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  • Proving that P <= NP

    - by Gail
    As most people know, P = NP is unproven and seems unlikely to be true. The proof would prove that P <= NP and NP <= P. Only one of those is hard, though. P <= NP is almost by definition true. In fact, that's the only way that I know how to state that P <= NP. It's just intuitive. How would you prove that P <= NP?

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  • Power of programming languages

    - by Casebash
    Are there any objective measures for measuring the power of programming languages? Turing-completeness is one, but it is not particularly discriminating. I also remember there being a few others measures of power which are more limited versions (like finite-state-autonoma), but is there any objective measure that is more powerful?

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  • Code Golf: Shortest Turing-complete interpreter.

    - by ilya n.
    I've just tried to create the smallest possible language interpreter. Would you like to join and try? Rules of the game: You should specify a programming language you're interpreting. If it's a language you invented, it should come with a list of commands in the comments. Your code should start with example program and data assigned to your code and data variables. Your code should end with output of your result. It's preferable that there are debug statements at every intermediate step. Your code should be runnable as written. You can assume that data are 0 and 1s (int, string or boolean, your choice) and output is a single bit. The language should be Turing-complete in the sense that for any algorithm written on a standard model, such as Turing machine, Markov chains, or similar of your choice, it's reasonably obvious (or explained) how to write a program that after being executred by your interpreter performs the algorithm. The length of the code is defined as the length of the code after removal of input part, output part, debug statements and non-necessary whitespaces. Please add the resulting code and its length to the post. You can't use functions that make compiler execute code for you, such as eval(), exec() or similar. This is a Community Wiki, meaning neither the question nor answers get the reputation points from votes. But vote anyway!

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  • Complex behavior generated by simple computation

    - by Yuval A
    Stephen Wolfram gave a fascinating talk at TED about his work with Mathematica and Wolfram Alpha. Amongst other things, he pointed out how very simple computations can yield extremely complex behaviors. (He goes on to discuss his ambition for computing the entire physical universe. Say what you will, you gotta give the guy some credit for his wild ideas...) As an example he showed several cellular automata. What other examples of simple computations do you know of that yield fascinating results?

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  • Is it possible to use Regex through Hexadecimal to find email addresses

    - by LukeJenx
    Not sure if this is even possible but I have been looking at using Regex to get an email address that is in Hex. Basically this is to build up some of my automated forensic tools but I am having problems on making a suitable Regex algorithm. Regex for email: /^([a-z0-9_.-]+)@([\da-z.-]+).([a-z.]{2,6})$/ Hex values: @ = 40 . = 2E .com = 636f6d _ = 5f A/a = 41/61 [1] Z/z = 5a/7a - = 2d This is what I have got at the moment (it only takes into account lower case and .com). But it doesn't work! Have I messed something simple up? "/^([61-7a]+)40([61-7a]+)23(636f6d)$/" [1] I know email can only be lower case but I need to take uppercase into account too.

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  • Access modifiers in Object-Oriented Programming

    - by Imran
    I don't understand Access Modifiers in OOP. Why do we make for example in Java instance variables private and then use public getter and setter methods to access them? I mean what's the reasoning/logic behind this? You still get to the instance variable but why use setter and getter methods when you can just make your variables public? please excuse my ignorance as I am simply trying to understand why? Thank you in advance. ;-)

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  • Can Haskell's Parsec library be used to implement a recursive descent parser with backup?

