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• Number of different elements in an array.

  - by AB

  Is it possible to compute the number of different elements in an array in linear time and constant space? Let us say it's an array of long integers, and you can not allocate an array of length sizeof(long). P.S. Not homework, just curious. I've got a book that sort of implies that it is possible.

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• What is the purpose of Java's unary plus operator?

  - by Syntactic

  Java's unary plus operator appears to have come over from C, via C++. As near as I can tell, it has the following effects: promotes its operand to int, if it's not already an int or wider unboxes its operand, if it's a wrapper object complicates slightly the parsing of evil expressions containing large numbers of consecutive plus signs It seems to me that there are better (or, at least, clearer) ways to do all of these things. In this SO question, concerning the counterpart operator in C#, someone said that "It's there to be overloaded if you feel the need." But in Java, one cannot overload any operator. So does this operator exist in Java just because it existed in C++?

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• A two way minimum spanning tree of a directed graph

  - by mvid

  Given a directed graph with weighted edges, what algorithm can be used to give a sub-graph that has minimum weight, but allows movement from any vertex to any other vertex in the graph (under the assumption that paths between any two vertices always exist). Does such an algorithm exist?

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• Lackadaisical One-to-One between Char and Byte Streams

  - by Vaibhav Bajpai

  I expected to have a one-to-one correspondence between the character streams and byte streams in terms of how the classes are organized in their hierarchy. FilterReader and FilterWriter (character streams) correspond back to FilterInputStream and FilterOutputStream (byte stream) classes. However I noticed few changes as - BufferedInputStream extends FilterInputStream, but BufferedReader does NOT extend FilterReader. BufferedOutputStream and PrintStream both extend FilterOutputStream, but BufferedWriter and PrintWriter does NOT extend FilterWriter. FilterInputStream and FilterOutputStream are not abstract classes, but FilterReader and FilterWriter are. I am not sure if I am being too paranoid to point out such differences, but was just curious to know if there was design reasoning behind such decision.

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• where does the professional sheen of a GUI application realistically come from?

  - by JW

  I have been playing around with php-gtk recently and in the past I have experimented with Java to make GUI 'hello world' apps. However both these types of applications have had a bit of a clunky (almost childish) look and feel to them. I cannot deny that they are handy for making apps for in-house use (and I totally respect the amount of community effort that goes into these projects). But I would not necessarily be proud to sell it as a commercial application with a price tag of, say, £450 or £1,000. If I wanted to make an application that had the look and feel of, say, Firefox for Windows, or Adobe xyz, what GUI/language should I use? Is the 'professional sheen' or smart look and feel down to the designer or is it the case that, no matter how good a designer is, picking the right GUI framework is essential to get that look?

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• javascript: execute a bunch of asynchronous method with one callback

  - by Samuel Michelot

  I need to execute a bunch of asynchronous methods (client SQLite database), and call only one final callback. Of course, the ugly way is: execAll : function(callBack) { asynch1(function() { asynch2(function() { ... asynchN(function() { callBack(); }) }) }); } But I know there are better ways to do it. Intuitively I would detect when all call back has been called with a counter to call the final callback. I think this is a common design-pattern, so if someone could point me in the right direction... Thanks in advance !

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• writing a program to ban a user (by MAC address) from a network

  - by Gail

  I'm not sure if this is possible at the application layer. Can a program be written to read and analyze packets (maybe interfacing with wireshark through it's lua api) and ban MAC addresses with suspicious network traffic? (defining suspicious network traffic as packet injection patterns similar to known attacks)

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• C# internal VS VBNET Friend

  - by Will Marcouiller

  To this SO question: What is the C# equivalent of friend?, I would personally have answered "internal", just like Ja did among the answers! However, Jon Skeet says that there is no direct equivalence of VB Friend in C#. If Jon Skeet says so, I won't be the one telling otherwise! ;P I'm wondering how can the keyword internal (C#) not be the equivalent of Friend (VBNET) when their respective definitions are: Friend VBNET The Friend (Visual Basic) keyword in the declaration statement specifies that the elements can be accessed from within the same assembly, but not from outside the assembly. [...] internal C# Internal: Access is limited to the current assembly. To my understanding, these definitions mean quite the same to me. Then, respectively, when I'm coding in VB.NET, I use the Friend keyword to specify that a class or a property shall be accessible only within the assembly where it is declared. The same in C#, I use the internal keyword to specify the same. Am I doing something or anything wrong from this perspective? What are the refinements I don't get? Might someone please explain how or in what Friend and internal are not direct equivalences? Thanks in advance for any of your answers!

