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• Need some help understanding this problem about maximizing graph connectivity

  - by Legend

  I was wondering if someone could help me understand this problem. I prepared a small diagram because it is much easier to explain it visually. Problem I am trying to solve: 1. Constructing the dependency graph Given the connectivity of the graph and a metric that determines how well a node depends on the other, order the dependencies. For instance, I could put in a few rules saying that node 3 depends on node 4 node 2 depends on node 3 node 3 depends on node 5 But because the final rule is not "valuable" (again based on the same metric), I will not add the rule to my system. 2. Execute the request order Once I built a dependency graph, execute the list in an order that maximizes the final connectivity. I am not sure if this is a really a problem but I somehow have a feeling that there might exist more than one order in which case, it is required to choose the best order. First and foremost, I am wondering if I constructed the problem correctly and if I should be aware of any corner cases. Secondly, is there a closely related algorithm that I can look at? Currently, I am thinking of something like Feedback Arc Set or the Secretary Problem but I am a little confused at the moment. Any suggestions? PS: I am a little confused about the problem myself so please don't flame on me for that. If any clarifications are needed, I will try to update the question.

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• Understanding CLR 2.0 Memory Model

  - by Eloff

  Joe Duffy, gives 6 rules that describe the CLR 2.0+ memory model (it's actual implementation, not any ECMA standard) I'm writing down my attempt at figuring this out, mostly as a way of rubber ducking, but if I make a mistake in my logic, at least someone here will be able to catch it before it causes me grief. Rule 1: Data dependence among loads and stores is never violated. Rule 2: All stores have release semantics, i.e. no load or store may move after one. Rule 3: All volatile loads are acquire, i.e. no load or store may move before one. Rule 4: No loads and stores may ever cross a full-barrier (e.g. Thread.MemoryBarrier, lock acquire, Interlocked.Exchange, Interlocked.CompareExchange, etc.). Rule 5: Loads and stores to the heap may never be introduced. Rule 6: Loads and stores may only be deleted when coalescing adjacent loads and stores from/to the same location. I'm attempting to understand these rules. x = y y = 0 // Cannot move before the previous line according to Rule 1. x = y z = 0 // equates to this sequence of loads and stores before possible re-ordering load y store x load 0 store z Looking at this, it appears that the load 0 can be moved up to before load y, but the stores may not be re-ordered at all. Therefore, if a thread sees z == 0, then it also will see x == y. If y was volatile, then load 0 could not move before load y, otherwise it may. Volatile stores don't seem to have any special properties, no stores can be re-ordered with respect to each other (which is a very strong guarantee!) Full barriers are like a line in the sand which loads and stores can not be moved over. No idea what rule 5 means. I guess rule 6 means if you do: x = y x = z Then it is possible for the CLR to delete both the load to y and the first store to x. x = y z = y // equates to this sequence of loads and stores before possible re-ordering load y store x load y store z // could be re-ordered like this load y load y store x store z // rule 6 applied means this is possible? load y store x // but don't pop y from stack (or first duplicate item on top of stack) store z What if y was volatile? I don't see anything in the rules that prohibits the above optimization from being carried out. This does not violate double-checked locking, because the lock() between the two identical conditions prevents the loads from being moved into adjacent positions, and according to rule 6, that's the only time they can be eliminated. So I think I understand all but rule 5, here. Anyone want to enlighten me (or correct me or add something to any of the above?)

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• understanding this regex

  - by DarthVader

  I m trying to understand what the following does. ^([^=]+)(?:(?:\\=)(.+))?$ Any ideas? This is being used here. Obviously it s command line parser but i m trying to understand the syntax so i can actually run the program. This is from commandline-jmxclient , they have no documents on setting JMX properties but in their source code, there is such an option, so i just want to understand how i can invoke that method. Matcher m = Client.CMD_LINE_ARGS_PATTERN.matcher(command); if ((m == null) || (!m.matches())) { throw new ParseException("Failed parse of " + command, 0); } this.cmd = m.group(1); if ((m.group(2) != null) && (m.group(2).length() > 0)) this.args = m.group(2).split(","); else this.args = null;

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• Understanding SingleTableEntityPersister n QueryLoader

