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  • Malware vs Viruses

    - by Kyle Brandt
    Is there a legitimate technical difference between malware and viruses? I have looked at the Wikipedia entries but I am not really seeing the difference. Currently, I kind of feel like this is just Symantec's way of getting more money for products (features). But maybe someone involved more deeply in this area can explain this.

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  • Moving SQL 2008 from 2003 OS to virtual 2012 OS

    - by David
    If we wanted to move SQL 2008 from a 2003 OS to a virtual 2012 OS (using VM Ware), does anyone know if there are any licensing or technical problems that would get in the way? All the instructions I've seen on moving SQL server from one machine to another assume the new machine has the same OS. I realize that there are licences have more cores and fail over capability, but for now we are fine with a simple installation.

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  • I need a tutorial for IIS/ASP.Net administration

    - by TheCodeMonk
    My technical support staff is only a little familiar with IIS and that's only as far as install goes. They need to install/update and configure ASP.Net web applications and WCF services that we write. Does anyone know any good tutorials/books/web sites that can help them understand some of the basic concepts of web applications and maintaining them on IIS?

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  • What is the difference between "load" and "fetch"?

    - by DragonLord
    I often encounter the words load and fetch in contexts where data are being read from some source, and they seem to have slightly different meanings. What's the difference? I've done some research and couldn't find any specific technical difference in general usage. While the term fetch can refer to one stage in CPU instruction execution, I've seen it used in contexts not related to CPUs, and I'm looking for an answer that is not specific to CPUs.

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  • On-call EC2 System Administrator

    - by Ball
    Is there a company that can respond to critical alerts for our EC2-based web application? Does such a service exist? If not, is there a place where we can find individuals who are willing to do 5-10 hrs of work a month responding to issues? Thanks. I know this is not a technical question, but I know it's a useful question for many companies and I'm not sure where else to ask.

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  • Word caption style

    - by Theodor
    This should be a common problem for anyone involved in technical writing. When you insert an image (or table etc.) caption in Word, you get this dialog. You enter your caption, in this case "A happy bicycle" (in yellow). This might be fine for a happy bicycle, but I want to have captions that have one style for the "enumeration" and one style for the description, as such: I have tried searching for this but I haven't found anything yet...

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  • 403 error with codeignitor

    - by DJB
    When I type in the standard web address for my site, I get a 403 error. However, when I type in a more exact address, say pointing to an index.php file, everything shows up fine. I'm using Anodyne Productions' Nova (SMS 3) which uses codeignitor. All accompanying software (PHP/MySQL) is compatible. I'm not a very technical person, so I'm hoping that this is an easy fix. Thanks for taking the time to answer.

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  • Sharing www folder breaks IIS sites

    - by SixfootJames
    I would like to share my www folder on my local machine with others in my office but when I do this and try navigate back to my localhost sites, I get the error HTTP Error 401.3 - Unauthorized. Now I have done a search already and found this http://forums.iis.net/t/1048841.aspx but it's a bit too technical for my skill level and was hoping someone here might be able to give me an easier way to understand this?

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  • Advice on Auditing VPN Connections

    - by WhiteAdam
    I've inherited a network that is truly a mess and the VPN config is amazingly long with confusing group names and very little documentation. I am curious to see how others have gone about auditing these types of connections. Are there any forms out there? What information should I collect? I assume things like Customer, Department, Technical Lead, and etc. Thanks in advance for any advice on this nightmare.

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  • How to connect to MySQL server in LAN

    - by waelbk
    Ok, Here is the technical description. My laptop's config: Ip Adress:192.168.2.5 Mysqlserver 5.0 on port : 3306 Operating system: Ubuntu the database is in this machine My friend's laptop config: Ip Adress:192.168.2.4 Mysqlserver 5.0 on port : 3306 Operating system: Windows XP Both are on a wireless LAN connected through a belkin router (192.168.2.1) I put this but its not working: url = "jdbc:mysql://192.168.2.5:3306/Database" so how configure to connect to this database?

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  • Does frequently formatting and reinstalling an operating system damage a hard drive?

