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• Finding recurrence relations of an algorithm

  - by Roarke

  I'm reading my algorithms text book, and I'm reading about recurrence relations and finding the algorithms big O complexity. I run across this line "In the case of the merge-sort algorithm, we get the recurrence equation: t(n) = b if n < 2 = 2t(n/2) +bn if n >= 2 for b > 0 my response was "how the heck did we know that?!?!" So i'm wondering if there is a systematic approach, or just a logical way of getting these recurrence relations from the algorithms can some one explain where the b and the two 2's come from?

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• Optimal password salt length

  - by Juliusz Gonera

  I tried to find the answer to this question on Stack Overflow without any success. Let's say I store passwords using SHA-1 hash (so it's 160 bits) and let's assume that SHA-1 is enough for my application. How long should be the salt used to generated password's hash? The only answer I found was that there's no point in making it longer than the hash itself (160 bits in this case) which sounds logical, but should I make it that long? E.g. Ubuntu uses 8-byte salt with SHA-512 (I guess), so would 8 bytes be enough for SHA-1 too or maybe it would be too much?

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• Python threads all executing on a single core

  - by Rob Lourens

  I have a Python program that spawns many threads, runs 4 at a time, and each performs an expensive operation. Pseudocode: for object in list: t = Thread(target=process, args=(object)) # if fewer than 4 threads are currently running, t.start(). Otherwise, add t to queue But when the program is run, Activity Monitor in OS X shows that 1 of the 4 logical cores is at 100% and the others are at nearly 0. Obviously I can't force the OS to do anything but I've never had to pay attention to performance in multi-threaded code like this before so I was wondering if I'm just missing or misunderstanding something. Thanks.

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• Encrypt text using a number

  - by Adam Matan

  Project Euler I have recently begun to solve some of the Project Euler riddles. I found the discussion forum in the site a bit frustrating (most of the discussions are closed and poorly-threaded), So I have decided to publish my Python solutions on launchpad for discussion. The problem is that it seems quite unethical to publish these solutions, as it would let other people gain reputation without doing the programming work, which the site deeply discourages. My Encryption problem I want to encrypt my answers so that only those who have already solved the riddles can see my code. The logical key would be the answer to the riddle, which is always numeric. In order to prevent brute-force attacks on my answers, I want to find an encryption algorithm that takes a significantly long time (few seconds) to run. Do you know any such algorithm? I would fancy a Python package, which I can attach to the code, over an external program that might have portability issues. Thanks, Adam

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• Why will this code compile using ifort compiler and not when using gfortran compiler? Help!

  - by CuriousCompiler

  I'm rewriting some code to make a program compile with the gfortran compiler as opposed to ifort compiler I usually use. The code follows: _Subroutine SlideBits (WORD, BITS, ADDR) Implicit None Integer(4) WORD Integer(4) BITS Integer(4) ADDR Integer(4) ADDR1 ADDR1 = 32 - ADDR WORD = (WORD .And. (.Not.ISHFT(1,ADDR1))) .Or. ISHFT(BITS,ADDR1) End_ When I compile the above code using the gfortran compiler, I recieve this error: WORD = (WORD .And. (.Not.ISHFT(1,ADDR1))) .Or. ISHFT(BITS,ADDR1) Error: Operand of .NOT. operator at (1) is INTEGER(4) All three of the variables coming into the subroutine are integers. I've looked around a bit and the gfortran wiki states that the gfortran compiler should be able to handle logical statments being applied to integer values. Several other sites I've visited either quote from the gnu wiki or agree with it. This is the first time I've seen this error as the Intel Fortran compiler (ifort) I normally use compiles cleanly.

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• What is a Web Framework ? How does it compare with LAMP

  - by Nishant

  I started web development in LAMP/WAMP and it was logical to me . There is a Web Server program called Apache which does the networking part of setting up a service on port 80 ( common port ) . If the request is regular HTML it uses the HTTP headers to transport files .And if the request for the file is a PHP one , it has a mod_php with which Apache invokes the PHP interpreter to process the file and it gives back HTML which is again transferred as usual HTML . Now the question is what is a Web Framework ? I came across Python based website creation and there is Flask . What is a flask , how does it compare with LAMP . Further are DJango / Ruby on Rails different from flask ? Can someone answer me and also give some good places to read on these .Thanks for your answers in advance . Further is things like LAMP slower than the common FRAMEWORKS because they claimn easy deplyment fo web apps .