  - by Iapilgrim

  Hi, I have the Hibernate model @Cache(usage = CacheConcurrencyStrategy.NONE, region = SitesConstants.CACHE_REGION) public class Node extends StatefulEntity implements Inheritable, Cloneable { private Node _parent; private List<Node> _childNodes; .. } @Cache(usage = CacheConcurrencyStrategy.NONE, region = SitesConstants.CACHE_REGION) public class Page extends Node implements Defaultable, Securable { private RootZone _rootZone; ...... @OneToOne(fetch = FetchType.LAZY) @JoinColumn(name = "root_zone_id", insertable = false, updatable = false) public RootZone getRootZone() { return _rootZone; } public void setRootZone(RootZone rootZone) { if (rootZone != null) { rootZone.setPageId(this.getId()); _rootZone = rootZone; } } I want to get all pages ( call getSiteTree), so I using this query String hpql = "SELECT n FROM Node n "; See the trace I find Page.setRootZone(RootZone) line: 155 NativeMethodAccessorImpl.invoke0(Method, Object, Object[]) line: not available [native method] NativeMethodAccessorImpl.invoke(Object, Object[]) line: 39 DelegatingMethodAccessorImpl.invoke(Object, Object[]) line: 25 Method.invoke(Object, Object...) line: 597 BasicPropertyAccessor$BasicSetter.set(Object, Object, SessionFactoryImplementor) line: 66 PojoEntityTuplizer(AbstractEntityTuplizer).setPropertyValues(Object, Object[]) line: 352 PojoEntityTuplizer.setPropertyValues(Object, Object[]) line: 232 SingleTableEntityPersister(AbstractEntityPersister).setPropertyValues(Object, Object[], EntityMode) line: 3580 TwoPhaseLoad.initializeEntity(Object, boolean, SessionImplementor, PreLoadEvent, PostLoadEvent) line: 152 QueryLoader(Loader).initializeEntitiesAndCollections(List, Object, SessionImplementor, boolean) line: 877 QueryLoader(Loader).doQuery(SessionImplementor, QueryParameters, boolean) line: 752 QueryLoader(Loader).doQueryAndInitializeNonLazyCollections(SessionImplementor, QueryParameters, boolean) line: 259 QueryLoader(Loader).doList(SessionImplementor, QueryParameters) line: 2232 QueryLoader(Loader).listIgnoreQueryCache(SessionImplementor, QueryParameters) line: 2129 QueryLoader(Loader).list(SessionImplementor, QueryParameters, Set, Type[]) line: 2124 QueryLoader.list(SessionImplementor, QueryParameters) line: 401 QueryTranslatorImpl.list(SessionImplementor, QueryParameters) line: 363 HQLQueryPlan.performList(QueryParameters, SessionImplementor) line: 196 SessionImpl.list(String, QueryParameters) line: 1149 QueryImpl.list() line: 102 QueryImpl.getResultList() line: 67 NodeDaoImpl.getSiteTree(long) line: 358 PageNodeServiceImpl.getSiteTree(long) line: 797 NativeMethodAccessorImpl.invoke0(Method, Object, Object[]) line: not available [native method] NativeMethodAccessorImpl.invoke(Object, Object[]) line: 39 DelegatingMethodAccessorImpl.invoke(Object, Object[]) line: 25 Method.invoke(Object, Object...) line: 597 AopUtils.invokeJoinpointUsingReflection(Object, Method, Object[]) line: 307 JdkDynamicAopProxy.invoke(Object, Method, Object[]) line: 198 $Proxy100.getSiteTree(long) line: not available the calling setRootZone in Page makes Hibernate issue a hit to database. I don't want this. So my question is + Why query String hpql = "SELECT n FROM Node n "; issues un-expected trace logs like above. Why the query String hpql = "SELECT n.nodename FROM Node n " not? What is the mechanism behind? Note: Im using hibernate caching level 2. In case I don't want to see that trace logs. I mean I just get Node data only. How to do ? Thanks for your help. Sorry for my bad english :( Van

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• Difficulty in Understanding Slideshow script