    - by Closure Cowboy
    I tend to format and reinstall my operating system fairly frequently (about once a month). I apologize for my lack of technical terms, but I do not perform a "full format" (the type of formatting that zeros-out the existing data). Regardless, I know that most modern operating systems consume several GBs of data. Is doing this particular damaging to my hard drives? Would it matter whether I'm using a solid state drive (I'm not)?

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  • Tools for reformatting a spreadsheet into PDF?

    - by Todd
    I'm looking for an easy way to produce nicely formatted PDF documents using a subset of data available in a spreadsheet. Ideally, I would be able to produce one or more PDF documents for each row int e spreadsheet. Each row contains scientific data about a sample. There are numbers, text and images in the Spreadsheet that need to be reformatted in a nice way in a PDF. Ideally, the solution would be a piece of software that a non-technical but highly intelligent scientist could operate.

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  • Windows 7 Home Premium or better on a Netbook?

    - by Michael Stum
    I have a Netbook with Windows XP Home. Processor is an Atom N280 (1.66 GHz) with 2 GB of RAM. I noticed that newer Netbooks come with Windows 7, but as Starter Edition (which kinda sucks). I wonder if there is a technical reason for using Windows 7 Starter? Or would a better edition (x86) perform equally well? I'm currently considering Home Premium, but BitLocker and Offline Files might have me get Ultimate.

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  • What is a good operating system for relative newbies for setting up a website and source control? [on hold]

    - by Zeroth
    I'm part of a small group of people working on video games. We want to set up our own boxes to serve the website and handle the source control for development work. I have the most technical expertise, but its been a few years since I've set up or played with Linux or other open source operating systems. So I'm a bit out of the loop on what are the user-friendly open source operating systems out there right now?

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  • Java Generics, JPA 2, J2EE, JSF 2, GWT, Ajax, Oracle's Java Strategies, Flex, iPhone, Agile ALM, Gra

