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  • Confused over behavior of List.mapi in F#

    - by James Black
    I am building some equations in F#, and when working on my polynomial class I found some odd behavior using List.mapi Basically, each polynomial has an array, so 3*x^2 + 5*x + 6 would be [|6, 5, 3|] in the array, so, when adding polynomials, if one array is longer than the other, then I just need to append the extra elements to the result, and that is where I ran into a problem. Later I want to generalize it to not always use a float, but that will be after I get more working. So, the problem is that I expected List.mapi to return a List not individual elements, but, in order to put the lists together I had to put [] around my use of mapi, and I am curious why that is the case. This is more complicated than I expected, I thought I should be able to just tell it to make a new List starting at a certain index, but I can't find any function for that. type Polynomial() = let mutable coefficients:float [] = Array.empty member self.Coefficients with get() = coefficients static member (+) (v1:Polynomial, v2:Polynomial) = let ret = List.map2(fun c p -> c + p) (List.ofArray v1.Coefficients) (List.ofArray v2.Coefficients) let a = List.mapi(fun i x -> x) match v1.Coefficients.Length - v2.Coefficients.Length with | x when x < 0 -> ret :: [((List.ofArray v1.Coefficients) |> a)] | x when x > 0 -> ret :: [((List.ofArray v2.Coefficients) |> a)] | _ -> [ret]

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  • Sparse constrained linear least-squares solver

    - by Jacob
    This great SO answer points to a good sparse solver, but I've got constraints on x (for Ax = b) such that each element in x is >=0 an <=N. The first thing which comes to mind is an QP solver for large sparse matrices. Also, A is huge (around 2e6x2e6) but very sparse with <=4 elements per row. Any ideas/recommendations? I'm looking for something like MATLAB's lsqlin but with huge sparse matrices.

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  • How "healthy" is the haskell community

    - by Asher
    I came across a post where someone wrote something inaccurate about haskell (won't go into it) and he got flammed for it. Which (pleasantly) surprised me. About 3 years ago I read this joke about haskell: All the haskell programmers in the world can fit into a 747 and if that plane were to crash no one would care... or something along those lines. Which brings me to my question: is haskell "mainstream"? Is it mainly used by hobbiest and academics or someone making some serious money from it (which is the true yardstick of how good a language is - just kidding, geez!)?

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  • How do I evaluate my skillset against the current market to see what needs improvement and where my

    - by baijajusav
    First of all, this question may be out of bounds for this site. If so, remove it. I say this because this site seems to be a place for more concrete questions that are not so relative in nature. And before I begin, for those of you whom just prefer a question and not this sort of dialog, here is my question: How can I assess my current skills as a programmer and decide where and what areas to improve upon? That said, here's what I'm asking/talking about, in essence. The market is always in constant flux. As programmers we're always having to learn new things, update our skills, push ourselves into that next project. There's not a very good litmus test that I know of for us to get an idea of where we stand as programmers. I came across this blog post by Jeff Atwood talking about why can't programmers code. Instinctively (and as the post goes on to state) I rushed through the program in about 4 minutes (most of that time was b/c I was hand writing it out. Still, this doesn't really answer the question of where do my skills need to be to succeed in today's world. I real blogs, listen to podcasts, try to keep up on the latest things coming out. It has only been in the past couple of months that I made a decision to pick a focus area for my learning as I can't learn everything and trying to do so is to spread myself too thin. I chose ASP.NET MVC & C#. I plan to stick with Microsoft technologies, not out of some sense of loyalty or stubbornness, but rather because they seem to stream together and have a unifying connection between them. With Windows Phone 7 coming out, it seems that now is the obvious time to pick up WPF and Silverlight as well. Still, if you asked me to code something apart from intellisense and the internet, I probably couldn't get the syntax right. I don't have libraries memorized or know precisely where the classes I use exist within the .Net framework, namely because I haven't had to pull that knowledge out of the air. In a way, I suppose Visual Studio has insulated me, which isn't a good thing, but, at the same time, I've still been able to be productive. I'm working on my own side project to try and help my learning. In doing so, I'm trying to make use of best practices and 3rd party frameworks where I can. I'm using automapper and EF 1.0. I know everyone in the .net community seems to cry foul at the sound of EF 1.0, but I can't say why because I've never used it. There's no lazy loading and that has proven rather annoying; however, aside from that, I haven't had that much of an issue. Granted this is probably because I'm not writing tests as I go (which I'm not doing because I don't know how to test EF in tests and don't really have a clue how to write tests for ASP.NET MVC 1.0). I'm also using a custom membership provider; granted, it's a barebone implementation, but I'm using it still. My thinking in all of this is, while I am neglecting a great many important technologies that are in the mainstream, I'll have a working project in the end. I can come back and add those things after I finish. Doing it all now and at once seems like too much. I know how I work and I don't think I'd ever get it done that way. I've elected to make this a community wiki as I think this question might fight better there. If a moderator disagrees with that choice or the decision to post this here, the just delete the question. I'm not trying to make undue work for anyone. I'm just a programmer trying to assess my where his skills are now and where I should be improving.

