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• Really "wow" them in the interview

  - by Juliet

  Let me put it to you this way: I'm a top-notch programmer, but a notoriously bad interviewee. I've flunked 3 interviews consecutively because I get so nervous that my voice tightens at least 2 octaves higher and I start visibly shaking -- mind you, I can handle whatever technical questions the interviewer throws at me in that state, but I think it looks bad to come off as a quivering, squeaky-voiced young woman during a job interview. I've just got the personality type of a shy computer programmer. No matter how technical I am, I'm going to get passed up in favor of a smooth talker. I have another interview coming up shortly, and I want to really impress the company. Here are my trouble spots: What can I do to be less nervous during my interview? I always get really excited when I hear I have a face-to-face interview, but get more and more anxious as D-Day the interview approaches. My employers wants me to explain what I used to do at my prior employment. I'm a very chatty person and tend to talk/squeak for 10 minutes at a time. How long or short should I time my answers? On that note, when I'm explaining what I did at prior jobs, what exactly is my interviewer looking for? At some point, my interviewer will ask "do you have any questions for me while you're here?" I should, but what kinds of questions should I ask to show that I'm interested in being employed? My interviewer always asks why I'm looking for a new job. The real reason is that my present salary is $27K/yr [Edit to add: and I've yet to get a raise since I started], and I want to make more money -- otherwise the work environment is fine. How do I sugarcoat "I want to make more money" into something that sounds nicer? I have only one prior programmer job, and I've worked there for 18 months, but I have the skill of someone with 4 to 6 years of experience. What can I say to compete against applicants with more work experience? I took a low-paying $27K/yr programming job just to get my foot in IT, and I've been trying to leverage that job as a stepping stone to better opportunities. I get interviews because I consistently out-score senior-level developers in aptitude tests, and my desired salary range is right in the ballpark of what most companies want to offer. Unfortunately, while I've been a programming as a hobby for 10 years and I'm geared to graduate with my BA in Comp Sci in May '09, employers see me as a junior-level programmer with no degree. I want to prove them wrong and get a job that matches my skill level. I'd appreciate any advice anyone has to offer, especially if they can help me get a better job in the process.

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• Miller-rabin exception number?

  - by nightcracker

  Hey everyone. This question is about the number 169716931325235658326303. According to http://www.alpertron.com.ar/ECM.HTM it is prime. According to my miller-rabin implementation in python with 7 repetitions is is composite. With 50 repetitions it is still composite. With 5000 repetitions it is STILL composite. I thought, this might be a problem of my implementation. So I tried GNU MP bignum library, which has a miller-rabin primality test built-in. I tested with 1000000 repetitions. Still composite. This is my implementation of the miller-rabin primality test: def isprime(n, precision=7): if n == 1 or n % 2 == 0: return False elif n < 1: raise ValueError("Out of bounds, first argument must be > 0") d = n - 1 s = 0 while d % 2 == 0: d //= 2 s += 1 for repeat in range(precision): a = random.randrange(2, n - 2) x = pow(a, d, n) if x == 1 or x == n - 1: continue for r in range(s - 1): x = pow(x, 2, n) if x == 1: return False if x == n - 1: break else: return False return True And the code for the GMP test: #include <gmp.h> #include <stdio.h> int main(int argc, char* argv[]) { mpz_t test; mpz_init_set_str(test, "169716931325235658326303", 10); printf("%d\n", mpz_probab_prime_p(test, 1000000)); mpz_clear(test); return 0; } As far as I know there are no "exceptions" (which return false positives for any amount of repetitions) to the miller-rabin primality test. Have I stumpled upon one? Is my computer broken? Is the Elliptic Curve Method wrong? What is happening here? EDIT I found the issue, which is http://www.alpertron.com.ar/ECM.HTM. I trusted this applet, I'll contact the author his applet's implementation of the ECM is faulty for this number. Thanks. EDIT2 Hah, the shame! In the end it was something that went wrong with copy/pasting on my side. NOR the applet NOR the miller-rabin algorithm NOR my implementation NOR gmp's implementation of it is wrong, the only thing that's wrong is me. I'm sorry.

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• Beautifulsoup recursive attribute

  - by Marcos Placona

  Hi, trying to parse an XML with Beautifulsoup, but hit a brick wall when trying to use the "recursive" attribute with findall() I have a pretty odd xml format shown below: <?xml version="1.0"?> <catalog> <book id="bk101"> <author>Gambardella, Matthew</author> <title>XML Developer's Guide</title> <genre>Computer</genre> <price>44.95</price> <publish_date>2000-10-01</publish_date> <description>An in-depth look at creating applications with XML.</description> <catalog>true</catalog> </book> <book id="bk102"> <author>Ralls, Kim</author> <title>Midnight Rain</title> <genre>Fantasy</genre> <price>5.95</price> <publish_date>2000-12-16</publish_date> <description>A former architect battles corporate zombies, an evil sorceress, and her own childhood to become queen of the world.</description> <catalog>false</catalog> </book> </catalog> As you can see, the catalog tag repeats inside the book tag, which causes an error when I try to to something like: from BeautifulSoup import BeautifulStoneSoup as BSS catalog = "catalog.xml" def open_rss(): f = open(catalog, 'r') return f.read() def rss_parser(): rss_contents = open_rss() soup = BSS(rss_contents) items = soup.findAll('catalog', recursive=False) for item in items: print item.title.string rss_parser() As you will see, on my soup.findAll I've added recursive=false, which in theory would make it no recurse through the item found, but skip to the next one. This doesn't seem to work, as I always get the following error: File "catalog.py", line 17, in rss_parser print item.title.string AttributeError: 'NoneType' object has no attribute 'string' I'm sure I'm doing something stupid here, and would appreciate if someone could give me some help on how to solve this problem. Changing the HTML structure is not an option, this this code needs to perform well as it will potentially parse a large XML file. Thanks in advance, Marcos

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• java.lang.ClassNotFoundException: org.apache.struts2.dispatcher.FilterDispatcher