    - by Thor Thurn
    I've been considering using Haskell's Parsec parsing library to parse a subset of Java as a recursive descent parser as an alternative to more traditional parser-generator solutions like Happy. Parsec seems very easy to use, and parse speed is definitely not a factor for me. I'm wondering, though, if it's possible to implement "backup" with Parsec, a technique which finds the correct production to use by trying each one in turn. For a simple example, consider the very start of the JLS Java grammar: Literal: IntegerLiteral FloatingPointLiteral I'd like a way to not have to figure out how I should order these two rules to get the parse to succeed. As it stands, a naive implementation like this: literal = do { x <- try (do { v <- integer; return (IntLiteral v)}) <|> (do { v <- float; return (FPLiteral v)}); return(Literal x) } Will not work... inputs like "15.2" will cause the integer parser to succeed first, and then the whole thing will choke on the "." symbol. In this case, of course, it's obvious that you can solve the problem by re-ordering the two productions. In the general case, though, finding things like this is going to be a nightmare, and it's very likely that I'll miss some cases. Ideally, I'd like a way to have Parsec figure out stuff like this for me. Is this possible, or am I simply trying to do too much with the library? The Parsec documentation claims that it can "parse context-sensitive, infinite look-ahead grammars", so it seems like something like I should be able to do something here.

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  • Dynamic programming solution to the subset-sum decision problem

    - by Gail
    How can a dynamic programming solution for the unbounded knapsack decision problem be used to come up with a dynamic programming solution to the subset-sum decision problem? This limitation seems to render the unbounded knapsack problem useless. In the unbounded knapsack, we simply store true or false for if some subset of integers sum up to our target value. However, if we have a limit on the frequency of the use of these integers, the optimal substructure at least appears to fail. How can this be done?

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  • Proving that the distance values extracted in Dijkstra's algorithm is non-decreasing?

    - by Gail
    I'm reviewing my old algorithms notes and have come across this proof. It was from an assignment I had and I got it correct, but I feel that the proof certainly lacks. The question is to prove that the distance values taken from the priority queue in Dijkstra's algorithm is a non-decreasing sequence. My proof goes as follows: Proof by contradiction. Fist, assume that we pull a vertex from Q with d-value 'i'. Next time, we pull a vertex with d-value 'j'. When we pulled i, we have finalised our d-value and computed the shortest-path from the start vertex, s, to i. Since we have positive edge weights, it is impossible for our d-values to shrink as we add vertices to our path. If after pulling i from Q, we pull j with a smaller d-value, we may not have a shortest path to i, since we may be able to reach i through j. However, we have already computed the shortest path to i. We did not check a possible path. We no longer have a guaranteed path. Contradiction.

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  • Stereo Matching - Dynamic Programming

    - by Varun
    Hi, I am supposed to implement Dynamic programming algorithm for Stereo matching problem. I have read 2 research papers but still haven't understood as to how do I write my own c++ program for that ! Is there any book or resource that's available somewhere that I can use to get an idea as to how to start coding actually ? Internet search only gives me journal and conference papers regarding Dynamic Programming but not how to implement the algorithm step by step. Thanks Varun

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  • Which operating systems book should I go for?

    - by pecker
    Hi, I'm in a confusion. For our course (1 year ago) I used Stallings. I read it. It was fine. But I don't own any operating system's book. I want to buy a book on operating systems. I'm confused!! which one to pick? Modern Operating Systems (3rd Edition) ~ Andrew S. Tanenbaum (Author) Operating System Concepts ~ Abraham Silberschatz , Peter B. Galvin, Greg Gagne Operating Systems: Internals and Design Principles (6th Edition) ~ William Stallings I've plans of getting into development of realworld operating systems : Linux, Unix & Windows Driver Development. I know that for each of these there are specific books available. But I feel one should have a basic book on the shelf. So, which one to go for?

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  • Generating All Permutations of Character Combinations when # of arrays and length of each array are