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  • SocketChannel in Java sends data, but it doesn't get to destination application

    - by Peterson
    Hi Everybody, I'm suffering a lot to create a simple ChatServer in Java, using the NIO libraries. Wonder if someone could help me. I am doing that by using SocketChannel and Selector to handle multiple clients in a single thread. The problem is: I am able to accept new connections and get it's data, but when I try to send data back, the SocketChannel simply doesn't work. In the method write(), it returns a integer that is the same size of the data i'm passing to it, but the client never receives that data. Strangely, when I close the server application, the client receives the data. It's like the socketchannel maintains a buffer, and it only get flushed when I close the application. Here are some more details, to give you more information to help. I'm handling the events in this piece of code: private void run() throws IOException { ServerSocketChannel ssc = ServerSocketChannel.open(); // Set it to non-blocking, so we can use select ssc.configureBlocking( false ); // Get the Socket connected to this channel, and bind it // to the listening port this.serverSocket = ssc.socket(); InetSocketAddress isa = new InetSocketAddress( this.port ); serverSocket.bind( isa ); // Create a new Selector for selecting this.masterSelector = Selector.open(); // Register the ServerSocketChannel, so we can // listen for incoming connections ssc.register( masterSelector, SelectionKey.OP_ACCEPT ); while (true) { // See if we've had any activity -- either // an incoming connection, or incoming data on an // existing connection int num = masterSelector.select(); // If we don't have any activity, loop around and wait // again if (num == 0) { continue; } // Get the keys corresponding to the activity // that has been detected, and process them // one by one Set keys = masterSelector.selectedKeys(); Iterator it = keys.iterator(); while (it.hasNext()) { // Get a key representing one of bits of I/O // activity SelectionKey key = (SelectionKey)it.next(); // What kind of activity is it? if ((key.readyOps() & SelectionKey.OP_ACCEPT) == SelectionKey.OP_ACCEPT) { // Aceita a conexão Socket s = serverSocket.accept(); System.out.println( "LOG: Conexao TCP aceita de " + s.getInetAddress() + ":" + s.getPort() ); // Make sure to make it non-blocking, so we can // use a selector on it. SocketChannel sc = s.getChannel(); sc.configureBlocking( false ); // Registra a conexao no seletor, apenas para leitura sc.register( masterSelector, SelectionKey.OP_READ ); } else if ( key.isReadable() ) { SocketChannel sc = null; // It's incoming data on a connection, so // process it sc = (SocketChannel)key.channel(); // Verifica se a conexão corresponde a um cliente já existente if((clientsMap.getClient(key)) != null){ boolean closedConnection = !processIncomingClientData(key); if(closedConnection){ int id = clientsMap.getClient(key); closeClient(id); } } else { boolean clientAccepted = processIncomingDataFromNewClient(key); if(!clientAccepted){ // Se o cliente não foi aceito, sua conexão é simplesmente fechada sc.socket().close(); sc.close(); key.cancel(); } } } } // We remove the selected keys, because we've dealt // with them. keys.clear(); } } This piece of code is simply handles new clients that wants to connect to the chat. So, a client makes a TCP connection to the server, and once it gets accepted, it sends data to the server following a simply text protocol, informing his id and asking to get registrated to the server. I handle this in the method processIncomingDataFromNewClient(key). I'm also keeping a map of clients and its connections in a data structure similar to a hashtable. I? doing that because I need to recover a client Id from a connection and a connection from a client Id. This is can be shown in: clientsMap.getClient(key). But the problem itself resides in the method