  - by user714698

  log when start the tomcat Apr 28, 2011 10:52:57 AM org.apache.catalina.core.AprLifecycleListener init INFO: The APR based Apache Tomcat Native library which allows optimal performance in production environments was not found on the java.library.path: D:\software\jdk1.5.0_06\bin;.;C:\WINDOWS\system32;C:\WINDOWS;D:/software/jdk1.5.0_06/bin/../jre/bin/client;D:/software/jdk1.5.0_06/bin/../jre/bin;C:\WINDOWS\system32;C:\WINDOWS;C:\WINDOWS\System32\Wbem;C:\Program Files\TortoiseSVN\bin.;D:\software\jdk1.5.0_06\bin;D:\software\Ant 1.7\bin;D:\software\Axis2-1.5.4\axis2-1.5.4-bin\axis2-1.5.4\bin;C:\Program Files\IDM Computer Solutions\UltraEdit\ Apr 28, 2011 10:52:58 AM org.apache.tomcat.util.digester.SetPropertiesRule begin WARNING: [SetPropertiesRule]{Server/Service/Engine/Host/Context} Setting property 'source' to 'org.eclipse.jst.jee.server:StrutsHelloWorld' did not find a matching property. Apr 28, 2011 10:52:58 AM org.apache.tomcat.util.digester.SetPropertiesRule begin WARNING: [SetPropertiesRule]{Server/Service/Engine/Host/Context} Setting property 'source' to 'org.eclipse.jst.jee.server:StrutsHelloWorld1' did not find a matching property. Apr 28, 2011 10:53:00 AM org.apache.coyote.http11.Http11Protocol init INFO: Initializing Coyote HTTP/1.1 on http-8080 Apr 28, 2011 10:53:00 AM org.apache.catalina.startup.Catalina load INFO: Initialization processed in 3657 ms Apr 28, 2011 10:53:00 AM org.apache.catalina.core.StandardService start INFO: Starting service Catalina Apr 28, 2011 10:53:00 AM org.apache.catalina.core.StandardEngine start INFO: Starting Servlet Engine: Apache Tomcat/6.0.32 Apr 28, 2011 10:53:01 AM org.apache.catalina.core.StandardContext filterStart SEVERE: Exception starting filter struts2 java.lang.ClassNotFoundException: org.apache.struts2.dispatcher.FilterDispatcher at org.apache.catalina.loader.WebappClassLoader.loadClass(WebappClassLoader.java:1680) at org.apache.catalina.loader.WebappClassLoader.loadClass(WebappClassLoader.java:1526) at org.apache.catalina.core.ApplicationFilterConfig.getFilter(ApplicationFilterConfig.java:269) at org.apache.catalina.core.ApplicationFilterConfig.setFilterDef(ApplicationFilterConfig.java:422) at org.apache.catalina.core.ApplicationFilterConfig.<init>(ApplicationFilterConfig.java:115) at org.apache.catalina.core.StandardContext.filterStart(StandardContext.java:4071) at org.apache.catalina.core.StandardContext.start(StandardContext.java:4725) at org.apache.catalina.core.ContainerBase.start(ContainerBase.java:1053) at org.apache.catalina.core.StandardHost.start(StandardHost.java:840) at org.apache.catalina.core.ContainerBase.start(ContainerBase.java:1053) at org.apache.catalina.core.StandardEngine.start(StandardEngine.java:463) at org.apache.catalina.core.StandardService.start(StandardService.java:525) at org.apache.catalina.core.StandardServer.start(StandardServer.java:754) at org.apache.catalina.startup.Catalina.start(Catalina.java:595) at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39) at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25) at java.lang.reflect.Method.invoke(Method.java:585) at org.apache.catalina.startup.Bootstrap.start(Bootstrap.java:289) at org.apache.catalina.startup.Bootstrap.main(Bootstrap.java:414) Apr 28, 2011 10:53:01 AM org.apache.catalina.core.StandardContext start SEVERE: Error filterStart Apr 28, 2011 10:53:01 AM org.apache.catalina.core.StandardContext start SEVERE: Context [/StrutsHelloWorld1] startup failed due to previous errors log4j:WARN No appenders could be found for logger (com.opensymphony.xwork2.config.providers.XmlConfigurationProvider). log4j:WARN Please initialize the log4j system properly. Apr 28, 2011 10:53:05 AM org.apache.coyote.http11.Http11Protocol start INFO: Starting Coyote HTTP/1.1 on http-8080 Apr 28, 2011 10:53:05 AM org.apache.jk.common.ChannelSocket init INFO: JK: ajp13 listening on /0.0.0.0:8009 Apr 28, 2011 10:53:05 AM org.apache.jk.server.JkMain start INFO: Jk running ID=0 time=0/46 config=null Apr 28, 2011 10:53:05 AM org.apache.catalina.startup.Catalina start INFO: Server startup in 4951 ms

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• Publishing WCF .NET 3.5 to IIS 6 (Windows Server 2003)

  - by Adam

  I've been developing a WCF web service using .NET 3.5 with IIS7 and it works perfectly on my local computer. I tried publishing it to a server running IIS 6 and even though I can view the WSDL in my browser, the client application doesn't seem to be connecting to it correctly. I launched a packet sniffing app (Charles Proxy) and the response for the first message comes back to the client empty (0 bytes). Every message after the first one times out. The WCF service is part of a larger application that uses ASP .NET 3.5. That application has been working fine on IIS 6 for awhile now so I think it's something specific to WCF. I also tried throwing an exception in the SVC file to see if it made it that far and the exception never got thrown so I have a feeling it's something more low level that's not working. Any thoughts? Is there anything I need to install on the IIS5 server? If so how am I still able to view the WSDL in my browser? The service is being consumed via an SVC file using basicHttpBinding Here's the meat of the Web.Config (let me know if you need any other part of it): <system.net> <defaultProxy> <proxy usesystemdefault="False" proxyaddress="http://127.0.0.1:80" bypassonlocal="True"/> </defaultProxy> </system.net> ... <system.serviceModel> <services> <service name="Nexternal.Service.XMLTools.VNService" behaviorConfiguration="VNServiceBehavior"> <!--The first endpoint would be picked up from the confirg this shows how the config can be overriden with the service host--> <endpoint address="" binding="basicHttpBinding" contract="Nexternal.Service.XMLTools.IVNService" /> <endpoint address="mex" binding="mexHttpBinding" contract="IMetadataExchange" name="mexHttpBinding" /> </service> </services> <behaviors> <serviceBehaviors> <behavior name="VNServiceBehavior"> <serviceMetadata httpGetEnabled="true" /> <serviceDebug includeExceptionDetailInFaults="true" /> </behavior> </serviceBehaviors> </behaviors> <serviceHostingEnvironment aspNetCompatibilityEnabled="true" /> </system.serviceModel>

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• CSS selectors : should I make my CSS easier to read or optimise the speed

  - by Laurent Bourgault-Roy

  As I was working on a small website, I decided to use the PageSpeed extension to check if their was some improvement I could do to make the site load faster. However I was quite surprise when it told me that my use of CSS selector was "inefficient". I was always told that you should keep the usage of the class attribute in the HTML to a minimum, but if I understand correctly what PageSpeed tell me, it's much more efficient for the browser to match directly against a class name. It make sense to me, but it also mean that I need to put more CSS classes in my HTML. It make my .css file harder to read. I usually tend to mark my CSS like this : #mainContent p.productDescription em.priceTag { ... } Which make it easy to read : I know this will affect the main content and that it affect something in a paragraph tag (so I wont start to put all sort of layout code in it) that describe a product and its something that need emphasis. However it seem I should rewrite it as .priceTag { ... } Which remove all context information about the style. And if I want to use differently formatted price tag (for example, one in a list on the sidebar and one in a paragraph), I need to use something like that .paragraphPriceTag { ... } .listPriceTag { ... } Which really annoy me since I seem to duplicate the semantic of the HTML in my classes. And that mean I can't put common style in an unqualified .priceTag { ... } and thus I need to replicate the style in both CSS rule, making it harder to make change. (Altough for that I could use multiple class selector, but IE6 dont support them) I believe making code harder to read for the sake of speed has never been really considered a very good practice . Except where it is critical, of course. This is why people use PHP/Ruby/C# etc. instead of C/assembly to code their site. It's easier to write and debug. So I was wondering if I should stick with few CSS classes and complex selector or if I should go the optimisation route and remove my fancy CSS selectors for the sake of speed? Does PageSpeed make over the top recommandation? On most modern computer, will it even make a difference?