  - by shining star

  I have taken slide show script from net. But There some functions i cannot understand here is script <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd" > <html lang="en"> <head> <title></title> <script> var interval = 1500; var random_display = 0; var imageDir = "my_images/"; var imageNum = 0; imageArray = new Array(); imageArray[imageNum++] = new imageItem(imageDir + "01.jpg"); imageArray[imageNum++] = new imageItem(imageDir + "02.jpg"); imageArray[imageNum++] = new imageItem(imageDir + "03.jpg"); imageArray[imageNum++] = new imageItem(imageDir + "04.jpg"); imageArray[imageNum++] = new imageItem(imageDir + "05.jpg"); var totalImages = imageArray.length; function imageItem(image_location) { this.image_item = new Image(); this.image_item.src = image_location; return this.image_item.src; } function get_ImageItemLocation(imageObj) { return(imageObj.image_item.src) } alert(imageArray[imageNum].image_item.src); function randNum(x, y) { var range = y - x + 1; return Math.floor(Math.random() * range) + x; } function getNextImage() { if (random_display) { imageNum = randNum(0, totalImages-1); } else { imageNum = (imageNum+1) % totalImages; } var new_image = get_ImageItemLocation(imageArray[imageNum]); //alert(new_image) return(new_image); } function getPrevImage() { imageNum = (imageNum-1) % totalImages; var new_image = get_ImageItemLocation(imageArray[imageNum]); return(new_image); } function prevImage(place) { var new_image = getPrevImage(); document[place].src = new_image; } function switchImage(place) { var new_image = getNextImage(); document[place].src = new_image; var recur_call = "switchImage('"+place+"')"; timerID = setTimeout(recur_call, interval); } </script> </head> <body onLoad="switchImage('slideImg')"> <img name="slideImg" src="27.jpg" width=500 height=375 border=0> <a href="#" onClick="switchImage('slideImg')">play slide show</a> <a href="#" onClick="clearTimeout(timerID)"> pause</a> <a href="#" onClick="prevImage('slideImg'); clearTimeout(timerID)"> previous</a> <a href="#" onClick="switchImage('slideImg'); clearTimeout(timerID)">next </a> </body> </html> here exactly i dont know what does acctually function of get_ImageItemLocation(imageObj) and imageItem(image_location) what does these two functions seperately? Thanks in advance for attention

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• Need help understanding _set_security_error_handler()

  - by Emil D

  So , I've been reading this article: http://msdn.microsoft.com/en-us/library/aa290051%28VS.71%29.aspx And I would like to define my custom handler.However, I'm not sure I understand the mechanics well.What happens after a call is made to the user-defined function ( e.g. the argument of _set_security_error_handler() ) ? Does the program still terminate afterward ? If that is the case, is it possible to terminate only the current thread(assuming that it is not the main thread of the application).AFAIK, each thread has its own stack , so if the stack of a thread gets corrupted, the rest of the application shouldn't be affected. Finally, if it is indeed possible to only terminate the current thread of execution, what potential problems could such an action cause? I'm trying to do all this inside an unmanaged C++ dll that I would like to use in my C# code.

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• Understanding CUDA grid dimensions, block dimensions and threads organization (simple explanation)

  - by cibercitizen1

  How threads are organized to be executed by a GPU?

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• Understanding an interesting array update/replace function

  - by dave

  I'm a PHP amateur. This array function is an adaption of a function I noticed while reading this article. I thought this was an interesting type of array function, but I have a question about the way it works. my_func( array( 'sky' => 'blue' ) ); function my_func( array $settings = array() ) { $settings = $settings + array( 'grass'=>'green','sky'=>'dark' ); print_r( $settings ) ; // outputs: Array ( [sky] => blue [grass] => green ) } but..................... my_func( array( 'sky' => 'blue' ) ); function my_func( array $settings = array() ) { $settings = array( 'clock'=>'time' ) ; $settings = $settings + array( 'grass'=>'green','sky'=>'dark' ); print_r( $settings ) ; // outputs: Array ( [clock] => time [grass] => green [sky] => dark ) } Why does [sky] not equal 'blue' in the second instance? Thanks.