    - by Jay
    Hi everyone, I'm not sure how to ask my question in a succinct way, so I'll start with examples and expand from there. I am working with VBA, but I think this problem is non language specific and would only require a bright mind that can provide a pseudo code framework. Thanks in advance for the help! Example: I have 3 Character Arrays Like So: Arr_1 = [X,Y,Z] Arr_2 = [A,B] Arr_3 = [1,2,3,4] I would like to generate ALL possible permutations of the character arrays like so: XA1 XA2 XA3 XA4 XB1 XB2 XB3 XB4 YA1 YA2 . . . ZB3 ZB4 This can be easily solved using 3 while loops or for loops. My question is how do I solve for this if the # of arrays is unknown and the length of each array is unknown? So as an example with 4 character arrays: Arr_1 = [X,Y,Z] Arr_2 = [A,B] Arr_3 = [1,2,3,4] Arr_4 = [a,b] I would need to generate: XA1a XA1b XA2a XA2b XA3a XA3b XA4a XA4b . . . ZB4a ZB4b So the Generalized Example would be: Arr_1 = [...] Arr_2 = [...] Arr_3 = [...] . . . Arr_x = [...] Is there a way to structure a function that will generate an unknown number of loops and loop through the length of each array to generate the permutations? Or maybe there's a better way to think about the problem? Thanks Everyone!

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  • Context-sensitive grammar for specific language

    - by superagio
    How can I construct a grammar that generates this language? Construct a grammar that generates L: L = {a^n b^m c^k|k>n, k>m} I believe my productions should go along this lines: S-> ABCC A-> a|aBC|BC B-> b|bBC C-> c|Cc CB->BC The idea is to start with 2 c and keep always one more c, and then with C-c|Cc ad as much c as i want. How can my production for C remember the numbers of m and n.

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  • Finding the nth number of primes

    - by Braxton Smith
    I can not figure out why this won't work. Please help me from math import sqrt pN = 0 numPrimes = 0 num = 1 def checkPrime(x): '''Check\'s whether a number is a prime or not''' prime = True if(x==2): prime = True elif(x%2==0): prime=False else: root=int(sqrt(x)) for i in range(3,root,2): if(x%i==0): prime=False break return prime n = int(input("Find n number of primes. N being:")) while( numPrimes != n ): if( checkPrime( num ) == True ): numPrimes += 1 pN = num print("{0}: {1}".format(numPrimes,pN)) num += 1 print("Prime {0} is: {1}".format(n,pN))

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  • Examples of monoids/semigroups in programming

    - by jkff
    It is well-known that monoids are stunningly ubiquitous in programing. They are so ubiquitous and so useful that I, as a 'hobby project', am working on a system that is completely based on their properties (distributed data aggregation). To make the system useful I need useful monoids :) I already know of these: Numeric or matrix sum Numeric or matrix product Minimum or maximum under a total order with a top or bottom element (more generally, join or meet in a bounded lattice, or even more generally, product or coproduct in a category) Set union Map union where conflicting values are joined using a monoid Intersection of subsets of a finite set (or just set intersection if we speak about semigroups) Intersection of maps with a bounded key domain (same here) Merge of sorted sequences, perhaps with joining key-equal values in a different monoid/semigroup Bounded merge of sorted lists (same as above, but we take the top N of the result) Cartesian product of two monoids or semigroups List concatenation Endomorphism composition. Now, let us define a quasi-property of an operation as a property that holds up to an equivalence relation. For example, list concatenation is quasi-commutative if we consider lists of equal length or with identical contents up to permutation to be equivalent. Here are some quasi-monoids and quasi-commutative monoids and semigroups: Any (a+b = a or b, if we consider all elements of the carrier set to be equivalent) Any satisfying predicate (a+b = the one of a and b that is non-null and satisfies some predicate P, if none does then null; if we consider all elements satisfying P equivalent) Bounded mixture of random samples (xs+ys = a random sample of size N from the concatenation of xs and ys; if we consider any two samples with the same distribution as the whole dataset to be equivalent) Bounded mixture of weighted random samples Which others do exist?

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  • Why is it useful to count the number of bits?

    - by Scorchin
    I've seen the numerous questions about counting the number of set bits in a insert type of input, but why is it useful? For those looking for algorithms about bit counting, look here: http://stackoverflow.com/questions/1517848/counting-common-bits-in-a-sequence-of-unsigned-longs http://stackoverflow.com/questions/472325/fastest-way-to-count-number-of-bit-transitions-in-an-unsigned-int http://stackoverflow.com/questions/109023/best-algorithm-to-count-the-number-of-set-bits-in-a-32-bit-integer

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  • Annoying Captcha >> How to programm a form that can SMELL difference between human and robot?