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• Is this slow WPF TextBlock performance expected?

  - by Ben Schoepke

  Hi, I am doing some benchmarking to determine if I can use WPF for a new product. However, early performance results are disappointing. I made a quick app that uses data binding to display a bunch of random text inside of a list box every 100 ms and it was eating up ~15% CPU. So I made another quick app that skipped the data binding/data template scheme and does nothing but update 10 TextBlocks that are inside of a ListBox every 100 ms (the actual product wouldn't require 100 ms updates, more like 500 ms max, but this is a stress test). I'm still seeing ~10-15% CPU usage. Why is this so high? Is it because of all the garbage strings? Here's the XAML: <Grid> <ListBox x:Name="numericsListBox"> <ListBox.Resources> <Style TargetType="TextBlock"> <Setter Property="FontSize" Value="48"/> <Setter Property="Width" Value="300"/> </Style> </ListBox.Resources> <TextBlock/> <TextBlock/> <TextBlock/> <TextBlock/> <TextBlock/> <TextBlock/> <TextBlock/> <TextBlock/> <TextBlock/> <TextBlock/> </ListBox> </Grid> Here's the code behind: public partial class Window1 : Window { private int _count = 0; public Window1() { InitializeComponent(); } private void OnLoad(object sender, RoutedEventArgs e) { var t = new DispatcherTimer(TimeSpan.FromSeconds(0.1), DispatcherPriority.Normal, UpdateNumerics, Dispatcher); t.Start(); } private void UpdateNumerics(object sender, EventArgs e) { ++_count; foreach (object textBlock in numericsListBox.Items) { var t = textBlock as TextBlock; if (t != null) t.Text = _count.ToString(); } } } Any ideas for a better way to quickly render text? My computer: XP SP3, 2.26 GHz Core 2 Duo, 4 GB RAM, Intel 4500 HD integrated graphics. And that is an order of magnitude beefier than the hardware I'd need to develop for in the real product.

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• WMI: Editing the Registry Remotely

  - by Quanta

  I'm trying to use the following code (poorly written, but it's just a proof of concept) to edit the registry of a computer on a domain. I have a domain account, and I've verified that the domain admin group is present in the local admin group on the machines I'm attempting to affect. I've connected to these other machines to perform other administrative type tasks, so I'm sure that I have administrative privileges on these hosts. All of StdRegProv's "get" methods work fine (http://msdn.microsoft.com/en-us/library/aa393664%28VS.85%29.aspx) but the "set" or "create" methods as well as check access all return "5" which is "Error_Access_Denied" according to winerror.h. So there's the problem: why do I get access denied when attempting to modify the registry? If anyone can help me figure this out, you'd have my utmost appreciation! I almost forgot, when I fire up Visual Studio in admin mode on my local machine and run the code against the local machine, it works flawlessly. If I don't start in admin mode on the local machine, the code fails, so I suspect there's a UAC problem maybe? using System; using System.Management; public class EditRemoteRegistry { public static void Main(string[] args) { ConnectionOptions options = new ConnectionOptions(); options.EnablePrivileges = true; options.Impersonation = ImpersonationLevel.Impersonate; options.Password = "password goes here"; //options.Authority = "my company's domain"; //options.Username = "Admin username"; ManagementScope scope = new ManagementScope("\\\\arbitraryhost\\root\\default", options); scope.Connect(); ManagementClass mc = new ManagementClass("StdRegProv"); ManagementBaseObject inParams = mc.GetMethodParameters("CreateKey"); inParams["hDefKey"] = (UInt32)2147483650; inParams["sSubKeyName"] = "Software\\Test"; ManagementBaseObject outParams = mc.InvokeMethod("CreateKey", inParams, null); //Should return a 0, but returns a 5, "Error_Access_Denied" Console.WriteLine("CreateKey Method returned " + outParams["returnValue"]); //This chunk works fine ManagementBaseObject inParams5 = mc.GetMethodParameters("GetDWORDValue"); inParams5["hDefKey"] = 2147483650; inParams5["sSubKeyName"] = "Software\\Test"; inParams5["sValueName"] = "testDWORDvalue"; ManagementBaseObject outParams5 = mc.InvokeMethod("GetDWORDValue", inParams5, null); Console.WriteLine("GetDWORDValue returned " + (UInt32)outParams5["returnValue"] + " "); Console.WriteLine((UInt32)outParams5["uValue"]); ManagementBaseObject inParams6 = mc.GetMethodParameters("SetStringValue"); inParams6["hDefKey"] = 2147483650; inParams6["sSubKeyName"] = "Software\\Test"; inParams6["sValueName"] = "TestStringValue"; inParams6["sValue"] = "Hello World!"; ManagementBaseObject outParams6 = mc.InvokeMethod("SetStringValue", inParams6, null); //Should return a 0, but returns a 5, "Error_Access_Denied" Console.WriteLine("SetStringValue returned " + outParams6["returnValue"]); Console.ReadKey(); } }

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• compiling opencv 2.4 on a 64 bit mac in Xcode

  - by Walt

  I have an opencv project that I've been developing under ubuntu 12.04, on a parellels VM on a mac which has an x86_64 architecture. There have been many screen switching performance issues that I believe are due to the VM, where linux video modes flip around for a couple seconds while camera access is made by the opencv application. I decided to moved the project into Xcode on the mac side of the computer to continue the opencv development. However, I'm not that familiar with xcode and am having trouble getting the project to build correctly there. I have xcode installed. I downloaded and decompressed the latest version of opencv on the mac, and ran: ~/src/opencv/build/cmake-gui -G Xcode .. per the instructions from willowgarage and various other locations. This appeared to work fine (however I'm wondering now if I'm missing an architecture setting in here, although it is 64-bit intel in Xcode). I then setup an xcode project with the source files from the linux project and changed the include directories to use /opt/local/include/... rather than the /usr/local/include/... I switched xcode to use the LLVM GCC compiler in the build settings for the project then set the Apple LLVM Dialog for C++ to Language Dialect to GNU++11 (which seems possibly inconsistant with the line above) I'm not using a makefile in xcode, (that I'm aware of - it has its own project file...) I was also running into a linker issue that looked like they may be resolved with the addition of this linker flag: -lopencv_video based on a similar posting here: other thread however in that case the person was using a Makefile in their project. I've tried adding this linker flag under "Other Linker Flags" in xcode build settings but still get build errors. I think I may have two issues here, one with the architecture settings when building the opencv libraries with Cmake, and one with the linker flag settings in my project. Currently the build error list looks like this: Undefined symbols for architecture x86_64: "cv::_InputArray::_InputArray(cv::Mat const&)", referenced from: _main in main.o "cv::_OutputArray::_OutputArray(cv::Mat&)", referenced from: _main in main.o "cv::Mat::deallocate()", referenced from: cv::Mat::release() in main.o "cv::Mat::copySize(cv::Mat const&)", referenced from: cv::Mat::Mat(cv::Mat const&)in main.o cv::Mat::operator=(cv::Mat const&)in main.o "cv::Mat::Mat(_IplImage const*, bool)", referenced from: _main in main.o "cv::imread(std::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, int)", referenced from: _main in main.o ---SNIP--- ld: symbol(s) not found for architecture x86_64 collect2: ld returned 1 exit status Can anyone provide some guidance on what to try next? Thanks, Walt