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• stored as array understanding mongoid

  - by Gagan

  Hello frens, This is not a problem, but I just want to know stored as array of Mongoid better. I have following code in my model. class Company include Mongoid::Document include Mongoid::Timestamps references_many :people, :stored_as => :array, :inverse_of => :companies end class Person include Mongoid::Document include Sunspot::Mongoid references_many :companies, :stored_as => :array, :inverse_of => :people end Now in Company object we get person_ids as a result of stored_as array and company_ids in Person object. Now initially I inserted lots of person in company and the ids in person_ids fields is huge. Now I deleted most of person from company down to 8 people. Now I don't get why person_ids fields of Company object storing all the deleted ids of person. My console snapshot is follwing ruby-1.9.2-head Company.first.person_ids = [BSON::ObjectId('4d12d2907adf350695000025'), BSON::ObjectId('4d12d2907adf35069500002c'), BSON::ObjectId('4d12d2907adf350695000035'), BSON::ObjectId('4d12d2907adf35069500003f'), BSON::ObjectId('4d12d2907adf350695000048'), BSON::ObjectId('4d12d2907adf350695000052'), BSON::ObjectId('4d12d2907adf350695000059'), BSON::ObjectId('4d12d2907adf350695000062'), BSON::ObjectId('4d12d4017adf35069500008d'), BSON::ObjectId('4d12d4017adf350695000094'), BSON::ObjectId('4d12d4017adf35069500009d'), BSON::ObjectId('4d12d4017adf3506950000a7'), BSON::ObjectId('4d12d4017adf3506950000b0'), BSON::ObjectId('4d12d4017adf3506950000ba'), BSON::ObjectId('4d12d4017adf3506950000c1'), BSON::ObjectId('4d12d4017adf3506950000ca'), BSON::ObjectId('4d12d48a7adf3506950000f5'), BSON::ObjectId('4d12d48a7adf3506950000fc'), BSON::ObjectId('4d12d48a7adf350695000108'), BSON::ObjectId('4d12d48b7adf350695000115'), BSON::ObjectId('4d12d48b7adf350695000121'), BSON::ObjectId('4d12d48b7adf35069500012e'), BSON::ObjectId('4d12d48b7adf350695000135'), BSON::ObjectId('4d12d48b7adf350695000141'), BSON::ObjectId('4d12d53e7adf35069500016f'), BSON::ObjectId('4d12d53e7adf350695000176'), BSON::ObjectId('4d12d53e7adf350695000182'), BSON::ObjectId('4d12d53e7adf35069500018f'), BSON::ObjectId('4d12d53e7adf35069500019b'), BSON::ObjectId('4d12d53f7adf3506950001a8'), BSON::ObjectId('4d12d53f7adf3506950001af'), BSON::ObjectId('4d12d53f7adf3506950001bb'), BSON::ObjectId('4d12d8587adf3506950001e9'), BSON::ObjectId('4d12d8587adf3506950001f0'), BSON::ObjectId('4d12d8587adf3506950001ff'), BSON::ObjectId('4d12d8597adf35069500020f'), BSON::ObjectId('4d12d8597adf35069500021e'), BSON::ObjectId('4d12d8597adf35069500022e'), BSON::ObjectId('4d12d8597adf350695000235'), BSON::ObjectId('4d12d85a7adf350695000244'), BSON::ObjectId('4d12d9587adf35069500025b'), BSON::ObjectId('4d12db8b7adf35069500026a'), BSON::ObjectId('4d12de6f7adf3509c9000024'), BSON::ObjectId('4d12de6f7adf3509c900002b'), BSON::ObjectId('4d12de6f7adf3509c900003a'), BSON::ObjectId('4d12de707adf3509c900004a'), BSON::ObjectId('4d12de707adf3509c9000059'), BSON::ObjectId('4d12de707adf3509c9000069'), BSON::ObjectId('4d12de707adf3509c9000070'), BSON::ObjectId('4d12de717adf3509c900007f'), BSON::ObjectId('4d12e7f27adf350bd2000009'), BSON::ObjectId('4d12e81f7adf350bd2000015'), BSON::ObjectId('4d12e87f7adf350bd2000024'), BSON::ObjectId('4d12e8b87adf350bd200004c'), BSON::ObjectId('4d12e8b97adf350bd2000053'), BSON::ObjectId('4d12e8b97adf350bd200005c'), BSON::ObjectId('4d12e8b97adf350bd2000066'), BSON::ObjectId('4d12e8b97adf350bd200006f'), BSON::ObjectId('4d12e8b97adf350bd2000079'), BSON::ObjectId('4d12e8ba7adf350bd2000080'), BSON::ObjectId('4d12e8ba7adf350bd2000089'), BSON::ObjectId('4d12ee6b7adf350bd2000198'), BSON::ObjectId('4d12ee6b7adf350bd200019f'), BSON::ObjectId('4d12ee6c7adf350bd20001a5'), BSON::ObjectId('4d12ee6c7adf350bd20001ac'), BSON::ObjectId('4d12ee6c7adf350bd20001b2'), BSON::ObjectId('4d12ee6c7adf350bd20001b9'), BSON::ObjectId('4d12ee6c7adf350bd20001c0'), BSON::ObjectId('4d12ee6c7adf350bd20001c6'), BSON::ObjectId('4d141ca57adf35033e00006e'), BSON::ObjectId('4d141ca57adf35033e000075'), BSON::ObjectId('4d1420aa7adf350705000003'), BSON::ObjectId('4d1420aa7adf35070500000a'), BSON::ObjectId('4d1420f47adf350705000011'), BSON::ObjectId('4d1420f57adf350705000015'), BSON::ObjectId('4d1420f57adf350705000018'), BSON::ObjectId('4d1420f57adf35070500001c'), BSON::ObjectId('4d1420f57adf350705000023'), BSON::ObjectId('4d1420f57adf350705000026'), BSON::ObjectId('4d14215f7adf35070500004b'), BSON::ObjectId('4d14215f7adf350705000052'), BSON::ObjectId('4d14215f7adf350705000055'), BSON::ObjectId('4d14215f7adf350705000059'), BSON::ObjectId('4d14215f7adf35070500005c'), BSON::ObjectId('4d14215f7adf350705000060'), BSON::ObjectId('4d14215f7adf350705000067'), BSON::ObjectId('4d14215f7adf35070500006a')] Company.first.people.collect(&:id) = [BSON::ObjectId('4d14215f7adf35070500004b'), BSON::ObjectId('4d14215f7adf350705000052'), BSON::ObjectId('4d14215f7adf350705000055'), BSON::ObjectId('4d14215f7adf350705000059'), BSON::ObjectId('4d14215f7adf35070500005c'), BSON::ObjectId('4d14215f7adf350705000060'), BSON::ObjectId('4d14215f7adf350705000067'), BSON::ObjectId('4d14215f7adf35070500006a')] Isn't the Company.first.person_ids array be only storing the ids shown by Company.first.people.collect(&:id) It would be helpful if some one tell me when to best use stored_as = :array method. Do stored_as = :array increase querying performance? Thanks