    - by Kim Won
    Great Indian Developer Summit 2010 – India's Biggest Polyglot Conference and Workshops for IT Software Professionals Bangalore, April 9, 2010: The GIDS.Java Conference and Workshops has announced the complete program of over 50 sessions on the present and future of the Java language and VM, how they are evolving to meet the community's ever-changing needs, and some of the cutting-edge tools, technologies & techniques used for building robust enterprise Java applications today. The GIDs.Java track at Great Indian Developer Summit takes place 22 and 23 April 2010, at the Indian Institute of Science in Bangalore. As one of the longest running independent developer conferences in India, GIDS.Java at the Great Indian Developer Summit 2010 is uniquely positioned to provide a blend of practical, pragmatic and immediately applicable knowledge and a glimpse of the future of technology. During 22 and 23 April 2010, GIDS.Java offers a multi-track conference, workshops, expo show floor, and networking opportunities. The first keynote at GIDS.Java "Pointy Haired Bosses and Pragmatic Programmers" is led by Dr. Venkat Subramaniam. He speaks about how each of us has a professional responsibility to be objective and make decisions that will help us and our teams be productive and deliver results. Venkat will pick on some fallacies, lay down facts, and discuss how to stay professional and objective in our daily efforts. The second keynote of the day explains the practical features that make the Cloud so interesting, and why everyone should start using it in their everyday life. Simone Brunozzi, Amazon Web Services Technology Evangelist, will detail technical examples, business details all mixed with a lot of Italian humor to ensure audience enjoy this talk without a single line of code. The third keynote of the day gives an exciting overview of directions in the Java space for Oracle, featuring concrete signs of Oracles heavy investment, a clear concise strategy overview, and deep dives into some of the most interesting pieces of technology being developed in the Java Platform Group today; such as JavaEE, JDK7, JavaFX, and our exciting new visual tools. Featuring demos by a Java evangelism team star, Simon Ritter, this talk takes you top to bottom in Java Technology. Featured talks at GID.Web include: Good, Bad, and Ugly of Java Generics, Venkat Subramaniam Pure Java Ajax: An Overview of GWT 2.0, Marty Hall How JPA 2.0 Makes a Good Thing Even Better, Mike Keith Building Enterprise RIAs with Adobe Flex and Java, Sujit Reddy G Integrated Ajax Support in JSF 2.0, Marty Hall Design Patterns in Java and Groovy, Venkat Subramaniam A Gentle Introduction to iPhone and Obj-C for Java Developers, Matthew McCullough Cloud Computing: Azure for Java Developers, Janakiram MSV Ajax Support in the Prototype JavaScript Library, Marty Hall First steps to IT Heaven Through the Cloud. Part III: .Java, Simone Brunozi Building Web 2.0 User Interfaces for Web Service Models using JSF, Frank Nimphius and Jobinesh P Acceptance Test Driven Development, John Tobin and Mohammed Mohsinali Architecting Your Java Applications for the Cloud, Praveen Srivatsa Effective Java, Venkat Subramaniam The Amazing Groovy Weight-loss Plan, Scott Davis Enterprise Modeling - from Conceptual Planning to Technical Blueprints, J Sripad Java Collections Renaissance, Donald Raab and Vlad Zakharov Power 7 and IBM J9VM, Himanshu Goyal A Whistle-stop Tour of Maven 3.0, Matthew McCullough Mass Volume Opportunities for Java Developers, Jouko Nuottila Emerging Technology Complex Event Processing, Duvvuri Srinivas Agile ALM for Distributed Development, Karthi Swaminathan Dim Sum Grails - A Sampler of Practical Non Database-Driven Grails Applications, Scott Davis Diagnosing Performance Bottlenecks in J2EE, Deepak Kaul Business Driven Identity Management, Suneet Agera Combining Java EE with OSGi using Eclipse Gemini, Mike Keith Workshop: Essence of Functional Programming, Venkat Subramaniam Workshop: Agile Development, Tools, and Teams and Scrum Certification, Stephen Forte Workshop: Cloud Computing Boot Camp on the Google App Engine, Matthew McCullough Workshop: Building Your First Amazon App, Simone Brunozzi Workshop: The 180-min AJAX and JSON Spike Class, Scott Davis Workshop: PHP + Adobe Flex = Killer RIA, Shyamprasad P Workshop: User Expereince Evaluation Model Walkthrough, Sanna Häiväläinen Workshop: Building Data Centric Applications using Adobe Flex and Java, Prashant Singh Workshop: Monetizing your Apps with PayPal X Payments Platform, Khurram Khan, Praveen Alavilli Sponsors of Great Indian Developer Summit 2010 include: Platinum sponsors Microsoft, Oracle Forum Nokia and Adobe; Gold sponsors Intel and SAP; Silver sponsors Quest Software, PayPal, Telerik and AMT. About Great Indian Developer Summit Great Indian Developer Summit is the gold standard for India's software developer ecosystem for gaining exposure to and evaluating new projects, tools, services, platforms, languages, software and standards. Packed with premium knowledge, action plans and advise from been-there-done-it veterans, creators, and visionaries, the 2010 edition of Great Indian Developer Summit features focused sessions, case studies, workshops and power panels that will transform you into a force to reckon with. Featuring 3 co-located conferences: GIDS.NET, GIDS.Web, GIDS.Java and an exclusive day of in-depth tutorials - GIDS.Workshops, from 20 April to 24 April at the IISc campus in Bangalore. At GIDS you'll participate in hundreds of sessions encompassing the full range of Microsoft computing, Java, Agile, RIA, Rich Web, open source/standards, languages, frameworks and platforms, practical tutorials that deep dive into technical skill and best practices, inspirational keynote presentations, an Expo Hall featuring dozens of the latest projects and products activities, engaging networking events, and the interact with the best and brightest of speakers from around the world. For further information on GIDS 2010, please visit the summit on the web http://www.developersummit.com/ A Saltmarch Media Press Release E: [email protected] Ph: +91 80 4005 1000

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  • to understand the code- how the heap is written in process migration in solaris