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• Linked List Design

  - by Jim Scott

  The other day in a local .NET group I attend the following question came up: "Is it a valid interview question to ask about Linked Lists when hiring someone for a .NET development position?" Not having a computer sciense degree and being a self taught developer my response was that I did not feel it was appropriate as I in 5 years of developer with .NET had never been exposed to linked lists and did not hear any compeling reason for a use for one. However the person commented that it is a very common interview question so I decided when I left that I would do some reasearch on linked lists and see what I might be missing. I have read a number of posts on stack overflow and various google searches and decided the best way to learn about them was to write my own .NET classes to see how they worked from the inside out. Here is my class structure Single Linked List Constructor public SingleLinkedList(object value) Public Properties public bool IsTail public bool IsHead public object Value public int Index public int Count private fields not exposed to a property private SingleNode firstNode; private SingleNode lastNode; private SingleNode currentNode; Methods public void MoveToFirst() public void MoveToLast() public void Next() public void MoveTo(int index) public void Add(object value) public void InsertAt(int index, object value) public void Remove(object value) public void RemoveAt(int index) Questions I have: What are typical methods you would expect in a linked list? What is typical behaviour when adding new records? For example if I have 4 nodes and I am currently positioned in the second node and perform Add() should it be added after or before the current node? Or should it be added to the end of the list? Some of the designs I have seen explaining things seem to expose outside of the LinkedList class the Node object. In my design you simply add, get, remove values and know nothing about any node object. Should the Head and Tail be placeholder objects that are only used to define the head/tail of the list? I require my Linked List be instantiated with a value which creates the first node of the list which is essentially the head and tail of the list. Would you change that ? What should the rules be when it comes to removing nodes. Should someone be able to remove all nodes? Here is my Double Linked List Constructor public DoubleLinkedList(object value) Properties public bool IsHead public bool IsTail public object Value public int Index public int Count Private fields not exposed via property private DoubleNode currentNode; Methods public void AddFirst(object value) public void AddLast(object value) public void AddBefore(object existingValue, object value) public void AddAfter(object existingValue, object value) public void Add(int index, object value) public void Add(object value) public void Remove(int index) public void Next() public void Previous() public void MoveTo(int index)

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• Revision control for writing programming lessons

  - by Dietrich Epp

  I'd like to write a series programming lessons that guide programmers to build a certain kind of program. After each lesson, I'd like to provide sample code that implements what that lesson covered, and the next lesson would use that code as a starting point. Right now I'm using Git to keep track of the code from lesson to lesson. Each lesson has its own branch. lesson1: A--B--C \ lesson2: D--E--F \ lesson3: G--H--I However, suppose that now I want to make it easier on the Windows programmers using my lessons, so I add a Visual Studio project to lesson 1 and then merge it into lessons 2 and 3. lesson1: A--B--C--------------J \ \ lesson2: D--E--F--------K \ \ lesson3: G--H--I--L And then someone points out a bug in lesson 2 that causes crashes on certain systems. (This diagram is where I am right now, and I'm having doubts about continuing along this path.) lesson1: A--B--C--------------J \ \ lesson2: D--E--F--------K--M \ \ \ lesson3: G--H--I--L--N Here are the problems I imagine having: If I had many lessons, and I fix something in lesson 1, am I going to have to spend fifteen minutes or more just merging that one simple change? I know I'll probably have to test all of those lessons again, but I can put that off. When I make a bunch of changes to various lessons on one computer, how do I pull all of the branches at the same time? If I decide to publish these lessons, I'd like a way to tag all of the branches to correspond with what I publish. I figure I'll just need to tag each branch separately, but it would be nice if there were a better way. When I look at the history, I imagine becoming terribly confused about what I've done. Compare the above diagram to a hypothetical diagram below, where I use rebase instead of merge (and rebase has its own problems): lesson1: A--B--C--J \ lesson2: D2--E2--F2--M \ lesson3: G2--H2--I2 Do any of you have experience working with a project like this? Should I consider using a different VCS, such as Darcs? (Note: it would be a real pain to use centralized VCS, so don't suggest one of those unless the benefits are clear.) Should I consider writing plugins or extra tools for a VCS (such as a "meta tag" which tags several branches)?

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• FASM vc MASM trasnlation problem in mov si, offset msg

  - by Ruben Trancoso

  hi folks, just did my first test with MASM and FASM with the same code (almos) and I falled in trouble. The only difference is that to produce just the 104 bytes I need to write to MBR in FASM I put org 7c00h and in MASM 0h. The problem is on the mov si, offset msg that in the first case transletes it to 44 7C (7c44h) and with masm translates to 44 00 (0044h)! but just when I change org 7c00h to org 0h in MASM. Otherwise it will produce the entire segment from 0 to 7dff. how do I solve it? or in short, how to make MASM produce a binary that begins at 7c00h as it first byte and subsequent jumps remain relative to 7c00h? .model TINY .code org 7c00h ; Boot entry point. Address 07c0:0000 on the computer memory xor ax, ax ; Zero out ax mov ds, ax ; Set data segment to base of RAM jmp start ; Jump to the first byte after DOS boot record data ; ---------------------------------------------------------------------- ; DOS boot record data ; ---------------------------------------------------------------------- brINT13Flag db 90h ; 0002h - 0EH for INT13 AH=42 READ brOEM db 'MSDOS5.0' ; 0003h - OEM name & DOS version (8 chars) brBPS dw 512 ; 000Bh - Bytes/sector brSPC db 1 ; 000Dh - Sectors/cluster brResCount dw 1 ; 000Eh - Reserved (boot) sectors brFATs db 2 ; 0010h - FAT copies brRootEntries dw 0E0h ; 0011h - Root directory entries brSectorCount dw 2880 ; 0013h - Sectors in volume, < 32MB brMedia db 240 ; 0015h - Media descriptor brSPF dw 9 ; 0016h - Sectors per FAT brSPH dw 18 ; 0018h - Sectors per track brHPC dw 2 ; 001Ah - Number of Heads brHidden dd 0 ; 001Ch - Hidden sectors brSectors dd 0 ; 0020h - Total number of sectors db 0 ; 0024h - Physical drive no. db 0 ; 0025h - Reserved (FAT32) db 29h ; 0026h - Extended boot record sig brSerialNum dd 404418EAh ; 0027h - Volume serial number (random) brLabel db 'OSAdventure' ; 002Bh - Volume label (11 chars) brFSID db 'FAT12 ' ; 0036h - File System ID (8 chars) ;------------------------------------------------------------------------ ; Boot code ; ---------------------------------------------------------------------- start: mov si, offset msg call showmsg hang: jmp hang msg db 'Loading...',0 showmsg: lodsb cmp al, 0 jz showmsgd push si mov bx, 0007 mov ah, 0eh int 10h pop si jmp showmsg showmsgd: retn ; ---------------------------------------------------------------------- ; Boot record signature ; ---------------------------------------------------------------------- dw 0AA55h ; Boot record signature END