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• Trying to get a better understanding of SelectedValuePath and IsSynchronizedWithCurrentItem

  - by rasx

  The following XAML produces a run-time Binding error when I click on an item in the ListBox: <Window xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" xmlns:sys="clr-namespace:System;assembly=mscorlib" xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" x:Class="WpfApplication1.MainWindow" x:Name="Window" Title="MainWindow" Width="640" Height="480"> <Window.Resources> <x:Array x:Key="strings" Type="{x:Type sys:String}"> <sys:String>one</sys:String> <sys:String>two</sys:String> <sys:String>three</sys:String> <sys:String>four</sys:String> </x:Array> </Window.Resources> <Grid> <ListBox DataContext="{StaticResource strings}" IsSynchronizedWithCurrentItem="True" ItemsSource="{Binding}" SelectedValuePath="{Binding /Length}"> <ListBox.ItemTemplate> <DataTemplate> <Grid> <Grid.Resources> <Style TargetType="{x:Type Label}"> <Setter Property="Background" Value="Yellow"/> <Setter Property="Margin" Value="0,0,4,0"/> <Setter Property="Padding" Value="0"/> </Style> </Grid.Resources> <Grid.ColumnDefinitions> <ColumnDefinition/> <ColumnDefinition/> </Grid.ColumnDefinitions> <Grid.RowDefinitions> <RowDefinition/> <RowDefinition/> </Grid.RowDefinitions> <!-- Row 0 --> <Label Grid.Column="0" Grid.Row="0">String:</Label> <TextBlock Grid.Column="1" Grid.Row="0" Text="{Binding}"/> <!-- Row 1 --> <Label Grid.Column="0" Grid.Row="1">Length:</Label> <TextBlock Grid.Column="1" Grid.Row="1" Text="{Binding Length, Mode=Default}"/> </Grid> </DataTemplate> </ListBox.ItemTemplate> </ListBox> </Grid> </Window> This is the run-time Binding error message: System.Windows.Data Error: 39 : BindingExpression path error: '3' property not found on 'object' ''String' (HashCode=1191344027)'. BindingExpression:Path=3; DataItem='String' (HashCode=1191344027); target element is 'ListBox' (Name=''); target property is 'NoTarget' (type 'Object') I would like the selected value of the ListBox to be the Length of the selected String object. What is wrong with my SelectedValuePath Binding syntax? Are there any related issues with IsSynchronizedWithCurrentItem?