    - by akshay
    hi guys i need help understanding what this piece of code actually does as it is a part of my project i am stuck here. the code is from libckpt on solaris. /********************************** * function: write_heap * args: map_fd -- file descriptor for map file * data_fd -- file descriptor for data file * returns: no. of chunks written on success, -1 on failure * side effects: writes all included segments of the heap to ckpt files * misc.: If we are forking and copyonwrite is set, we will write the heap from bottom to top, moving the brk pointer up each time so that we don't get a page copied if the * called from: take_ckpt() ***********************************/ static int write_heap(int map_fd, int data_fd) { Dlist curptr, endptr; int no_chunks=0, pn; long size; caddr_t stop, addr; if(ckptflags.incremental){ /-- incremental checkpointing on? --/ endptr = ckptglobals.inc_list-main-flink; /*-- for each included chunk of the heap --*/ for(curptr = ckptglobals.inc_list->main->blink->blink; curptr != endptr; curptr = curptr->blink){ /*-- write out the last page in the included chunk --*/ stop = curptr->addr; pn = ((long)curptr->stop - (long)sys.DATASTART) / PAGESIZE; if(isdirty(pn)){ addr = (caddr_t)max((long)curptr->addr, (long)((pn * PAGESIZE) + sys.DATASTART)); size = (long)curptr->stop - (long)addr; debug(stderr, "DEBUG: Writing heap from 0x%x to 0x%x, pn = %d\n", addr, addr+size, pn); if(write_chunk(addr, size, map_fd, data_fd) == -1){ return -1; } if((int)addr > (int)(&end) && ckptflags.enhanced_fork){ brk(addr); } no_chunks++; } /*-- write out all the whole pages in the middle of the chunk --*/ for(pn--; pn * PAGESIZE + sys.DATASTART >= stop; pn--){ if(isdirty(pn)){ addr = (caddr_t)((pn * PAGESIZE) + sys.DATASTART); debug(stderr, "DEBUG: Writing heap from 0x%x to 0x%x, pn = %d\n", addr, addr+PAGESIZE, pn); if(write_chunk(addr, PAGESIZE, map_fd, data_fd) == -1){ return -1; } if((int)addr > (int)(&end) && ckptflags.enhanced_fork){ brk(addr); } no_chunks++; } } /*-- write out the first page in the included chunk --*/ addr = curptr->addr; size = ((pn+1) * PAGESIZE + sys.DATASTART) - addr; if(size > 0 && (isdirty(pn))){ debug(stderr, "DEBUG: Writing heap from 0x%x to 0x%x\n", addr, addr+size); if(write_chunk(addr, size, map_fd, data_fd) == -1){ return -1; } if((int)addr > (int)(&end) && ckptflags.enhanced_fork){ brk(addr); } no_chunks++; } } } else{ /-- incremental checkpointing off! --/ endptr = ckptglobals.inc_list-main-blink; /*-- for each included chunk of the heap --*/ for(curptr = ckptglobals.inc_list->main->flink->flink; curptr != endptr; curptr = curptr->flink){ debug(stderr, "DEBUG: saving memory from 0x%x to 0x%x\n", curptr->addr, curptr->addr+curptr->size); if(write_chunk(curptr->addr, curptr->size, map_fd, data_fd) == -1){ return -1; } if((int)addr > (int)(&end) && ckptflags.enhanced_fork){ brk(addr); } no_chunks++; } } return no_chunks; }

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  • Why does Saxon evaluate the result-document URI to be the same?

