do this: public class Employee{ public string Name { get; private set; } public Employee(string name){ Name = name; } } which means that the Name property can be set within the class Employee & can be read publicly. But, if I want to restrict the set to only within the constructors of the Employee class, I need to do: public class Employee{ public readonly string Name = String.Empty; public Employee(string name){ Name = name; } } But, for this case, I had to change the property to a field. Is there any reason this is not possible/allowed in C#: public class Employee{ public string Name { get; private readonly set; } public Employee(string name){ Name = name; } } IMO this will allow us to have properties which can be set only in the constructor & does not require us to change properties to fields... Thanks!

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  • Linux Kernel - Slab Allocator Question

    - by Drex
    I am playing around with the kernel and am looking at the kmem_cache files_cachep belonging to fork.c. It detects the sizeof(files_struct). My question is this: I have altered files_struct and added a rb_root (red/black tree root) using the built-in functionality in linux/rbtree.h. I can properly insert values into this tree. However, at some point, a segfault occurs and GDB backtraces the following information: (gdb) backtrace 0 0x08066ad7 in page_ok (page=) at arch/um/os-Linux/sys-i386/task_size.c:31 1 0x08066bdf in os_get_top_address () at arch/um/os-Linux/sys-i386/task_size.c:100 2 0x0804a216 in linux_main (argc=1, argv=0xbfb05f14) at arch/um/kernel/um_arch.c:277 3 0x0804acdc in main (argc=1, argv=0xbfb05f14, envp=0xbfb05f1c) at arch/um/os-Linux/main.c:150 I have spent many hours trying to figure out why there is a segfault given that the red/black tree inserts properly. I'm thinking it's a memory allocation issue with new processes made by fork() of a parent process. Could this be the case and could it have something to do with kmem_cache files_cachep?

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  • What languages should a microISV use to write commercial software?

    - by Wal
    I've been writing software in Java for many years now, but it was always for internal applications that would be deployed to a server. I'd like to get into writing desktop applications now but I don't know where to start. I've written a few Java/Swing applications but again they were for internal use. My understanding is that Java and other semi-compiled and interpreted languages are too easy to reverse engineer, making them unsuitable for commercial software. I am aware that there are compilers for Java and some other interpreted language, but I've also heard that they are pricey and/or unreliable. Assuming I start a microISV and wish to develop and sell applications to a broad audience, what's my best bet? I would prefer something that can be written close to once, and compiled for different operating systems but I am not opposed to .NET and a Windows-only audience if other languages would compromise the experience (installation ease & user experience) in Windows. My only issue there is that I don't have a large starting budget and paying out the wazoo for the required development tools is not really in the cards.

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  • Should I invest time in learning about OR\M or LINQ?

    - by Peter Smith
    I'm a .NET web developer primarily who occasionally writes console applications to mine data, cleanup tasks, etc. Most of what I do winds up involving a database which I currently design via sql server management studio, using stored procedures, and query analyzer. I also create a lot of web services which are consumed via AJAX applications. Do these technologies really help you in speeding up development times? Do you still have to build the database or object code first?

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  • Generate an image / thumbnail of a webpage using X/Gui-less linux

    - by dassouki
    There are a lot of solutions out there on generating a thumbnail or an image preview of a webpage. Some of these are websites like websnapshots, windows libraries such as PHP's imagegrabscreen (only works on windows), and KDE's wkhtml. Many more do exist. However, I'm looking for a GUI-less solution. Something I can create an API around and link it to php or python. I'm comfortable with python, php, C, and shell. This is a personal project, so I'm not interested in commercial applications as I'm aware of their existence. Any ideas?

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  • Naming convention in Objective C /C , start with "_"?

    - by Tattat
    Something I see ppl define the variable like this: b2World *_world; b2Body *_body; CCSprite *_ball; instead of b2World *world; b2Body *body; CCSprite *ball; I familiar with the second one, but not the first one. So, I checked the Wikipedia about naming convention: Names beginning with double underscore or an underscore and a capital letter are reserved for implementation (compiler, standard library) and should not be used (e.g. __reserved or _Reserved). So, is that any special meaning which is start with "_"? The wiki page.

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  • Object serialization practical uses?

    - by nash
    How many software projects have you worked on used object serialization? I personally never came across a scenario where object serialization was used. One use case i can think of is, a server software storing objects to disk to save memory. Are there other types of software where object serialization is essential or preferred over a database?