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• Understanding pattern matching with cons operator

  - by Mathias

  In "Programming F#" I came across a pattern-matching like this one (I simplified a bit): let rec len list = match list with | [] -> 0 | [_] -> 1 | head :: tail -> 1 + len tail;; Practically, I understand that the last match recognizes the head and tail of the list. Conceptually, I don't get why it works. As far as I understand, :: is the cons operator, which appends a value in head position of a list, but it doesn't look to me like it is being used as an operator here. Should I understand this as a "special syntax" for lists, where :: is interpreted as an operator or a "match pattern" depending on context? Or can the same idea be extended for types other than lists, with other operators?

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• Understanding addSubview: memoryLeak

  - by Leandros

  I don't really understand, why this code leaks. ParentViewController *parentController = [[ParentViewController alloc] init]; ChildViewController *childController = [[ChildViewController alloc] init]; [parentController containerAddChildViewController:childController]; [[self window] setRootViewController:parentController]; - (void)containerAddChildViewController:(UIViewController *)childViewController { [self addChildViewController:childViewController]; [self.view addSubview:childViewController.view]; // Instruments is telling me, the leak occurs here! [childViewController didMoveToParentViewController:self]; } According to Instruments, this line: [self.view addSubview:childViewController.view]; is leaking. The whole code is called once in application:didFinishLaunchingWithOptions:, but it is shown that this code is responsible for 30 leaks (approx. 1.12 kB).

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• Understanding syntax in c#

  - by user279521

  I am hoping someone can help me understand what is going on in the code line below: Table t = (Table)Page.FindControl("Panel1").FindControl("tbl"); I understand Page.FindControl("Panel1").FindControl("tbl"); Why is there a (Table) before the Page.FindControl?

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• Help understanding .NET delegates, events, and eventhandlers

  - by Seth Spearman

  Hello, In the last couple of days I asked a couple of questions about delegates HERE and HERE. I confess...I don't really understand delegates. And I REALLY REALLY REALLY want to understand and master them. (I can define them--type safe function pointers--but since I have little experience with C type languages it is not really helpful.) Can anyone recommend some online resource(s) that will explain delegates in a way that presumes nothing? This is one of those moments where I suspect that VB actually handicaps me because it does some wiring for me behind the scenes. The ideal resource would just explain what delegates are, without reference to anything else like (events and eventhandlers), would show me how all everything is wired up, explain (as I just learned) that delegates are types and what makes them unique as a type (perhaps using a little ildasm magic)). That foundation would then expand to explain how delegates are related to events and eventhandlers which would need a pretty good explanation in there own right. Finally this resource could tie it all together using real examples and explain what wiring DOES happen automatically by the compiler, how to use them, etc. And, oh yeah, when you should and should not use delegates, in other words, downsides and alternatives to using delegates. What say ye? Can any of you point me to resource(s) that can help me begin my journey to mastery? EDIT One last thing. The ideal resource will explain how you can and cannot use delegates in an interface declaration. That is something that really tripped me up. Thanks for your help. Seth

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• Help on understanding multiple columns on an index?