    - by Sam
    Hi folks. On the comment of my old form needing a CAPTHA, I felt I share my problem, perhaps you recognize it and find its time we had better solutions: FACTUAL PROBLEM I know most of my clients (typical age= 40~60) hate CAPTCHA things. Now, I myself always feel like a robot, when I have to sueeze my eyes and fill in the strange letters from the Capcha... Sometimes I fail! Go back etc. Turnoff. I mean comon its 2011, shouldnt the forms have better A.I. by now? MY NEW IDEA (please dont laugh) Ive thought about it and this is my idea's to tell difference between human and robot: My idea is to give credibility points. 100 points = human 0% = robot. require real human mouse movements require mousemovements that dont follow any mathematical pattern require non-instantaneous reading delays, between load and first input in form when typing in form, delays are measured between letters and words approve as human when typical human behaviour measured (deleting, rephrasing etc) dont allow instant pasting or all fields give points for real keyboard pressures retract points for credibility when hyperlinks in form Test wether fake email field (invisible by human) is populated (suggested by Tomalak) when more than 75% human cretibility, allow to be sent without captcha when less than 25% human crecibility, force captcha puzzle to be sure Could we write a A.I. PHP that replaces the human-annoying capthas, meanwhile stopping most spamservers filing in the data? Not only for the fun of it, but also actually to provide a 99% better alternative than CAPTHCA's. Imagine the userfriendlyness of your forms! Your site distinguishing itself from others, showing your audience your sites KNOWS the difference between a robot and a human. Imagine the advangage. I am trying to capture the essense of that distinguishing edge. PROGRAMMING QUESTION: 1) Are such things possible to programm? 2) If so how would you start such programm? 3) Are there already very good working solutions available elsewhere? 4) If it isn't so hard, your are welcome to share your answer/solutions below. 5) upon completion of hints and new ideas, could this page be the start of a new AI captcha, OR should I forget about it and just go with the flow, forget about the whole AI dream, and use captcha like everyone else.

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  • Using Dijkstra's algorithm with negative edges?

    - by Riddler
    Most books explain the reason the algorithm doesn't work with negative edges as nodes are deleted from the priority queue after the node is arrived at since the algorithm assumes the shortest distance has been found. However since negative edges can reduce the distance, a future shorter distance might be found; but since the node is deleted it cannot be updated. Wouldn't an obvious solution to this be to not delete the node? Why not keep the node in the queue, so if a future shorter distance is found, it can be updated? If I am misunderstanding the problem, what is preventing the algorithm from being used with negative edges?

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  • Who in the software world do you admire the most?

    - by David McGraw
    In an effort to spark some discussion and to find interesting people that I didn't know about, is there anybody around the software industry that you really admire? Perhaps admire is the wrong choice of word, but I'm sure there is somebody out there that has impacted you in a minor way. What did you learn from this individual that defines what you try to achieve today?

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  • Real world example of Unification in First Order Logic?

    - by Sebi
    I know this is only part of a programming question, but at the moment, I'm doing a little bit of logic programming. One thing I still don't understand correctly is Unification in First Order Logic. I read the Wikipedia article and it is more or less clear that the purpose is searching a term that unifies two sentences... There are also examples in this article but I just don't get the point why this should be useful. Can anyone give an example with real world objects instead of A, B, C,, etc.? I hope this will help me to understand. Thanks

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  • Cpu schedule, removing thread from queue

    - by Kamil
    I'm implementing now CPU schedule algorithms FCFS, SJF and Round Robin. Could somebody tell when process is removed from queue (FCFS,SJF,RR)? I mean, first CPU execute thread and after executing remove from queue or the other way around?

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