    - by Jan
    My XSL source document looks like this <Topology> <Environment> <Id>test</Id> <Machines> <Machine> <Id>machine1</Id> <modules> <module>m1</module> <module>m2</module> </modules> </Machine> </Machines> </Environment> <Environment> <Id>production</Id> <Machines> <Machine> <Id>machine1</Id> <modules> <module>m1</module> <module>m2</module> </modules> </Machine> <Machine> <Id>machine2</Id> <modules> <module>m3</module> <module>m4</module> </modules> </Machine> </Machines> </Environment> </Topology> I want to create one result-document per machine, so I use the following stylesheet giving modelDir as path for the result-documents as parameter. <xsl:output method="xml" version="1.0" encoding="UTF-8" indent="yes" name="myXML" doctype-system="http://java.sun.com/dtd/properties.dtd"/> <xsl:template match="/"> <xsl:for-each-group select="/Topology/Environment/Machines/Machine" group-by="Id"> <xsl:variable name="machine" select="Id"/> <xsl:variable name="filename" select="concat($modelDir,$machine,'.xml')" /> <xsl:message terminate="no">Writing machine description to <xsl:value-of select="$filename"/></xsl:message> <xsl:result-document href="$filename" format="myXML"> <xsl:variable name="currentMachine" select="Id"/> <xsl:for-each select="current-group()/LogicalHosts/LogicalHost"> <xsl:variable name="environment" select="normalize-space(../../../../Id)"/> <xsl:message terminate="no">Module <xsl:value-of select="."/> for <xsl:value-of select="$environment"/></xsl:message> </xsl:for-each> </xsl:result-document> </xsl:for-each-group> </xsl:template> As my messages show me this seems to work fine - if saxon would not evaluate the URI of the result-document to be the same and thus give the following output. Writing machine description to target/build/model/m1.xml Module m1 for test Module m2 for test Module m1 for production Module m2 for production Writing machine description to target/build/model/m2.xml Error at xsl:result-document on line 29 of file:/C:/Projekte/.../machine.xsl: XTDE1490: Cannot write more than one result document to the same URI, or write to a URI that has been read: file:/C:/Projekte/.../$filename file:/C:/Projekte/.../machine.xsl(29,-1) : here Cannot write more than one result document to the same URI, or write to a URI that has been read: file:/C:/Projekte/.../$filename ; SystemID: file:/C:/Projekte/.../machine.xsl; Line#: 29; Column#: -1 net.sf.saxon.trans.DynamicError: Cannot write more than one result document to the same URI, or write to a URI that has been read: file:/C:/Projekte/.../$filename at net.sf.saxon.instruct.ResultDocument.processLeavingTail(ResultDocument.java:300) at net.sf.saxon.instruct.Block.processLeavingTail(Block.java:365) at net.sf.saxon.instruct.Instruction.process(Instruction.java:91) Any ideas on how to solve this?

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  • Is there a library available which easily can record and replay results of API calls?

    - by Billy ONeal
    I'm working on writing various things that call relatively complicated Win32 API functions. Here's an example: //Encapsulates calling NtQuerySystemInformation buffer management. WindowsApi::AutoArray NtDll::NtQuerySystemInformation( SystemInformationClass toGet ) const { AutoArray result; ULONG allocationSize = 1024; ULONG previousSize; NTSTATUS errorCheck; do { previousSize = allocationSize; result.Allocate(allocationSize); errorCheck = WinQuerySystemInformation(toGet, result.GetAs<void>(), allocationSize, &allocationSize); if (allocationSize <= previousSize) allocationSize = previousSize * 2; } while (errorCheck == 0xC0000004L); if (errorCheck != 0) { THROW_MANUAL_WINDOWS_ERROR(WinRtlNtStatusToDosError(errorCheck)); } return result; } //Client of the above. ProcessSnapshot::ProcessSnapshot() { using Dll::NtDll; NtDll ntdll; AutoArray systemInfoBuffer = ntdll.NtQuerySystemInformation( NtDll::SystemProcessInformation); BYTE * currentPtr = systemInfoBuffer.GetAs<BYTE>(); //Loop through the results, creating Process objects. SYSTEM_PROCESSES * asSysInfo; do { // Loop book keeping asSysInfo = reinterpret_cast<SYSTEM_PROCESSES *>(currentPtr); currentPtr += asSysInfo->NextEntryDelta; //Create the process for the current iteration and fill it with data. std::auto_ptr<ProcImpl> currentProc(ProcFactory( static_cast<unsigned __int32>(asSysInfo->ProcessId), this)); NormalProcess* nptr = dynamic_cast<NormalProcess*>(currentProc.get()); if (nptr) { nptr->SetProcessName(asSysInfo->ProcessName); } // Populate process threads for(ULONG idx = 0; idx < asSysInfo->ThreadCount; ++idx) { SYSTEM_THREADS& sysThread = asSysInfo->Threads[idx]; Thread thread( currentProc.get(), static_cast<unsigned __int32>(sysThread.ClientId.UniqueThread), sysThread.StartAddress); currentProc->AddThread(thread); } processes.push_back(currentProc); } while(asSysInfo->NextEntryDelta != 0); } My problem is in mocking out the NtDll::NtQuerySystemInformation method -- namely, that the data structure returned is complicated (Well, here it's actually relatively simple but it can be complicated), and writing a test which builds the data structure like the API call does can take 5-6 times as long as writing the code that uses the API. What I'd like to do is take a call to the API, and record it somehow, so that I can return that recorded value to the code under test without actually calling the API. The returned structures cannot simply be memcpy'd, because they often contain inner pointers (pointers to other locations in the same buffer). The library in question would need to check for these kinds of things, and be able to restore pointer values to a similar buffer upon replay. (i.e. check each pointer sized value if it could be interpreted as a pointer within the buffer, change that to an offset, and remember to change it back to a pointer on replay -- a false positive rate here is acceptable) Is there anything out there that does anything like this?

