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What is a loop invariant?

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问题 I\'m reading \"Introduction to Algorithm\" by CLRS. In chapter 2, the authors mention \"loop invariants\". What is a loop invariant? 回答1: In simple words, a loop invariant is some predicate (condition) that holds for every iteration of the loop. For example, let's look at a simple for loop that looks like this: int j = 9; for(int i=0; i<10; i++) j--; In this example it is true (for every iteration) that i + j == 9 . A weaker invariant that is also true is that i >= 0 && i <= 10 . 回答2: I like

What is AJAX, really?

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I have to start using AJAX in a project and I don't know where to start. Can someone please help? A synchronous J avaScript A nd X ml. A technique for achieving bi-directional, script-driven communications between Web browsers and servers via HTTP. See also: definition on Wikipedia AJAX Introduction on w3schools Ajax Workshop 1 on Ajax Lessons Edit: As pointed out by Nosredna, JSON is often used in place of XML. The rough idea in English: You have a web page. Some event (can be a button press or other form event, or just something triggered by a timer) occurs and triggers JavaScript code that

What are public, private and protected in object oriented programming?

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What are public, private and protected in object oriented programming? They are access modifiers and help us implement Encapsulation (or information hiding). They tell the compiler which other classes should have access to the field or method being defined. private - Only the current class will have access to the field or method. protected - Only the current class and subclasses (and sometimes also same-package classes) of this class will have access to the field or method. public - Any class can refer to the field or call the method. This assumes these keywords are used as part of a field or

What is a Y-combinator? [closed]

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A Y-combinator is a computer science concept from the “functional” side of things. Most programmers don't know much at all about combinators, if they've even heard about them. What is a Y-combinator? How do combinators work? What are they good for? Are they useful in procedural languages? Nicholas Mancuso If you're ready for a long read, Mike Vanier has a great explanation . Long story short, it allows you to implement recursion in a language that doesn't necessarily support it natively. Chris Ammerman A Y-combinator is a "functional" (a function that operates on other functions) that enables

ReferenceError: variable is not defined

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问题 I met this issue sometimes but still don\'t know what causes it. I have this script in the page: $(function(){ var value = \"10\"; }); But the browser says \"ReferenceError: value is not defined\". However if I go to the browser console and input either 10 or var value = \"10\"; either of them can return 10. What is the problem with my script? Edit:just get rid of \"var\" can solve the problem. 回答1: It's declared inside a closure, which means it can only be accessed there. If you want a

Error with multiple definitions of function

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问题 I am trying to relearn C++ after taking an intro course a few years ago and I’m having some basic problems. My current problem occurs when trying to use a friend function. Here is my code in 2 files. First: // fun.cpp #include <iostream> using namespace std; class classA { friend void funct(); public: classA(int a=1,int b=2):propa(a),propb(b){cout<<\"constructor\\n\";} private: int propa; int propb; void outfun(){ cout<<\"propa=\"<<propa<<endl<<\"propb=\"<<propb<<endl; } }; void funct(){ //

In CUDA, what is memory coalescing, and how is it achieved?

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问题 What is \"coalesced\" in CUDA global memory transaction? I couldn\'t understand even after going through my CUDA guide. How to do it? In CUDA programming guide matrix example, accessing the matrix row by row is called \"coalesced\" or col.. by col.. is called coalesced? Which is correct and why? 回答1: It's likely that this information applies only to compute capabality 1.x, or cuda 2.0. More recent architectures and cuda 3.0 have more sophisticated global memory access and in fact "coalesced

can size of array be determined at run time in c?

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问题 As I know, an array needs to have a specific size before compiling time in c. I wonder why this code still works? int s; printf(\"enter the array size: \"); scanf(\"%d\",&s); int a[s]; // Isn\'t s value determined at run time? 回答1: Array sizes need to be known with ANSI 89 C. The 99 version of the spec removed this limitation and allowed for variable sized arrays. Here is the documentation no the GNU version of this feature http://www.cs.utah.edu/dept/old/texinfo/glibc-manual-0.02/library_toc

What is an example of the Single Responsibility Principle? [closed]

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问题 Can someone give me an example of the Single Responsibility Principle? I am trying to understand what it means, in practice, for a class to have a single responsibility as I fear I probably break this rule daily. 回答1: Check out the Solid description. Unless you ask for something more specific, it will be hard to help more. Single responsibility is the concept of a Class doing one specific thing (responsibility) and not trying to do more than it should, which is also referred to as High

Add Auto-Increment ID to existing table?

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问题 I have a pre-existing table, containing \'fname\', \'lname\', \'email\', \'password\' and \'ip\'. But now I want an auto-increment column. However, when I enter: ALTER TABLE users ADD id int NOT NULL AUTO_INCREMENT I get the following: #1075 - Incorrect table definition; there can be only one auto column and it must be defined as a key Any advice?:) 回答1: Try this ALTER TABLE `users` ADD `id` INT NOT NULL AUTO_INCREMENT PRIMARY KEY 回答2: If you don't care whether the auto-id is used as PRIMARY