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• Is it possible to create thread-safe collections without locks?

  - by Andrey

  This is pure just for interest question, any sort of questions are welcome. So is it possible to create thread-safe collections without any locks? By locks I mean any thread synchronization mechanisms, including Mutex, Semaphore, and even Interlocked, all of them. Is it possible at user level, without calling system functions? Ok, may be implementation is not effective, i am interested in theoretical possibility. If not what is the minimum means to do it? EDIT: Why immutable collections don't work. This of class Stack with methods Add that returns another Stack. Now here is program: Stack stack = new ...; ThreadedMethod() { loop { //Do the loop stack = stack.Add(element); } } this expression stack = stack.Add(element) is not atomic, and you can overwrite new stack from other thread. Thanks, Andrey

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• What do we log and why do we log it?

  - by Lucas

  This has been bugging me for quite some time. Reading various questions on SO, blogs and listening to colleagues, I keep hearing how important "logging" is. How various logging frameworks stack up against each other, and how there are so many to pick from it's (apparently) ridiculous. Now, I know what logging is. What I don't know is what is supposed to be logged and why. Sure, I can guess. Exceptions? Sounds like something one might want to log... but which exceptions? And is it only exceptions? And what do I do with the logged information? If it's an in-house app, then that could probably be put to good use, but if it's a commercial desktop application, how is the log of... whatever... helping anyone? I doubt regular users would be peeking inside. Is it then something you ask the users to provide on request? I'm deeply frustrated by my own ignorance in this. It's also surprising how little information there is about this. The info on the websites of the various logging frameworks is all written for an audience that already knows what it wants to log, and knows why it needs to do so. Same things goes for the various discussions on SO about logging, like for instance this highly voted up question on Logging best practices. For a question with so many votes, it's almost comical how there's next to nothing in there that would answer my what and why questions. So being finally fed up, I'm asking here: what do people log, and why do they log it?

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• How to create new account using servlets

  - by Alvin

  I want to know how can I create new account using servlets in post method? I'm currently following MVC design pattern, and I want to know if I pass the required data to register new account from JSP page, then how can I get that data in post method? As request.getParameter() method returning me null. How can I know that post method is calling to create new account? How can I pass all the relevant user information from servlets to model class for registering data to the database table?

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• Is there a way to programatically check dependencies of an EXE?

  - by Mason Wheeler

  I've got a certain project that I build and distribute to users. I have two build configurations, Debug and Release. Debug, obviously, is for my use in debugging, but there's an additional wrinkle: the Debug configuration uses a special debugging memory manager, with a dependency on an external DLL. There's been a few times when I've accidentally built and distributed an installer package with the Debug configuration, and it's then failed to run once installed because the users don't have the special DLL. I'd like to be able to keep that from happening in the future. I know I can get the dependencies in a program by running Dependency Walker, but I'm looking for a way to do it programatically. Specifically, I have a way to run scripts while creating the installer, and I want something I can put in the installer script to check the program and see if it has a dependency on this DLL, and if so, cause the installer-creation process to fail with an error. I know how to create a simple CLI program that would take two filenames as parameters, and could run a DependsOn function and create output based on the result of it, but I don't know what to put in the DependsOn function. Does anyone know how I'd go about writing it?

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• What is meant by porting an application X to a platform Y ?