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  • Opening a file with a variable as name & checking for undefined values

    - by Harm De Weirdt
    Hello everyone. I'm having some problems writing data into a file using perl. sub startNewOrder{ my $name = makeUniqueFileName(); open (ORDER, ">$name.txt") or die "can't open file: $!\n"; format ORDER_TOP = PRODUCT<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<CODE<<<<<<<<AANTAL<<<<EENHEIDSPRIJS<<<<<<TOTAAL<<<<<<< . format ORDER = @<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<< @<<<<<<<< @<<<< @<<<<<< @<<<<< $title, $code, $amount, $price, $total . close (ORDER); } This is the sub I use to make the file. (I translated most of it) The makeUniqueFileName method makes a fileName based upon current time("minuteshoursdayOrder"). The problem now is that I have to write in this file in another sub. sub addToOrder{ print "give productcode:"; $code = <STDIN>; chop $code; print "Give amount:"; $amount = <STDIN>; chop $amount; if($inventory{$code} eq undef){ #Does the product exist? print "This product does not exist"; }elsif($inventory{$code}[2] < $amount && !defined($inventaris{$code}[2]) ){ #Is there enough in the inventory? print "There is not enough in stock" }else{ $inventory{$code}[2] -= $amount; #write in order file open (ORDER ">>$naam.txt") or die "can't open file: $!\n"; $title = $inventory{$code}[0]; $code = $code; $amount = $inventory{$code}[2]; $price = $inventory{$code}[1]; $total = $inventory{$code}[1]; write; close(ORDER); } %inventory is a hashtable that has the productcode as key and an array with the title, price and amount as value. There are two problems here: when I enter an invalid product number, I still have to enter an amount even while my code says it should print the error directly after checking if there is a product with the given code. The second problem is that the writing doesn't seem to work. It always give's a "No such file or directory" error. Is there a way to open the ORDER file i made in the first sub without having to make $name not local? Or just a way to write in this file? I really don't know how to start here. I can't really find much info on writing a file that has been closed before, and in a different sub.. Any help is appreciated, Harm

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  • Opening a file with a variable as name and checking for undefined values