  - by Xaisoft

  Assume I have a table called "table" and I have 3 columns, a, b, and c. What does it mean to have a non-clustered index on columns a,b? Is a nonclustered index on columns a,b the same as a nonclustered index on columns b,a? (Note the order). Also, Is a nonclustered index on column a the same as a nonclustered index on a,c? I was looking at the website sqlserver performance and they had these dmv scripts where it would tell you if you had overlapping indexes and I believe it was saying that having an index on a is the same as a,b, so it is redundant. Is this true about indexes? One last question is why is the clustered index put on the primary key. Most of the time the primary key is not queried against, so shouldn't the clustered index be on the most queried column. I am probably missing something here like having it on the primary key speeds up joins? Great explanations. Should I turn this into a wiki and change the title index explanation?

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• Help understanding linux/tcp.h

  - by Chris

  I'm learning to use raw sockets, and im trying to prase out the tcp header data, but i can't seem to figure out what res1, ece, and cwr are. Through my networking book and google i know what the rest stand for, but can't seem to find anything on those three. Below is the tcphdr struct in my includes area. Ive commented the parts a bit as i was figureing out what they stood for. struct tcphdr { __be16 source; __be16 dest; __be32 seq; __be32 ack_seq; #if defined(__LITTLE_ENDIAN_BITFIELD) _u16 res1:4, doff:4,//tcp header length fin:1,//final syn:1,//synchronization rst:1,//reset psh:1,//push ack:1,//ack urg:1,// urge ece:1, cwr:1; #elif defined(_BIG_ENDIAN_BITFIELD) __u16 doff:4,//tcp header length res1:4, cwr:1, ece:1, urg:1,//urge ack:1,//ack psh:1,//push rst:1,//reset syn:1,//synchronization fin:1;//final #else #error "Adjust your defines" #endif __be16 window; __sum16 check; __be16 urg_ptr; };

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• understanding silverlight resource file format

  - by Quincy

  I'm trying to understand the format of silverlight resource files. There are 4 bytes of the data comes after PAD. I'd like to know what these values are, and how they are generated. here is the hex dump of a .g.resources file. Here is what I know: there is 0xbeefcace at the beginning, then there is dependancies, then padding. after that is the great unknown (but I really like to know). After 4 null bytes, are the file name and size of the resource. and After that is content of the said file. I'm not that familiar with .Net and silverlight resource management. would someone please tell me what the mystical 4 bytes are, or point me the url to the specification doc or something. Any help would be appreciated!

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• Understanding the Unix file system and ruby installs without Sudo

  - by JZ

  I'm trying to comprehend the Unix file system on my OSX. I'm following wikipedia Filesystem Hierarchy Standard. I understand when I install ruby gems I must use the command sudo gem install but if I omit sudo, problems may occur. Where are gems installed within the file system when I omit sudo? How can I delete these gems? A Fun side question: When I enter cd ~/.gem my terminal is directed to .gem user$, When I enter cd ~/ and list folders using the ls command I can't find a .gem folder. Where is the .gem folder? How does this fit into the Filesystem?

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• Help on understanding JS

  - by Anonymous

  I have thos piece of code: Math&&Math.random?Math.floor(Math.random()*10000000000000):Date.getTime(); And as far as i know && is logic operator for AND, so im trying to convert this into PHP and this is where i got: intval(floor(time()/10800000)%10+(rand()?floor(rand()*10000000000000):time())) The problem is that i can't understand the first part Math&& Can anyone help with this one cause i always get negative result, when i should get positive (probably the logic rand-time is not working in my php example)

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• Understanding Domain Driven Design

  - by Nihilist

  Hi I have been trying to understand DDD for few weeks now. Its very confusing. I dont understand how I organise my projects. I have lot of questions on UnitOfWork, Repository, Associations and the list goes on... Lets take a simple example. Album and Tracks. Album: AlbumId, Name, ListOf Tracks Tracks: TrackId, Name Question1: Should i expose Tracks as a IList/IEnumerabe property on Album ? If that how do i add an album ? OR should i expose a ReadOnlyCollection of Tracks and expose a AddTrack method? Question2: How do i load Tracks for Album [assuming lazy loading]? should the getter check for null and then use a repository to load the tracks if need be? Question3: How do we organise the assemblies. Like what does each assembly have? Model.dll - does it only have the domain entities? Where do the repositories go? Interfaces and implementations both. Can i define IAlbumRepository in Model.dll? Infrastructure.dll : what shold this have? Question4: Where is unit of work defined? How do repository and unit of work communicate? [ or should they ] for example. if i need to add multiple tracks to album, again should this be defined as AddTrack on Album OR should there a method in the repository? Regardless of where the method is, how do I implement unit of work here? Question5: Should the UI use Infrastructure..dll or should there be ServiceLayer? Do my quesitons make sense? Regards