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• In Django, why is user.is_authenticated a method and not a member variable like is_staff

  - by luc

  Hello all, I've lost some time with a bug in my app due to user authentication. I think that it's a bit confusing but maybe someone can explain the reason and it will appear to me very logical. The user.is_staff is a member variable while user.is_authenticated is a method. However is_authenticated only returns True or False depending if the class is User or AnonymousUser (see http://docs.djangoproject.com/en/dev/topics/auth/) Is there a reason for that? Why user.is_authenticated is a method? Thanks in advance

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• Flipping issue when interpolating Rotations using Quaternions

  - by uhuu

  I use slerp to interpolate between two quaternions representing rotations. The resulting rotation is then extracted as Euler angles to be fed into a graphics lib. This kind of works, but I have the following problem; when rotating around two (one works just fine) axes in the direction of the green arrow as shown in the left frame here the rotation soon jumps around to rotate from the opposite site to the opposite visual direction, as indicated by the red arrow in the right frame. This may be logical from a mathematical perspective (although not to me), but it is undesired. How could I achieve an interpolation with no visual flipping and changing of directions when rotating around more than one axis, following the green arrow at all times until the interpolation is complete? Thanks in advance.

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• Declaring pointers; asterisk on the left or right of the space between the type and name?

  - by GenTiradentes

  I've seen mixed versions of this in a lot of code. (This applies to C and C++, by the way.) People seem to declare pointers in one of two ways, and I have no idea which one is correct, of if it even matters. The first way it to put the asterisk adjacent the type name, like so: someType* somePtr; The second way is to put the asterisk adjacent the name of the variable, like so: someType *somePtr; This has been driving me nuts for some time now. Is there any standard way of declaring pointers? Does it even matter how pointers are declared? I've used both declarations before, and I know that the compiler doesn't care which way it is. However, the fact that I've seen pointers declared in two different ways leads me to believe that there's a reason behind it. I'm curious if either method is more readable or logical in some way that I'm missing.

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• Good excuses NOT to use version control

  - by Kent Fredric

  This question may seem backwards to all logical reasoning, but I encountered a team member who was working on a side project out of the main tree, and had decided ( rephrased ) I'm the only developer, we don't need version control, that's stupid Now personally, the idea horrifies me, and I can't for one justify it. So now my stance is clear, I want some legitimate answers why somebody can excuse themselves from using version control. The default answers will of course be "there is no excuse". Not what I want to see. If you can't see an excuse, then just don't post. ( Feel free to downmod excuses others post that are illegitimate/bad and give reasoning )

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• Should I use DTOs as my data models in MVVM?

  - by JonC

  I'm currently working on what will be my first real foray into using MVVM and have been reading various articles on how best to implement it. My current thoughts are to use my data models effectively as data transfer objects, make them serializable and have them exist on both the client and server sides. It seems like a logical step given that both object types are really just collections of property getters and setters and another layer in between seems like complete overkill. Obviously there would be issues with INotifyPropertyChanged not working correctly on the server side as there is no ViewModel to which to communicate, but as long as we are careful about constructing our proper domain model objects from data models in the service layer and not dealing the the data models on the server side I don't think it should be a big issue. I haven't found too much info about this approach in my reading, so I would like to know if this is a pretty standard thing, is this just assumed to be the de facto way of doing MVVM in a multi-tier environment? If I've got completely the wrong idea about things then thoughts on other approaches would be appreciated too.

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• unexpected result from gnu sort

  - by funkycat

  when I try to sort the following text file 'input': test1 3 test3 2 test 4 with the command sort input the output is exactly the input. Here is the output of od -bc input : 0000000 164 145 163 164 061 011 063 012 164 145 163 164 063 011 062 012 t e s t 1 \t 3 \n t e s t 3 \t 2 \n 0000020 164 145 163 164 011 064 012 t e s t \t 4 \n 0000027 It's just a tab separated file with two columns. When I do sort -k 2 The output changes to test3 2 test1 3 test 4 which is what I would expect. But if I do sort -k 1 nothing changes with respect to the input, whereas I would expect 'test' to sort before 'test1'. Finally, if I do cat input | cut -f 1 | sort I get test test1 test3 as expected. Is there a logical explanation for this? What exactly is sort supposed to do by default, something like: sort -k 1 ? My version of sort: sort (GNU coreutils) 7.4

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• Can FileOutputStream() take a relative path as an argument

  - by Ankur

  I am creating a FileOutputStream object. It takes a file or String as an argument in its constructor. My question is, can I give it a relative URL as an argument for the location of a file, it doesn't seem to work, but I am trying to work out if this is possible at all (if not I will stop trying). If it is not possible, how can I (from a servlet) get the absolute path (on the filesystem, not the logical URL) to the current location in such a way that I can pass that to the constructor. Part of my problem is that my dev box is Windows but I will publish this to a Unix box, so the paths cannot be the same i.e. on Windows C:/.... and on unix /usr/...