    - by Harm De Weirdt
    I'm having some problems writing data into a file using Perl. sub startNewOrder{ my $name = makeUniqueFileName(); open (ORDER, ">$name.txt") or die "can't open file: $!\n"; format ORDER_TOP = PRODUCT<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<CODE<<<<<<<<AANTAL<<<<EENHEIDSPRIJS<<<<<<TOTAAL<<<<<<< . format ORDER = @<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<< @<<<<<<<< @<<<< @<<<<<< @<<<<< $title, $code, $amount, $price, $total . close (ORDER); } This is the sub I use to make the file. (I translated most of it.) The makeUniqueFileName method makes a fileName based upon current time("minuteshoursdayOrder"). The problem now is that I have to write to this file in another sub. sub addToOrder{ print "give productcode:"; $code = <STDIN>; chop $code; print "Give amount:"; $amount = <STDIN>; chop $amount; if($inventory{$code} eq undef){ #Does the product exist? print "This product does not exist"; }elsif($inventory{$code}[2] < $amount && !defined($inventaris{$code}[2]) ){ #Is there enough in the inventory? print "There is not enough in stock" }else{ $inventory{$code}[2] -= $amount; #write in order file open (ORDER ">>$naam.txt") or die "can't open file: $!\n"; $title = $inventory{$code}[0]; $code = $code; $amount = $inventory{$code}[2]; $price = $inventory{$code}[1]; $total = $inventory{$code}[1]; write; close(ORDER); } %inventory is a hashtable that has the productcode as key and an array with the title, price and amount as value. There are two problems here: when I enter an invalid product number, I still have to enter an amount even while my code says it should print the error directly after checking if there is a product with the given code. The second problem is that the writing doesn't seem to work. It always give's a "No such file or directory" error. Is there a way to open the ORDER file I made in the first sub without having to make $name not local? Or just a way to write in this file? I really don't know how to start here. I can't really find much info on writing a file that has been closed before, and in a different sub. Any help is appreciated, Harm

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  • Parallelism in .NET – Part 2, Simple Imperative Data Parallelism