  - by Neeraj

  Pretty clear from the title itself, what is meant by porting an application X to a platform Y? Say for example I have an application X running on some OS, say Y, What do I do to port this application to another OS say Z? Does this mean rewriting a new application A for Operating system Z that necessarily imitates the behavior of application X on Operating System Y. Please explain.

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• How do we achieve "substring-match" under O(n) time?

  - by Pacerier

  I have an assignment that requires reading a huge file of random inputs, for example: Adana Izmir Adnan Menderes Apt Addis Ababa Aden ADIYAMAN ALDAN Amman Marka Intl Airport Adak Island Adelaide Airport ANURADHAPURA Kodiak Apt DALLAS/ADDISON Ardabil ANDREWS AFB etc.. If I specify a search term, the program is supposed to find the lines whereby a substring occurs. For example, if the search term is "uradha", the program is supposed to show ANURADHAPURA. If the search term is "airport", the program is supposed to show Amman Marka Intl Airport, Adelaide Airport A quote from the assignment specs: "You are to program this application taking efficiency into account as though large amounts of data and processing is involved.." I could easily achieve this functionality using a loop but the performance would be O(n). I was thinking of using a trie but it seems to only work if the substring starts from index 0. I was wondering what solutions are there which gives a performance better than O(n)?

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• Articles about replication schemes/algorithms?

  - by jkff

  I'm designing an hierarchical distributed system (every node has zero or more "master" nodes to which it propagates its current data). The data gets continuously updated and I'd like to guarantee that at least N nodes have almost-current data at any given time. I do not need complete consistency, only eventual consistency (t.i. for any time instant, the current snapshot of data should eventually appear on at least N nodes. It is tricky to define the term "current" here, but still). Nodes may fail and go back up at any moment, and there is no single "central" node. O overflowers! Point me to some good papers describing replication schemes. I've so far found one: Consistency Management in Optimistic Replication Algorithms

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• Does this data structure have a name? Sort of a "linked matrix"?

  - by Bob

  Let's say I wanted similar functionality to a doubly linked list but needed a matrix instead so that each node was structured like this: public class Node { Node Up, Down, Left, Right; object Value; } Is there a name for such a structure? I've looked through this Wikipedia listing of data structures but didn't see anything similar. Unless I just missed it.

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• Generalized plugable caching pattern?

  - by BCS

  Given that it's one of the hard things in computer science, does anyone know of a way to set up a plugable caching strategy? What I'm thinking of would allow me to write a program with minimal thought as to what needs to be cached (e.i. use some sort of boiler-plate, low/no cost pattern that compiles away to nothing anywhere I might want caching) and then when things are further along and I know where I need caching I can add it in without making invasive code changes. As an idea to the kind of solution I'm looking for; I'm working with the D programing language (but halfway sane C++ would be fine) and I like template.

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• Why can't I pass self as a named argument to an instance method in Python?

  - by Joseph Garvin

  This works: >>> def bar(x, y): ... print x, y ... >>> bar(y=3, x=1) 1 3 And this works: >>> class foo(object): ... def bar(self, x, y): ... print x, y ... >>> z = foo() >>> z.bar(y=3, x=1) 1 3 And even this works: >>> foo.bar(z, y=3, x=1) 1 3 But why doesn't this work? >>> foo.bar(self=z, y=3, x=1) Traceback (most recent call last): File "<stdin>", line 1, in <module> TypeError: unbound method bar() must be called with foo instance as first argument (got nothing instead) This makes metaprogramming more difficult, because it requires special case handling. I'm curious if it's somehow necessary by Python's semantics or just an artifact of implementation.

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• Why do Pascal control structures appear to be inconsistent?

  - by 70Mike

  Most Pascal control structures make sense to me, like: for ... do {statement}; if (condition) then {statement}; while (condition) do {statement}; where the {statement} is either a single statement, or a begin ... end block. I have a problem with: repeat {statement-list} until (expression); try {statement-list} except {statement-list} end; Wouldn't it be better that repeat and try have the same general structure, accepting only a single statement or a begin ... end block, instead of having a statement-list that's not formally blocked with a begin and an end?