processIncomingDataFromNewClient(key). There, I simply read the data that the client sent to me, validate it, and if it's ok, I send a message back to the client to tell that it is connected to the chat server. Here is a similar piece of code: private boolean processIncomingDataFromNewClient(SelectionKey key){ SocketChannel sc = (SocketChannel) key.channel(); String connectionOrigin = sc.socket().getInetAddress() + ":" + sc.socket().getPort(); int id = 0; //id of the client buf.clear(); int bytesRead = 0; try { bytesRead = sc.read(buf); if(bytesRead<=0){ System.out.println("Conexão fechada pelo: " + connectionOrigin); return false; } System.out.println("LOG: " + bytesRead + " bytes lidos de " + connectionOrigin); String msg = new String(buf.array(),0,bytesRead); // Do validations with the client sent me here // gets the client id }catch (Exception e) { e.printStackTrace(); System.out.println("LOG: Oops. Cliente não conhece o protocolo. Fechando a conexão: " + connectionOrigin); System.out.println("LOG: Primeiros 10 caracteres enviados pelo cliente: " + msg); return false; } } } catch (IOException e) { System.out.println("LOG: Erro ao ler dados da conexao: " + connectionOrigin); System.out.println("LOG: "+ e.getLocalizedMessage()); System.out.println("LOG: Fechando a conexão..."); return false; } // If it gets to here, the protocol is ok and we can add the client boolean inserted = clientsMap.addClient(key, id); if(!inserted){ System.out.println("LOG: Não foi possível adicionar o cliente. Ou ele já está conectado ou já têm clientes demais. Id: " + id); System.out.println("LOG: Fechando a conexão: " + connectionOrigin); return false; } System.out.println("LOG: Novo cliente conectado! Enviando mesnsagem de confirmação. Id: " + id + " Conexao: " + connectionOrigin); /* Here is the error */ sendMessage(id, "Servidor pet: connection accepted"); System.out.println("LOG: Novo cliente conectado! Id: " + id + " Conexao: " + connectionOrigin); return true; } And finally, the method sendMessage(SelectionKey key) looks like this: private void sendMessage(int destId, String msg) { Charset charset = Charset.forName("ISO-8859-1"); CharBuffer charBuffer = CharBuffer.wrap(msg, 0, msg.length()); ByteBuffer bf = charset.encode(charBuffer); //bf.flip(); int bytesSent = 0; SelectionKey key = clientsMap.getClient(destId); SocketChannel sc = (SocketChannel) key.channel(); try { / int total_bytes_sent = 0; while(total_bytes_sent < msg.length()){ bytesSent = sc.write(bf); total_bytes_sent += bytesSent; } System.out.println("LOG: Bytes enviados para o cliente " + destId + ": "+ total_bytes_sent + " Tamanho da mensagem: " + msg.length()); } catch (IOException e) { System.out.println("LOG: Erro ao mandar mensagem para: " + destId); System.out.println("LOG: " + e.getLocalizedMessage()); } } So, what is happening is that the server, when send a message, prints something like this: LOG: Bytes sent to the client: 28 Size of the message: 28 So, it tells that it sent the data, but the chat client keeps blocking, waiting in the recv() method. So, the data never gets to it. When I close the server application, though, all the data appears in the client. I wonder why. It is important to say that the client is in C and the server JAVA, and I'm running both in the same machine, an Ubuntu Guest in virtualbox under windows. I also run both under windows host and under linuxes hosts, and keep getting the same strange problem. I'm sorry for the great lenght of this question, but I already searched a lot of places for an answer, found a lot of tutorials and questions, including here at StackOverflow, but coundn't find a reasonable explanation. I am really not liking this Java NIO, and i saw a lot of people complaining about it too. I am thinking that if I had done that in C it would have been a lot easier :-D So, if someone could help me and even discuss this behavor, it would be great! :-) Thanks everybody in advance, Péterson