    - by Reed
    In my discussion of Decomposition of the problem space, I mentioned that Data Decomposition is often the simplest abstraction to use when trying to parallelize a routine.  If a problem can be decomposed based off the data, we will often want to use what MSDN refers to as Data Parallelism as our strategy for implementing our routine.  The Task Parallel Library in .NET 4 makes implementing Data Parallelism, for most cases, very simple. Data Parallelism is the main technique we use to parallelize a routine which can be decomposed based off data.  Data Parallelism refers to taking a single collection of data, and having a single operation be performed concurrently on elements in the collection.  One side note here: Data Parallelism is also sometimes referred to as the Loop Parallelism Pattern or Loop-level Parallelism.  In general, for this series, I will try to use the terminology used in the MSDN Documentation for the Task Parallel Library.  This should make it easier to investigate these topics in more detail. Once we’ve determined we have a problem that, potentially, can be decomposed based on data, implementation using Data Parallelism in the TPL is quite simple.  Let’s take our example from the Data Decomposition discussion – a simple contrast stretching filter.  Here, we have a collection of data (pixels), and we need to run a simple operation on each element of the pixel.  Once we know the minimum and maximum values, we most likely would have some simple code like the following: for (int row=0; row < pixelData.GetUpperBound(0); ++row) { for (int col=0; col < pixelData.GetUpperBound(1); ++col) { pixelData[row, col] = AdjustContrast(pixelData[row, col], minPixel, maxPixel); } } .csharpcode, .csharpcode pre { font-size: small; color: black; font-family: consolas, "Courier New", courier, monospace; background-color: #ffffff; /*white-space: pre;*/ } .csharpcode pre { margin: 0em; } .csharpcode .rem { color: #008000; } .csharpcode .kwrd { color: #0000ff; } .csharpcode .str { color: #006080; } .csharpcode .op { color: #0000c0; } .csharpcode .preproc { color: #cc6633; } .csharpcode .asp { background-color: #ffff00; } .csharpcode .html { color: #800000; } .csharpcode .attr { color: #ff0000; } .csharpcode .alt { background-color: #f4f4f4; width: 100%; margin: 0em; } .csharpcode .lnum { color: #606060; } This simple routine loops through a two dimensional array of pixelData, and calls the AdjustContrast routine on each pixel. As I mentioned, when you’re decomposing a problem space, most iteration statements are potentially candidates for data decomposition.  Here, we’re using two for loops – one looping through rows in the image, and a second nested loop iterating through the columns.  We then perform one, independent operation on each element based on those loop positions. This is a prime candidate – we have no shared data, no dependencies on anything but the pixel which we want to change.  Since we’re using a for loop, we can easily parallelize this using the Parallel.For method in the TPL: Parallel.For(0, pixelData.GetUpperBound(0), row => { for (int col=0; col < pixelData.GetUpperBound(1); ++col) { pixelData[row, col] = AdjustContrast(pixelData[row, col], minPixel, maxPixel); } }); Here, by simply changing our first for loop to a call to Parallel.For, we can parallelize this portion of our routine.  Parallel.For works, as do many methods in the TPL, by creating a delegate and using it as an argument to a method.  In this case, our for loop iteration block becomes a delegate creating via a lambda expression.  This lets you write code that, superficially, looks similar to the familiar for loop, but functions quite differently at runtime. We could easily do this to our second for loop as well, but that may not be a good idea.  There is a balance to be struck when writing parallel code.  We want to have enough work items to keep all of our processors busy, but the more we partition our data, the more overhead we introduce.  In this case, we have an image of data – most likely hundreds of pixels in both dimensions.  By just parallelizing our first loop, each row of pixels can be run as a single task.  With hundreds of rows of data, we are providing fine enough granularity to keep all of our processors busy. If we parallelize both loops, we’re potentially creating millions of independent tasks.  This introduces extra overhead with no extra gain, and will actually reduce our overall performance.  This leads to my first guideline when writing parallel code: Partition your problem into enough tasks to keep each processor busy throughout the operation, but not more than necessary to keep each processor busy. Also note that I parallelized the outer loop.  I could have just as easily partitioned the inner loop.  However, partitioning the inner loop would have led to many more discrete work items, each with a smaller amount of work (operate on one pixel instead of one row of pixels).  My second guideline when writing parallel code reflects this: Partition your problem in a way to place the most work possible into each task. This typically means, in practice, that you will want to parallelize the routine at the “highest” point possible in the routine, typically the outermost loop.  If you’re looking at parallelizing methods which call other methods, you’ll want to try to partition your work high up in the stack – as you get into lower level methods, the performance impact of parallelizing your routines may not overcome the overhead introduced. Parallel.For works great for situations where we know the number of elements we’re going to process in advance.  If we’re iterating through an IList<T> or an array, this is a typical approach.  However, there are other iteration statements common in C#.  In many situations, we’ll use foreach instead of a for loop.  This can be more understandable and easier to read, but also has the advantage of working with collections which only implement IEnumerable<T>, where we do not know the number of elements involved in advance. As an example, lets take the following situation.  Say we have a collection of Customers, and we want to iterate through each customer, check some information about the customer, and if a certain case is met, send an email to the customer and update our instance to reflect this change.  Normally, this might look something like: foreach(var customer in customers) { // Run some process that takes some time... DateTime lastContact = theStore.GetLastContact(customer); TimeSpan timeSinceContact = DateTime.Now - lastContact; // If it's been more than two weeks, send an email, and update... if (timeSinceContact.Days > 14) { theStore.EmailCustomer(customer); customer.LastEmailContact = DateTime.Now; } } Here, we’re doing a fair amount of work for each customer in our collection, but we don’t know how many customers exist.  If we assume that theStore.GetLastContact(customer) and theStore.EmailCustomer(customer) are both side-effect free, thread safe operations, we could parallelize this using Parallel.ForEach: Parallel.ForEach(customers, customer => { // Run some process that takes some time... DateTime lastContact = theStore.GetLastContact(customer); TimeSpan timeSinceContact = DateTime.Now - lastContact; // If it's been more than two weeks, send an email, and update... if (timeSinceContact.Days > 14) { theStore.EmailCustomer(customer); customer.LastEmailContact = DateTime.Now; } }); Just like Parallel.For, we rework our loop into a method call accepting a delegate created via a lambda expression.  This keeps our new code very similar to our original iteration statement, however, this will now execute in parallel.  The same guidelines apply with Parallel.ForEach as with Parallel.For. The other iteration statements, do and while, do not have direct equivalents in the Task Parallel Library.  These, however, are very easy to implement using Parallel.ForEach and the yield keyword. Most applications can benefit from implementing some form of Data Parallelism.  Iterating through collections and performing “work” is a very common pattern in nearly every application.  When the problem can be decomposed by data, we often can parallelize the workload by merely changing foreach statements to Parallel.ForEach method calls, and for loops to Parallel.For method calls.  Any time your program operates on a collection, and does a set of work on each item in the collection where that work is not dependent on other information, you very likely have an opportunity to parallelize your routine.

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