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• .NET Application broken on one PC, unhandleable exception

  - by Bobby

  Hello people. I have a .NET 2.0 application with nothing fancy in it. It worked until yesterday on every PC I installed or copied it to, no matter if 2.0, 3.0, 3.5 or 3.5 SP1 was installed, no matter if it was Win2000, XP or even Win7 (in total 100+ machines). Yesterday I did my normal installation procedure and wanted to start it one time to check if everything is working...and it wasn't. The program crashed hard leaving me with the uninformative "Do you wanna report this error?" dialog. The problem is an exception in the Main(String[] args) routine of my application. The event viewer is showing the following entry: Event Type: ErrorEvent Source: .NET Runtime 2.0 Error Reporting Event Category: None Event ID: 5000 Date: 05/05/2010 Time: 16:09:09 User: N/A Computer: myClientPC Description: EventType clr20r3, P1 apomenu.exe, P2 1.4.90.53, P3 4bdedea4, P4 system.configuration, P5 2.0.0.0, P6 4889de74, P7 1a6, P8 136, P9 ioibmurhynrxkw0zxkyrvfn0boyyufow, P10 NIL. Well...great information. After a lot of searching I finally was able to get further information about this exception (by adding a handler for UnhandledExceptions directly in My.MyApplication.New(), Application.Designer.vb): System.Configuration.ConfigurationErrorsException Configuration system failed to initialize at System.Configuration.ClientConfigurationSystem.EnsureInit(String configKey) at System.Configuration.ClientConfigurationSystem.PrepareClientConfigSystem(String sectionName) at System.Configuration.ClientConfigurationSystem.System.Configuration.Internal.IInternalConfigSystem.GetSection(String sectionName) at System.Configuration.ConfigurationManager.GetSection(String sectionName) at System.Configuration.PrivilegedConfigurationManager.GetSection(String sectionName) at System.Net.Configuration.SettingsSectionInternal.get_Section() at System.Net.Sockets.Socket.InitializeSockets() at System.Net.Sockets.Socket.get_SupportsIPv4() at Microsoft.VisualBasic.ApplicationServices.WindowsFormsApplicationBase.get_HostName() at Microsoft.VisualBasic.ApplicationServices.WindowsFormsApplicationBase.RegisterChannel(Boolean SecureChannel) at Microsoft.VisualBasic.ApplicationServices.WindowsFormsApplicationBase.Run(String[] commandLine) at MyAppNameHere.My.MyApplication.Main(String[] Args) in 17d14f5c-a337-4978-8281-53493378c1071.vb:Line 81. And at this point I'm stuck...I'm out of ideas. I'm not using any kind of configuration system from the framework (no reference to System.Configuration, and there never was a MyAppnameHere.exe.config generated or distributed, nor have I seen this error before). I also found a bug report at Microsoft (Google Cache) about this bug (in another context, though). But as it seems, they won't even look at it. Every help is greatly appreciated! Edit: I'm using Visual Studio 2008 Prof.. Crash happens in Release- and Debug-Build on the client machine. Debugging the application directly on this machine is out of question I fear, 300+ Miles and they only have two computers to work with.

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• agent-based simulation: performance issue: Python vs NetLogo & Repast

  - by max

  I'm replicating a small piece of Sugarscape agent simulation model in Python 3. I found the performance of my code is ~3 times slower than that of NetLogo. Is it likely the problem with my code, or can it be the inherent limitation of Python? Obviously, this is just a fragment of the code, but that's where Python spends two-thirds of the run-time. I hope if I wrote something really inefficient it might show up in this fragment: UP = (0, -1) RIGHT = (1, 0) DOWN = (0, 1) LEFT = (-1, 0) all_directions = [UP, DOWN, RIGHT, LEFT] # point is just a tuple (x, y) def look_around(self): max_sugar_point = self.point max_sugar = self.world.sugar_map[self.point].level min_range = 0 random.shuffle(self.all_directions) for r in range(1, self.vision+1): for d in self.all_directions: p = ((self.point[0] + r * d[0]) % self.world.surface.length, (self.point[1] + r * d[1]) % self.world.surface.height) if self.world.occupied(p): # checks if p is in a lookup table (dict) continue if self.world.sugar_map[p].level > max_sugar: max_sugar = self.world.sugar_map[p].level max_sugar_point = p if max_sugar_point is not self.point: self.move(max_sugar_point) Roughly equivalent code in NetLogo (this fragment does a bit more than the Python function above): ; -- The SugarScape growth and motion procedures. -- to M ; Motion rule (page 25) locals [ps p v d] set ps (patches at-points neighborhood) with [count turtles-here = 0] if (count ps > 0) [ set v psugar-of max-one-of ps [psugar] ; v is max sugar w/in vision set ps ps with [psugar = v] ; ps is legal sites w/ v sugar set d distance min-one-of ps [distance myself] ; d is min dist from me to ps agents set p random-one-of ps with [distance myself = d] ; p is one of the min dist patches if (psugar >= v and includeMyPatch?) [set p patch-here] setxy pxcor-of p pycor-of p ; jump to p set sugar sugar + psugar-of p ; consume its sugar ask p [setpsugar 0] ; .. setting its sugar to 0 ] set sugar sugar - metabolism ; eat sugar (metabolism) set age age + 1 end On my computer, the Python code takes 15.5 sec to run 1000 steps; on the same laptop, the NetLogo simulation running in Java inside the browser finishes 1000 steps in less than 6 sec. EDIT: Just checked Repast, using Java implementation. And it's also about the same as NetLogo at 5.4 sec. Recent comparisons between Java and Python suggest no advantage to Java, so I guess it's just my code that's to blame? EDIT: I understand MASON is supposed to be even faster than Repast, and yet it still runs Java in the end.