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  • install linux guest on windows host using a .iso file on vmserver 2.0

    - by Bob
    I have done this in vmserver 1.0. I set the Cd to point to the .iso file. Then I start the vm and it starts to install. I am on the summary screen. I have created a VM. I go to CD/DVD - edit I change the radio button to Host Medio Then I select ISO Image and Browse. Here is the problem: Then in my browser I get Inventory - Virtual Machine (directory) I don't know where that directory is. I have my ISO's in a folder on my desktop. I can't browse to my desktop. When I try to just put in a path such as c:\mydesktop\mylinuxdirectory\linux1.iso The following problems have been found:OKThe ISO Image File Path must be a valid file path in the format: "[datastore] /path/to/isoimage.iso". so how do I browse to an ISO? I can add the iso after I start the VM, but by then its too late and the VM says no Operating System installed.

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  • What makes Ometa special?

    - by Brian
    Ometa is "a new object-oriented language for pattern matching." I've encountered pattern matching in languages like Oz tools to parse grammars like Lexx/Yacc or Pyparsing before. Despite looking at example code, reading discussions, and talking to a friend, I still am not able to get a real understanding of what makes Ometa special (or at least, why some people think it is). Any explanation?

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  • unknown error in shell

    - by lego69
    can somebody explain me what does this error mean: > ./rank lines.in 'nknown option: `- Usage: tcsh [ -bcdefilmnqstvVxX ] [ argument ... ]. this is my script rank: #! /bin/tcsh -f set line = `cat ${1}` echo $line I think that the problem I have is with first row #! /bin/tcsh -f I'm working on Windows! but after I wrote script on windows editor, I converted it using dos2unix rank, what can be the problem, thanks in advance for any help

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  • Do you have any tips for comments code to keep them in step with the code?

    - by Rob Wells
    G'day, I've read both of Steve McConnell's excellent Code Complete books "Code Complete" and "Code Complete 2" and was wondering if people have any other suggestions for commenting code. My commenting mantra could be summed up by the basic idea of expressing "what the code below cannot say". While enjoying this interesting blog post by Jeff about commenting I was still left wondering "When coding, when do you feel a comment is required?" Edit: Oops. Seems to be a duplicate of this question http://stackoverflow.com/questions/121945/how-do-you-like-your-comments so sorry for the noise. Thanks to my, seemingly, SO shadow for pointing it out - wouldn't have thought I was that interesting. Now off to read the original post and see if it is relevant. Edit: I meant to emphasise the best appraoch to ensure that your comments will stay in step with the code. Maybe expressing an intent rather than the mechansim for instance.

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  • Bubble sort algorithm implementations (Haskell vs. C)