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• Understanding the workings of equals and hashCode in a HashMap

  - by andandandand

  I have this test code: import java.util.*; class MapEQ { public static void main(String[] args) { Map<ToDos, String> m = new HashMap<ToDos, String>(); ToDos t1 = new ToDos("Monday"); ToDos t2 = new ToDos("Monday"); ToDos t3 = new ToDos("Tuesday"); m.put(t1, "doLaundry"); m.put(t2, "payBills"); m.put(t3, "cleanAttic"); System.out.println(m.size()); } } class ToDos{ String day; ToDos(String d) { day = d; } public boolean equals(Object o) { return ((ToDos)o).day == this.day; } // public int hashCode() { return 9; } } When // public int hashCode() { return 9; } is uncommented m.size() returns 2, when it's left commented it returns three. Why?

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• Understanding Ruby class vs instance methods

  - by randombits

  I have the following code: #!/usr/bin/ruby class Person def self.speak p = self.new puts "Hello" p.chatter end private def chatter puts "Chattering" end end p = Person.new Person.speak I'd like to make chatter private, accessible only within p.. but I want p to be able to access it within the class method. Is there a better way to design this so chatter isn't available to the public, but a "factory" method like self.speak can call chatter?

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• Need some help understanding the MATLAB `cat` command in high dimensions

  - by Runner

  The commands a = magic(3); b = pascal(3); c = cat(4,a,b); produce a 3-by-3-by-1-by-2 array. Why is the result 3-3-1-2 when the dimension is 4?

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• Understanding Symbols In Ruby

  - by Kezzer

  Despite reading this article, I'm still confused as to the representation of the data in memory when it comes to using symbols. If a symbol, two of them contained in different objects, exist in the same memory location, then how is it that they contain different values? I'd have expected the same memory location to contain the same value. As a quote from the link: Unlike strings, symbols of the same name are initialized and exist in memory only once during a session of ruby I just don't understand how it manages to differentiate the values contained in the same memory location. EDIT So let's consider the example: patient1 = { :ruby => "red" } patient2 = { :ruby => "programming" } patient1.each_key {|key| puts key.object_id.to_s} 3918094 patient2.each_key {|key| puts key.object_id.to_s} 3918094 patient1 and patient2 are both hashes, that's fine. :ruby however is a symbol. If we were to output the following: patient1.each_key {|key| puts key.to_s} Then what will be output? "red", or "programming"? FURTHER EDIT I'm still really quite confused. I'm thinking a symbol is a pointer to a value. Let's forget hashes for a second. The questions I have are; can you assign a value to a symbol? Is a symbol just a pointer to a variable with a value in it? If symbols are global, does that mean a symbol always points to one thing?

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• Understanding MongoDB (and NoSQL in general) and how to make the best use of it

  - by Earlz

  Hello, I am beginning to think that my next project I am wanting to do would work better with a NoSQL solution. The project would either involve a ton of 2-column tables or a ton of dynamic queries with dynamically generated columns in a traditional SQL database. So I feel a NoSQL database would be much cleaner. I'm looking at MongoDB and it looks pretty promising. Anyway, I'm attempting to make sense of it all. Also, I will be using MongoMapper in Ruby. Anyway though, I'm confused as to how to layout things in such a freeform database. I've read http://stackoverflow.com/questions/2170152/nosql-best-practices and the answer there says that normalization is usually bad in a NoSQL DB. So how would be the best way of laying out say a simple blog with users, posts, and comments? My natural thought was to have three collections for each and then link them by a unique ID. But this apparently is wrong? So, what are some of the ways to lay out such a thing? My concern with the answer given in the other question is, what if the author's name changed? You'd have to go through updating a ton of posts and comments. But is this an okay thing to do with NoSQL?

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