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• QT vs. Net - REAL comparisons for R.A.D. projects

  - by Pirate for Profit

  Man in all these Qt vs. .NET discussions 90% these people argue about the dumbest crap. Trying to get a real comparison chart here, because I know a little about both frameworks but I don't know everything. I believe Qt and .NET both have strengths and weaknesses. This is to make a comparison that highlights these so people can make more informed decisions before embarking on a project, in the spirit of R.A.D. Event Handling In Qt the event handling system is very simple. You just emit signals when something cool happens and then catch them in slots. ie. // run some calculations, then emit valueChanged(30, false, 20.2); and then catching it, any object can make a slot to recieve that message easily void MyObj::valueChanged(int percent, bool ok, float timeRemaining). It's easy to "block" an event or "disconnect" when needed, and works seamlessly across threads... once you get the hang of it, it just seems a lot more natural and intuitive than the way the .NET event handling is set up (you know, void valueChanged(object sender, CustomEventArgs e). And I'm not just talking about syntax, because in the end the .NET anonymous delegates are the bomb. I'm also talking about in more than just reflection (because, yes, .NET obviously has much stronger reflection capabilities). I'm talking about in the way the system feels to a human being. Qt wins hands down for the simplest yet still flexible event handling system ever i m o. Plugins and such I do love some of the ease of C# compared to C++, as well as .NET's assembly architecture, even though it leads to a bunch of .dll's (there's ways to combine everything into a single exe though). That is a big bonus for modular projects, which are a PITA to import stuff in C++ as far as RAD is concerned. Database Ease of Doing Crap Also what about datasets and database manipulations. I think .net wins here but I'm not sure. Threading/Conccurency How do you guys think of the threading? In .NET, all I've ever done is make like a list of master worker threads with locks. I like QConcurrentFramework, you don't worry about locks or anything, and with the ease of the signal slot system across threads it's nice to get notified about the progress of things. QConcurrent is the simplest threading mechanism I've ever played with. Memory Usage Also what do you think of the overall memory usage comparison. Is the .NET garbage collector pretty on the ball and quick compared to the instantaneous nature of native memory management? Or does it just let programs leak up a storm and lag the computer then clean it up when it's about to really lag? Doesn't the just-in-time compiler make native code that is pretty good, like and that only happens the first time the program is run? However, I am a n00b who doesn't know what I'm talking about, please school me on the subject.

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• Using Excel VBA to send emails. Problem with attachments becoming embedded by accident.

  - by Alexei

  Hi, I am having an issue with an Excel macro I wrote that is used by several users within my company. It is used to send numerous emails daily with attachments that are also Excel workbooks. The issue is that sometimes, instead of the file simply being attached as it should be, it becomes an embedded object. This embedded object is openable by users on the email within the company (after clicking through the "YOu are about to activate an embedded object that may contain viruses or be otherwise harmful to your computer. It is important to be certain that it is from a trustworthy source. Do you want to continue?"), but those outside of the company do not see it at all. The email appears to have no attachment at all. Curiously, this appears to happen randomly, and only on some computers. So if the list has 15 email lists and attachments, it seems to happen randomly to anywhere between 0 and 15 of the emails. To be clear, my objective is to send emails with regular attachments. Running Excel 2003, Outlook 2003, and Windows XP. See code below. Please help! Sub Email() Dim P As String Dim N As String Dim M As String Dim Subject As String Dim Addresses As String Dim olApp As Outlook.Application Dim olNewMail As Outlook.MailItem Application.DisplayAlerts = False M = ActiveWorkbook.Name For c = 2 To 64000 If Range("B" & c) = "" Then Exit For If UCase(Range("E" & c)) = "Y" Then Workbooks(M).Sheets("Main").Activate Subject = Range("A" & c) Addresses = Range("B" & c) P = Range("C" & c) N = Range("D" & c) If Right(P, 1) <> "\" Then P = P & "\" If Right(N, 4) <> ".xls" Then N = N & ".xls" Set olApp = New Outlook.Application Set olNewMail = olApp.CreateItem(olMailItem) With olNewMail .Display .Recipients.Add Addresses Application.Wait (Now + TimeValue("0:00:01")) SendKeys ("{TAB}") .Subject = Subject .Attachments.Add P + N .Send End With Set olNewMail = Nothing Set olApp = Nothing End If Next c Range("E2:E65536").ClearContents Application.DisplayAlerts = True End Sub

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• Stack and queue operations on the same array.

  - by Passonate Learner

  Hi. I've been thinking about a program logic, but I cannot draw a conclusion to my problem. Here, I've implemented stack and queue operations to a fixed array. int A[1000]; int size=1000; int top; int front; int rear; bool StackIsEmpty() { return (top==0); } bool StackPush( int x ) { if ( top >= size ) return false; A[top++] = x; return true; } int StackTop( ) { return A[top-1]; } bool StackPop() { if ( top <= 0 ) return false; A[--top] = 0; return true; } bool QueueIsEmpty() { return (front==rear); } bool QueuePush( int x ) { if ( rear >= size ) return false; A[rear++] = x; return true; } int QueueFront( ) { return A[front]; } bool QueuePop() { if ( front >= rear ) return false; A[front++] = 0; return true; } It is presumed(or obvious) that the bottom of the stack and the front of the queue is pointing at the same location, and vice versa(top of the stack points the same location as rear of the queue). For example, integer 1 and 2 is inside an array in order of writing. And if I call StackPop(), the integer 2 will be popped out, and if I call QueuePop(), the integer 1 will be popped out. My problem is that I don't know what happens if I do both stack and queue operations on the same array. The example above is easy to work out, because there are only two values involved. But what if there are more than 2 values involved? For example, if I call StackPush(1); QueuePush(2); QueuePush(4); StackPop(); StackPush(5); QueuePop(); what values will be returned in the order of bottom(front) from the final array? I know that if I code a program, I would receive a quick answer. But the reason I'm asking this is because I want to hear a logical explanations from a human being, not a computer.

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• Qt vs .NET - a few comparisons [closed]

  - by Pirate for Profit

  Event Handling In Qt the event handling system you just emit signals when something cool happens and then catch them in slots, for instance emit valueChanged(int percent, bool something); and void MyCatcherObj::valueChanged(int p, bool ok){} blocking them and disconnecting them when needed, doing it across threads... once you get the hang of it, it just seems a lot more natural and intuitive than the way the .NET event handling is set up (you know, object sender, CustomEventArgs e). And I'm not just talking about syntax, because in the end the .NET delegate crap is the bomb. I'm also talking about in more than just reflection (because, yes, .NET obviously has much stronger reflection capabilities). I'm talking about in the way the system feels to a human being. Qt wins hands down i m o. Basically, the footprints make more sense and you can visualize the project easier without the clunky event handling system. I wish I could it explain it better. The only thing is, I do love some of the ease of C# compared to C++ and .NET's assembly architecture. That is a big bonus for modular projects, which are a PITA to do in C++. Database Ease of Doing Crap Also what about datasets and database manipulations. I think .net wins here but I'm not sure. Threading/Conccurency How do you guys think of the threading? In .NET, all I've ever done is make like a list of master worker threads with locks. I like QConcurrentFramework, you don't worry about locks or anything, and with the ease of the signal slot system across threads it's nice to get notified about the progress of things. Memory Usage Also what do you think of the overall memory usage comparison. Is the .NET garbage collector pretty on the ball and quick compared to the instantaneous nature of native memory management? Or does it just let programs leak up a storm and lag the computer then clean it up when it's about to really lag? However, I am a n00b who doesn't know what I'm talking about, please school me on the subject.