    - by kingping
    Hello. I have written 2 implementation of bubble sort algorithm in C and Haskell. Haskell implementation: module Main where main = do contents <- readFile "./data" print "Data loaded. Sorting.." let newcontents = bubblesort contents writeFile "./data_new_ghc" newcontents print "Sorting done" bubblesort list = sort list [] False rev = reverse -- separated. To see rev2 = reverse -- who calls the routine sort (x1:x2:xs) acc _ | x1 > x2 = sort (x1:xs) (x2:acc) True sort (x1:xs) acc flag = sort xs (x1:acc) flag sort [] acc True = sort (rev acc) [] False sort _ acc _ = rev2 acc I've compared these two implementations having run both on file with size of 20 KiB. C implementation took about a second, Haskell — about 1 min 10 sec. I have also profiled the Haskell application: Compile for profiling: C:\Temp ghc -prof -auto-all -O --make Main Profile: C:\Temp Main.exe +RTS -p and got these results. This is a pseudocode of the algorithm: procedure bubbleSort( A : list of sortable items ) defined as: do swapped := false for each i in 0 to length(A) - 2 inclusive do: if A[i] > A[i+1] then swap( A[i], A[i+1] ) swapped := true end if end for while swapped end procedure I wonder if it's possible to make Haskell implementation work faster without changing the algorithm (there's are actually a few tricks to make it work faster, but neither implementations have these optimizations)

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  • Coding guidelines + Best Practices?

    - by Chathuranga Chandrasekara
    I couldn't find any question that directly applies to my query so I am posting this as a new question. If there is any existing discussion that may help me, please point it out and close the question. Question: I am going to do a presentation on C# coding guidelines but it is not supposed to limit to coding standards. So I have a rough idea but I think I need to address good programing practices. So the contents will be something like this. Basic coding standards - Casing, Formatting etc. Good practices - Usage of Hashset over other data structures, String vs String Builder, String's immutability and using them effectively etc Really I would like to add more good practices (Especially to improve the performance.) So like to hear some more good practices to be used with C#. Any suggestions??? (No need of large descriptions :) Just the idea is sufficient.)

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  • Connecting to tcp server from client running in android device

    - by Andhravaala
    Hi, I am trying to connect to a standalone desktop tcp server(java) from tcp client application in android device. But I am not able to connect to that. I tried to write a desktop tcp client and tried to connect to server(remote server). It is working fine. But When I am trying to connecting from android, I am getting a IOException while creating Socket instance. Can anyone help me in this ? Thanks in advance....

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  • How to fake source ip-address of a udp-packet?

    - by youllknow
    Hi everyone! Think about the following: Your ISP offers you a dynamic ip-address (for example 123.123.123.123). My question is simple (the answer may not): Is it possible to send a single udp-packet with a outer source-ip (for example 124.124.124.124) to a fixed-ip server? I don't need to get a answer from the server. I just want to know if/how this one way communication can be done, using a faked source-ip address. I'm sorry for my bad English! Thanks for you help in advance!

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  • What is the exact problem with multiple inheritance?

    - by Totophil
    I can see people asking all the time whether multiple inheritance should be included into the next version of C# or Java and C++ folks, who are fortunate enough to have this ability, say that this is like giving someone a rope to eventually hang themselves. What’s the matter with the multiple inheritance? Are there any concrete samples?

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  • The Benefits of Constants

    - by onaclov2000
    I understand one of the big deals about constants is that you don't have to go through and update code where that constant is used all over the place. Thats great, but let's say you don't explicitly declare it as a constant. What benefit(s) exist(s) to take a variable that HAPPENS to actually not be changed and make it a constant, will this save on processing, and/or size of code...etc? Basically I have a program that the compiler is saying that a particular variable is not changed and thus can be declared a constant, I just wanted to know what the benefit to adding the constant qualifier to it would be, if it makes no difference then making that change adds no value and thus no point wasting time (this same scenario occurs in more then one place) going back and "fixing" all these variables. Thank you, onaclov

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  • Functional data structures in C++

    - by drg
    Does anyone know of a C++ data structure library providing functional (a.k.a. immutable, or "persistent" in the FP sense) equivalents of the familiar STL structures? By "functional" I mean that the objects themselves are immutable, while modifications to those objects return new objects sharing the same internals as the parent object where appropriate. Ideally, such a library would resemble STL, and would work well with Boost.Phoenix (caveat- I haven't actually used Phoenix, but as far as I can tell it provides many algorithms but no data structures, unless a lazily-computed change to an existing data structure counts - does it?)