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• 'Lexical' scoping of type parameters in C#

  - by leppie

  I have 2 scenarios. This fails: class F<X> { public X X { get; set; } } error CS0102: The type 'F' already contains a definition for 'X' This works: class F<X> { class G { public X X { get; set; } } } The only logical explanation is that in the second snippet the type parameter X is out of scope, which is not true... Why should a type parameter affect my definitions in a type? IMO, for consistency, either both should work or neither should work. Any other ideas? PS: I call it 'lexical', but it probably is not not the correct term.

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• Allow for modular development while still running in same JVM?

  - by Marcus

  Our current app runs in a single JVM. We are now splitting up the app into separate logical services where each service runs in its own JVM. The split is being done to allow a single service to be modified and deployed without impacting the entire system. This reduces the need to QA the entire system - just need to QA the interaction with the service being changed. For inter service communication we use a combination of REST, an MQ system bus, and database views. What I don't like about this: REST means we have to marshal data to/from XML DB views couple the systems together which defeats the whole concept of separate services MQ / system bus is added complexity There is inevitably some code duplication between services You have set up n JBoss server configurations, we have to do n number of deployments, n number of set up scripts, etc, etc. Is there a better way to structure an internal application to allow modular development and deployment while allowing the app to run in a single JVM (and achieving the associated benefits)?

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• Getting Parent Layout in Qt

  - by Austin

  Hi, quick question. Is there any way to (easily) retrieve the parent layout of a widget in Qt? PS: QObject::parent() won't work, for logical reasons. EDIT: I'm positive the widget has a parent layout, because I added it to a layout earlier in the code. Now, I have many other layouts in the window and while it is possible for me to keep track of them, I just want to know if there is an easy and clean way to get the parent layout. EDIT2: Sorry, "easy and clean" was probably not the best way of putting. I meant using the Qt API. EDIT3: I'm adding the widget to the layout like this: QHBoxLayout* layout = new QHBoxLayout; layout-addWidget(button);

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• How to allow for modular development while still running in same JVM?

  - by Marcus

  Our current app runs in a single JVM. We are now splitting up the app into separate logical services where each service runs in its own JVM. The split is being done to allow a single service to be modified and deployed without impacting the entire system. This reduces the need to QA the entire system - just need to QA the interaction with the service being changed. For interservice communication we use a combination of REST, an MQ system bus, and database views. What I don't like about this: REST means we have to marshal data to/from XML DB views couple the systems together which defeats the whole concept of separate services MQ / system bus is added complexity There is inevitably some code duplication between services You have set up n JBoss server configurations, we have to do n number of deployments, n number of set up scripts, etc, etc. Is there a better way to structure an internal application to allow modular development and deployment while allowing the app to run in a single JVM (and achieving the associated benefits)?

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• Download office document without the web server trying to render it

  - by Dan Revell

  I'm trying to download an InfoPath template that's hosted on SharePoint. If I hit the url in internet explorer it asks me where to save it and I get the correct file on my disk. If I try to do this programmatically with WebClient or HttpWebRequest then I get HTML back instead. How can I make my request so that the web server returns the actual xsn file and doesn't try to render it in html. If internet explorer can do this then it's logical to think that I can too. I've tried setting the Accept property of the request to application/x-microsoft-InfoPathFormTemplate but that hasn't helped. It was a shot in the dark.

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• What is the algorithm used by the memberwise equality test in .NET structs?