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• Would someone mind giving suggestions for this new assembly language?

  - by Noctis Skytower

  Greetings! Last semester in college, my teacher in the Computer Languages class taught us the esoteric language named Whitespace. In the interest of learning the language better with a very busy schedule (midterms), I wrote an interpreter and assembler in Python. An assembly language was designed to facilitate writing programs easily, and a sample program was written with the given assembly mnemonics. Now that it is summer, a new project has begun with the objective being to rewrite the interpreter and assembler for Whitespace 0.3, with further developments coming afterwards. Since there is so much extra time than before to work on its design, you are presented here with an outline that provides a revised set of mnemonics for the assembly language. This post is marked as a wiki for their discussion. Have you ever had any experience with assembly languages in the past? Were there some instructions that you thought should have been renamed to something different? Did you find yourself thinking outside the box and with a different paradigm than in which the mnemonics were named? If you can answer yes to any of those questions, you are most welcome here. Subjective answers are appreciated! hold N Push the number onto the stack copy Duplicate the top item on the stack copy N Copy the nth item on the stack (given by the argument) onto the top of the stack swap Swap the top two items on the stack drop Discard the top item on the stack drop N Slide n items off the stack, keeping the top item add Addition sub Subtraction mul Multiplication div Integer Division mod Modulo save Store load Retrieve L: Mark a location in the program call L Call a subroutine goto L Jump unconditionally to a label if=0 L Jump to a label if the top of the stack is zero if<0 L Jump to a label if the top of the stack is negative return End a subroutine and transfer control back to the caller exit End the program print chr Output the character at the top of the stack print int Output the number at the top of the stack input chr Read a character and place it in the location given by the top of the stack input int Read a number and place it in the location given by the top of the stack Question: How would you redesign, rewrite, or rename the previous mnemonics and for what reasons?

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• Classifying captured data in unknown format?

  - by monch1962

  I've got a large set of captured data (potentially hundreds of thousands of records), and I need to be able to break it down so I can both classify it and also produce "typical" data myself. Let me explain further... If I have the following strings of data: 132T339G1P112S 164T897F5A498S 144T989B9B223T 155T928X9Z554T ... you might start to infer the following: possibly all strings are 14 characters long the 4th, 8th, 10th and 14th characters may always be alphas, while the rest are numeric the first character may always be a '1' the 4th character may always be the letter 'T' the 14th character may be limited to only being 'S' or 'T' and so on... As you get more and more samples of real data, some of these "rules" might disappear; if you see a 15 character long string, then you have evidence that the 1st "rule" is incorrect. However, given a sufficiently large sample of strings that are exactly 14 characters long, you can start to assume that "all strings are 14 characters long" and assign a numeric figure to your degree of confidence (with an appropriate set of assumptions around the fact that you're seeing a suitably random set of all possible captured data). As you can probably tell, a human can do a lot of this classification by eye, but I'm not aware of libraries or algorithms that would allow a computer to do it. Given a set of captured data (significantly more complex than the above...), are there libraries that I can apply in my code to do this sort of classification for me, that will identify "rules" with a given degree of confidence? As a next step, I need to be able to take those rules, and use them to create my own data that conforms to these rules. I assume this is a significantly easier step than the classification, but I've never had to perform a task like this before so I'm really not sure how complex it is. At a guess, Python or Java (or possibly Perl or R) are possibly the "common" languages most likely to have these sorts of libraries, and maybe some of the bioinformatic libraries do this sort of thing. I really don't care which language I have to use; I need to solve the problem in whatever way I can. Any sort of pointer to information would be very useful. As you can probably tell, I'm struggling to describe this problem clearly, and there may be a set of appropriate keywords I can plug into Google that will point me towards the solution. Thanks in advance

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• Performance Optimization for Matrix Rotation

  - by Summer_More_More_Tea

  Hello everyone: I'm now trapped by a performance optimization lab in the book "Computer System from a Programmer's Perspective" described as following: In a N*N matrix M, where N is multiple of 32, the rotate operation can be represented as: Transpose: interchange elements M(i,j) and M(j,i) Exchange rows: Row i is exchanged with row N-1-i A example for matrix rotation(N is 3 instead of 32 for simplicity): ------- ------- |1|2|3| |3|6|9| ------- ------- |4|5|6| after rotate is |2|5|8| ------- ------- |7|8|9| |1|4|7| ------- ------- A naive implementation is: #define RIDX(i,j,n) ((i)*(n)+(j)) void naive_rotate(int dim, pixel *src, pixel *dst) { int i, j; for (i = 0; i < dim; i++) for (j = 0; j < dim; j++) dst[RIDX(dim-1-j, i, dim)] = src[RIDX(i, j, dim)]; } I come up with an idea by inner-loop-unroll. The result is: Code Version Speed Up original 1x unrolled by 2 1.33x unrolled by 4 1.33x unrolled by 8 1.55x unrolled by 16 1.67x unrolled by 32 1.61x I also get a code snippet from pastebin.com that seems can solve this problem: void rotate(int dim, pixel *src, pixel *dst) { int stride = 32; int count = dim >> 5; src += dim - 1; int a1 = count; do { int a2 = dim; do { int a3 = stride; do { *dst++ = *src; src += dim; } while(--a3); src -= dim * stride + 1; dst += dim - stride; } while(--a2); src += dim * (stride + 1); dst -= dim * dim - stride; } while(--a1); } After carefully read the code, I think main idea of this solution is treat 32 rows as a data zone, and perform the rotating operation respectively. Speed up of this version is 1.85x, overwhelming all the loop-unroll version. Here are the questions: In the inner-loop-unroll version, why does increment slow down if the unrolling factor increase, especially change the unrolling factor from 8 to 16, which does not effect the same when switch from 4 to 8? Does the result have some relationship with depth of the CPU pipeline? If the answer is yes, could the degrade of increment reflect pipeline length? What is the probable reason for the optimization of data-zone version? It seems that there is no too much essential difference from the original naive version. EDIT: My test environment is Intel Centrino Duo processor and the verion of gcc is 4.4 Any advice will be highly appreciated! Kind regards!