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• Technical choices in unmarshaling hash-consed data

  - by Pascal Cuoq

  There seems to be quite a bit of folklore knowledge floating about in restricted circles about the pitfalls of hash-consing combined with marshaling-unmarshaling of data. I am looking for citable references to these tidbits. For instance, someone once pointed me to library aterm and mentioned that the authors had clearly thought about this and that the representation on disk was bottom-up (children of a node come before the node itself in the data stream). This is indeed the right way to do things when you need to re-share each node (with a possible identical node already in memory). This re-sharing pass needs to be done bottom-up, so the unmarshaling itself might as well be, too, so that it's possible to do everything in a single pass. I am in the process of describing difficulties encountered in our own context, and the solutions we found. I would appreciate any citable reference to the kind of aforementioned folklore knowledge. Some people obviously have encountered the problems before (the aterm library is only one example). But I didn't find anything in writing. Even the little piece of information I have about aterm is hear-say. I am not worried it's not reliable (you can't make this up), but "personal communication" and "look how it's done in the source code" are considered poor form in citations. I have enough references on hash-consing alone. I am only interested in references where it interferes with other aspects of programming, such as marshaling or distribution.

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• Any benefit/disadvantages to open beta vs. closed beta for webapp

  - by Kamo

  This isn't a question about code, but it's programming related. We have a web app that's ready for beta testing. Has anyone noticed any difference between open beta vs. closed beta in terms of the quality or quantity of feedback the testers give or any other factors?

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• Nested class with hidden constructor impossible in c#?

  - by luckyluke

  I' ve been doing some programming lately and faced an issue which i found weird in c#. (at least for me) public class Foo { //whatever public class FooSpecificCollection : List<Bar> { //implementation details } public FooSpecificCollection GetFoosStuff() { //return the collection } } I want the consumer of Foo to be able to obtain a reference to FooSpecificCollection, even perform some operations on it. Maybe even set it to some other property of Foo or smth like that, but not To be able to CREATE an instance of this class. (the only class that should be able to instatiate this collection should be Foo. Is my request really that far-fetched? I know that people way smarter defined c# but shouldn't there be such an option that a parent class can create a nested class instance but nobody else can't. So far I created a solution to make an abstract class, or interface available through the property and implement a concrete private class that is not available anywhere else. Is this a correct way to handle such a situation.?

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• Issue when I'm trying to draw gradient in swift

  - by bagusflyer

  I got an error when I was trying to draw gradient in Swift code: GradientView.swift:31:40: Could not find an overload for '__conversion' that accepts the supplied arguments Here is my code: let context : CGContextRef = UIGraphicsGetCurrentContext() let locations :CGFloat[] = [ 0.0, 0.25, 0.5, 0.75 ] let colors = [UIColor.redColor().CGColor,UIColor.greenColor().CGColor,UIColor.blueColor().CGColor, UIColor.yellowColor().CGColor] let colorspace : CGColorSpaceRef = CGColorSpaceCreateDeviceRGB() let gradient : CGGradientRef = CGGradientCreateWithColors(colorspace, colors, locations) //CGGradientCreateWithColors(colorspace,colors,locations) let startPoint : CGPoint = CGPointMake(0, 0) let endPoint : CGPoint = CGPointMake(500,500) CGContextDrawLinearGradient(context, gradient,startPoint, endPoint, 0); The problem is the CGGradientCreateWithColors takes CFArray not a normal Swift Array. I have no idea how to convert CFArray to Array and can't find anything in Apple's document. Any idea? Thanks

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• How to write a spinlock without using CAS

  - by Martin

  Following on from a discussion which got going in the comments of this question. How would one go about writing a Spinlock without CAS operations? As the other question states: The memory ordering model is such that writes will be atomic (if two concurrent threads write a memory location at the same time, the result will be one or the other). The platform will not support atomic compare-and-set operations.

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• Why aren't constructors inherited?

  - by Mike

  In all OO languages I've worked with, constructors aren't inherited. Why is this?

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