  - by Damian Powell

  What is the algorithm used by the memberwise equality test in .NET structs? I would like to know this so that I can use it as the basis for my own algorithm. I am trying to write a recursive memberwise equality test for arbitrary objects (in C#) for testing the logical equality of DTOs. This is considerably easier if the DTOs are structs (since ValueType.Equals does mostly the right thing) but that is not always appropriate. I would also like to override comparison of any IEnumerable objects (but not strings!) so that their contents are compared rather than their properties. This has proven to be harder than I would expect. Any hints will be greatly appreciated. I'll accept the answer that proves most useful or supplies a link to the most useful information. Thanks.

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• Need help with ruby radiant cms

  - by AnimalCode

  I have some hidden pages like this: /staff John Bob Mike In all this pages i have extended content, like photo, bio and additional info. I make this, to have logical structure and quick edit without editing radiant code. My question is, how i can with base radius functional get access to this pages in this code: <r:find url="/staff"> <r:children:each by="title" order="asc"> <r:title /> </r:children:each> </r:find>

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• Django many-to-many relationship to self with extra data, how do I select from a certain direction?

  - by Jake

  I have some hierarchical data where each Set can have many members and can belong to more than one Set(group) Here are the models: class Set(models.Model): ... groups = models.ManyToManyField('self', through='Membership', symmetrical=False) members = models.ManyToManyField('self', through='Membership', symmetrical=False) class Membership(models.Model): group = models.ForeignKey( Set, related_name='Members' ) member = models.ForeignKey( Set, related_name='Groups' ) order = models.IntegerField( default=-1 ) I want to know how to get all the members or all the groups for a Set instance. I think I can do it as follows, but it's not very logical, can anyone tell me what's going on and how I should be doing it? # This gives me a set of Sets # Which seems to be the groups this Set belongs to set_instance.set_set.all() # These give me a set of Memberships, not Sets set_instance.Members.all() set_instance.Groups.all() # These they both return a set of Sets # which seem to be the members of this one set_instance.members.all() set_instance.groups.all()

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• Height of a binary tree

  - by Programmer

  Consider the following code: public int heightOfBinaryTree(Node node) { if (node == null) { return 0; } else { return 1 + Math.max(heightOfBinaryTree(node.left), heightOfBinaryTree(node.right)); } } I want to know the logical reasoning behind this code. How did people come up with it? Does some have an inductive proof? Moreover, I thought of just doing a BFS with the root of the binary tree as the argument to get the height of the binary tree. Is the previous approach better than mine?Why?

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• Oracle sequence but then in MS SQL Server

  - by Raymond

  In Oracle there is a mechanism to generate sequence numbers e.g.; CREATE SEQUENCE supplier_seq MINVALUE 1 MAXVALUE 999999999999999999999999999 START WITH 1 INCREMENT BY 1 CACHE 20; And then execute the statement supplier_seq.nextval to retrieve the next sequence number. How would you create the same functionality in MS SQL Server ? Edit: I'm not looking for ways to automaticly generate keys for table records. I need to generate a unique value that I can use as an (logical) ID for a process. So I need the exact functionality that Oracle provides.

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• iPhone UI: No edit button for UITableView, bad idea?

  - by Nic Hubbard

  I have a UITableViewController which lets the user drill down into different records. On the second level/view, the user can add and edit new records. But, I am not sure what to do, since the back button is on the top left, and I need to put the "Add" button on the top right, so there is no room (keeping to HIG) for the edit button, which would normally go where the back button is. (I am using a tab bar, so can't put it at the bottom.) Do you think that it is logical, to expect users to know to swipe to delete a record? Or, do I need to have an edit button? If I DO need an edit button, where should I put it if I am following HIG?

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• How to preserve object identity across different VMs

  - by wheleph

  To be specific let me illustrate the question with Spring http-remoting example. Suppose we have such implementation of a simple interface: public SearchServiceImpl implements SearchService { public SearchJdo processSearch(SearchJdo search) { search.name = "a funky name"; return search; } } SearchJdo is itself a simple POJO. Now when we call the method from a client through http-remoting we'll get: public class HTTPClient { public static void main(final String[] arguments) { final ApplicationContext context = new ClassPathXmlApplicationContext( "spring-http-client-config.xml"); final SearchService searchService = (SearchService) context.getBean("searchService"); SearchJdo search = new SearchJdo(); search.name = "myName"; // this method actually returns the same object it gets as an argument SearchJdo search2 = searchService.processSearch(search); System.out.println(search == search2); // prints "false" } } The problem is that the search objects are different because of serializaton although from logical prospective they are the same. The question is whether there are some technique that allows to support or emulate object identity across VMs.

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