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• TicTacToe AI Making Incorrect Decisions

  - by Chris Douglass

  A little background: as a way to learn multinode trees in C++, I decided to generate all possible TicTacToe boards and store them in a tree such that the branch beginning at a node are all boards that can follow from that node, and the children of a node are boards that follow in one move. After that, I thought it would be fun to write an AI to play TicTacToe using that tree as a decision tree. TTT is a solvable problem where a perfect player will never lose, so it seemed an easy AI to code for my first time trying an AI. Now when I first implemented the AI, I went back and added two fields to each node upon generation: the # of times X will win & the # of times O will win in all children below that node. I figured the best solution was to simply have my AI on each move choose and go down the subtree where it wins the most times. Then I discovered that while it plays perfect most of the time, I found ways where I could beat it. It wasn't a problem with my code, simply a problem with the way I had the AI choose it's path. Then I decided to have it choose the tree with either the maximum wins for the computer or the maximum losses for the human, whichever was more. This made it perform BETTER, but still not perfect. I could still beat it. So I have two ideas and I'm hoping for input on which is better: 1) Instead of maximizing the wins or losses, instead I could assign values of 1 for a win, 0 for a draw, and -1 for a loss. Then choosing the tree with the highest value will be the best move because that next node can't be a move that results in a loss. It's an easy change in the board generation, but it retains the same search space and memory usage. Or... 2) During board generation, if there is a board such that either X or O will win in their next move, only the child that prevents that win will be generated. No other child nodes will be considered, and then generation will proceed as normal after that. It shrinks the size of the tree, but then I have to implement an algorithm to determine if there is a one move win and I think that can only be done in linear time (making board generation a lot slower I think?) Which is better, or is there an even better solution?

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• Recursive N-way merge/diff algorithm for directory trees?

  - by BobMcGee

  What algorithms or Java libraries are available to do N-way, recursive diff/merge of directories? I need to be able to generate a list of folder trees that have many identical files, and have subdirectories with many similar files. I want to be able to use 2-way merge operations to quickly remove as much redundancy as possible. Goals: Find pairs of directories that have many similar files between them. Generate short list of directory pairs that can be synchronized with 2-way merge to eliminate duplicates Should operate recursively (there may be nested duplicates of higher-level directories) Run time and storage should be O(n log n) in numbers of directories and files Should be able to use an embedded DB or page to disk for processing more files than fit in memory (100,000+). Optional: generate an ancestry and change-set between folders Optional: sort the merge operations by how many duplicates they can elliminate I know how to use hashes to find duplicate files in roughly O(n) space, but I'm at a loss for how to go from this to finding partially overlapping sets between folders and their children. EDIT: some clarification The tricky part is the difference between "exact same" contents (otherwise hashing file hashes would work) and "similar" (which will not). Basically, I want to feed this algorithm at a set of directories and have it return a set of 2-way merge operations I can perform in order to reduce duplicates as much as possible with as few conflicts possible. It's effectively constructing an ancestry tree showing which folders are derived from each other. The end goal is to let me incorporate a bunch of different folders into one common tree. For example, I may have a folder holding programming projects, and then copy some of its contents to another computer to work on it. Then I might back up and intermediate version to flash drive. Except I may have 8 or 10 different versions, with slightly different organizational structures or folder names. I need to be able to merge them one step at a time, so I can chose how to incorporate changes at each step of the way. This is actually more or less what I intend to do with my utility (bring together a bunch of scattered backups from different points in time). I figure if I can do it right I may as well release it as a small open source util. I think the same tricks might be useful for comparing XML trees though.

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• Opinion on "loop invariants", and are these frequently used in the industry?

  - by Michael Aaron Safyan

  I was thinking back to my freshman year at college (five years ago) when I took an exam to place-out of intro-level computer science. There was a question about loop invariants, and I was wondering if loop invariants are really necessary in this case or if the question was simply a bad example... the question was to write an iterative definition for a factorial function, and then to prove that the function was correct. The code that I provided for the factorial function was as follows: public static int factorial(int x) { if ( x < 0 ){ throw new IllegalArgumentException("Parameter must be = 0"); }else if ( x == 0 ){ return 1; }else{ int result = 1; for ( int i = 1; i <= x; i++ ){ result*=i; } return result; } } My own proof of correctness was a proof by cases, and in each I asserted that it was correct by definition (x! is undefined for negative values, 0! is 1, and x! is 1*2*3...*x for a positive value of x). The professor wanted me to prove the loop using a loop invariant; however, my argument was that it was correct "by definition", because the definition of "x!" for a positive integer x is "the product of the integers from 1... x", and the for-loop in the else clause is simply a literal translation of this definition. Is a loop invariant really needed as a proof of correctness in this case? How complicated must a loop be before a loop invariant (and proper initialization and termination conditions) become necessary for a proof of correctness? Additionally, I was wondering... how often are such formal proofs used in the industry? I have found that about half of my courses are very theoretical and proof-heavy and about half are very implementation and coding-heavy, without any formal or theoretical material. How much do these overlap in practice? If you do use proofs in the industry, when do you apply them (always, only if it's complicated, rarely, never)?

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• SET game odds simulation (MATLAB)

  - by yuk

  Here is an interesting problem for your weekend. :) I recently find the great card came - SET. Briefly, there are 81 cards with the four features: symbol (oval, squiggle or diamond), color (red, purple or green), number (one, two or three) or shading (solid, striped or open). The task is to find (from selected 12 cards) a SET of 3 cards, in which each of the four features is either all the same on each card or all different on each card (no 2+1 combination). In my free time I've decided to code it in MATLAB to find a solution and to estimate odds of having a set in randomly selected cards. Here is the code: %% initialization K = 12; % cards to draw NF = 4; % number of features (usually 3 or 4) setallcards = unique(nchoosek(repmat(1:3,1,NF),NF),'rows'); % all cards: rows - cards, columns - features setallcomb = nchoosek(1:K,3); % index of all combinations of K cards by 3 %% test tic NIter=1e2; % number of test iterations setexists = 0; % test results holder % C = progress('init'); % if you have progress function from FileExchange for d = 1:NIter % C = progress(C,d/NIter); % cards for current test setdrawncardidx = randi(size(setallcards,1),K,1); setdrawncards = setallcards(setdrawncardidx,:); % find all sets in current test iteration for setcombidx = 1:size(setallcomb,1) setcomb = setdrawncards(setallcomb(setcombidx,:),:); if all(arrayfun(@(x) numel(unique(setcomb(:,x))), 1:NF)~=2) % test one combination setexists = setexists + 1; break % to find only the first set end end end fprintf('Set:NoSet = %g:%g = %g:1\n', setexists, NIter-setexists, setexists/(NIter-setexists)) toc 100-1000 iterations are fast, but be careful with more. One million iterations takes about 15 hours on my home computer. Anyway, with 12 cards and 4 features I've got around 13:1 of having a set. This is actually a problem. The instruction book said this number should be 33:1. And it was recently confirmed by Peter Norvig. He provides the Python code, but I didn't test it. So can you find an error?

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