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  • How to find minimum weight with maximum cost in 0-1 Knapsack algorithm?

    - by Nitin9791
    I am trying to solve a spoj problem Party Schedule the problem statement is- You just received another bill which you cannot pay because you lack the money. Unfortunately, this is not the first time to happen, and now you decide to investigate the cause of your constant monetary shortness. The reason is quite obvious: the lion's share of your money routinely disappears at the entrance of party localities. You make up your mind to solve the problem where it arises, namely at the parties themselves. You introduce a limit for your party budget and try to have the most possible fun with regard to this limit. You inquire beforehand about the entrance fee to each party and estimate how much fun you might have there. The list is readily compiled, but how do you actually pick the parties that give you the most fun and do not exceed your budget? Write a program which finds this optimal set of parties that offer the most fun. Keep in mind that your budget need not necessarily be reached exactly. Achieve the highest possible fun level, and do not spend more money than is absolutely necessary. Input The first line of the input specifies your party budget and the number n of parties. The following n lines contain two numbers each. The first number indicates the entrance fee of each party. Parties cost between 5 and 25 francs. The second number indicates the amount of fun of each party, given as an integer number ranging from 0 to 10. The budget will not exceed 500 and there will be at most 100 parties. All numbers are separated by a single space. There are many test cases. Input ends with 0 0. Output For each test case your program must output the sum of the entrance fees and the sum of all fun values of an optimal solution. Both numbers must be separated by a single space. Example Sample input: 50 10 12 3 15 8 16 9 16 6 10 2 21 9 18 4 12 4 17 8 18 9 50 10 13 8 19 10 16 8 12 9 10 2 12 8 13 5 15 5 11 7 16 2 0 0 Sample output: 49 26 48 32 now I know that it is an advance version of 0/1 knapsack problem where along with maximum cost we also have to find minimum weight that is less than a a given weight and have maximum cost. so I have used dp to solve this problem but still get a wrong awnser on submission while it is perfectly fine with given test cases. My code is typedef vector<int> vi; #define pb push_back #define FOR(i,n) for(int i=0;i<n;i++) int main() { //freopen("input.txt","r",stdin); while(1) { int W,n; cin>>W>>n; if(W==0 && n==0) break; int K[n+1][W+1]; vi val,wt; FOR(i,n) { int x,y; cin>>x>>y; wt.pb(x); val.pb(y); } FOR(i,n+1) { FOR(w,W+1) { if(i==0 || w==0) { K[i][w]=0; } else if (wt[i-1] <= w) { if(val[i-1] + K[i-1][w-wt[i-1]]>=K[i-1][w]) { K[i][w]=val[i-1] + K[i-1][w-wt[i-1]]; } else { K[i][w]=K[i-1][w]; } } else { K[i][w] = K[i-1][w]; } } } int a1=K[n][W],a2; for(int j=0;j<W;j++) { if(K[n][j]==a1) { a2=j; break; } } cout<<a2<<" "<<a1<<"\n"; } return 0; } Could anyone suggest what am I missing??

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  • Socket read() hangs for a while when there is no data to read.

    - by janesconference
    Hi' I'm writing a simple http port forwarder. I read data from port 80, and pass the data to my lighttpd server, on port 8080. As long as I write() data on the socket on port 8080 (forwarding the request) there's no problem, but when I read() data from that socket (forwarding the response), the last read() hangs a lot (about 1 or 2 seconds) before realizing there's no more data and returning 0. I tried to set the socket to non-blocking, but this doesn't work, as sometimes it returns EWOULDBLOCKING even if there's some data left (lighttpd + cgi can be quite slow). I tried to set a timeout with select(), but, as above, a slow cgi could timeout the socket when there's actually some data to transmit. How would you do?

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  • Does MATLAB perform tail call optimization?

    - by Shea Levy
    I've recently learned Haskell, and am trying to carry the pure functional style over to my other code when possible. An important aspect of this is treating all variables as immutable, i.e. constants. In order to do so, many computations that would be implemented using loops in an imperative style have to be performed using recursion, which typically incurs a memory penalty due to the allocation a new stack frame for each function call. In the special case of a tail call (where the return value of a called function is immediately returned to the callee's caller), however, this penalty can be bypassed by a process called tail call optimization (in one method, this can be done by essentially replacing a call with a jmp after setting up the stack properly). Does MATLAB perform TCO by default, or is there a way to tell it to?

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