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  • Functional way to get a matrix from text

    - by Elazar Leibovich
    I'm trying to solve some Google Code Jam problems, where an input matrix is typically given in this form: 2 3 #matrix dimensions 1 2 3 4 5 6 7 8 9 # all 3 elements in the first row 2 3 4 5 6 7 8 9 0 # each element is composed of three integers where each element of the matrix is composed of, say, three integers. So this example should be converted to #!scala Array( Array(A(1,2,3),A(4,5,6),A(7,8,9), Array(A(2,3,4),A(5,6,7),A(8,9,0), ) An imperative solution would be of the form #!python input = """2 3 1 2 3 4 5 6 7 8 9 2 3 4 5 6 7 8 9 0 """ lines = input.split('\n') print lines[0] m,n = (int(x) for x in lines[0].split()) array = [] row = [] A = [] for line in lines[1:]: for elt in line.split(): A.append(elt) if len(A)== 3: row.append(A) A = [] array.append(row) row = [] from pprint import pprint pprint(array) A functional solution I've thought of is #!scala def splitList[A](l:List[A],i:Int):List[List[A]] = { if (l.isEmpty) return List[List[A]]() val (head,tail) = l.splitAt(i) return head :: splitList(tail,i) } def readMatrix(src:Iterator[String]):Array[Array[TrafficLight]] = { val Array(x,y) = src.next.split(" +").map(_.trim.toInt) val mat = src.take(x).toList.map(_.split(" "). map(_.trim.toInt)). map(a => splitList(a.toList,3). map(b => TrafficLight(b(0),b(1),b(2)) ).toArray ).toArray return mat } But I really feel it's the wrong way to go because: I'm using the functional List structure for each line, and then convert it to an array. The whole code seems much less efficeint I find it longer less elegant and much less readable than the python solution. It is harder to which of the map functions operates on what, as they all use the same semantics. What is the right functional way to do that?

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  • UML Diagrams. When are they used?

    - by user69514
    So ok I understand that UML diagrams help for the construction of a program, but when are they used? Are they needed before you start coding? do you need them so you know what you have to code? Or do you use them after you have coded the program?

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  • Scala/Erlang use something like greenThread or not ?

    - by CHAPa
    Hi all, Im reading a lot about how scala/Erlang does lightweight threads and your concurrency model ( Actor Model ). Off course, some doubts appear in my head. Scala/Erlang use a approach similar to the old thread model used by java (greenThread) ? for example, suppose that there is a machine with 2 cores, so the scala/erlang environment will fork one thread per processor ? The other threads will be scheduled by user-space( scala VM / erlang vm ) environment. is it correct ? how under the hood that really work ? thanks a lot.

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  • pagesize and its ratio to 4096

    - by osgx
    Hello Are there systems, where minimal page of memory (pagesize) has a size not divisible by 2, or by 1024, 4096? Can it be 3000 or 3500? Will any posix program break, if pagesize will be not divisible by 1024?

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  • Writing an OS kernel in assembly with NASM

    - by Betamoo
    I want to know what is the standard way for writing a -simple- kernel to be compiled on NASM? To get it clearer: I was able to define the code block with all the following ways: [segment code] [segment .code] segment code segment .code [section code] [section .code] section code section .code I need to know what is the standard way to do that, And what is the difference between them... Thanks

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  • 'foreach' failing when using Parallel Task Library

    - by Chris Arnold
    The following code creates the correct number of files, but every file contains the contents of the first list. Can anyone spot what I've done wrong please? private IList<List<string>> GetLists() { // Code omitted for brevity... } private void DoSomethingInParallel() { var lists = GetLists(); var tasks = new List<Task>(); var factory = new TaskFactory(); foreach (var list in lists) { tasks.Add(factory.StartNew(() => { WriteListToLogFile(list); })); } Task.WaitAll(tasks.ToArray()); }

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