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• What are the preferred versions of Vim and Emacs on Mac OS X?

  - by Michiel de Mare

  For those of us that like to use the graphical version of Vim or Emacs, instead of the console version, which version do you recommend? For Vim, there's Mac OS X Vim, MacVim, Vim-Cocoa. For Emacs, CarbonEmacs, XEmacs, and Aquamacs. Are there more? Which of these are ready for prime-time? If it's a tough call, what are the trade-offs? Are all of these still being maintained? No discussion of Vim vs. Emacs, if you don't mind, or comparisons with other editors.

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• No speed-up with useless printf's using OpenMP

  - by t2k32316

  I just wrote my first OpenMP program that parallelizes a simple for loop. I ran the code on my dual core machine and saw some speed up when going from 1 thread to 2 threads. However, I ran the same code on a school linux server and saw no speed-up. After trying different things, I finally realized that removing some useless printf statements caused the code to have significant speed-up. Below is the main part of the code that I parallelized: #pragma omp parallel for private(i) for(i = 2; i <= n; i++) { printf("useless statement"); prime[i-2] = is_prime(i); } I guess that the implementation of printf has significant overhead that OpenMP must be duplicating with each thread. What causes this overhead and why can OpenMP not overcome it?

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• The Community-Driven GDB Primer

  - by fbrereto

  I was reading this question and realized it might be helpful for entry- and pro-level developers alike (including myself) to have a common reference for best practices in using gdb. Many questions asked on Stack Overflow could easily be solved by taking some time to step some code in a debugger, and it would be good to have a community-approved resource to "teach them how to fish", so to speak. Even for those seasoned veterans who occasionally find themselves in gdb when they are accustomed to a GUI-tastic debugger might benefit from those who are much more familiar with the command line tool. For starters (both to gdb and to prime this thread) I submit: Ninefinger's gdb primer The gdb quick reference guide, which is useful for telling you what commands are available but not how best to use them. My hope is this thread is a seed planted that is of continued value to the community. If by "continued value" the community decides to nix it altogether, well then the masses have spoken.

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• Modifying an old Windows program not to call exit after a keypress without source access

  - by rustyn2

  I have an older C++ Windows program that I've been asked to get run in a kiosk style environment for a student project. From the main menu, hitting ESC will exit the program, which is undesirable. In ye olden days I would have trapped the keyboard interrupt or whatever and dug around to NOOP whatever JMP or CALL was getting referenced in the case statement that likely decides all that, but on Windows everything goes through various registered event handlers, and I haven't done any windows internals work in about 10 years. Is there a good tool to breakpoint a program on certain WM_EVENT (WM_KEYDOWN being a prime target) messages or similar, so that I can narrow down where in the executable the check is made? I'm currently stepping back from various potential system calls made before the various thread cleanups and final exit calls but it seems like there has to be a better way to do this that I'm forgetting.

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• Pohlig–Hellman algorithm for computing discrete logarithms

  - by drelihan

  Hi Folks, I'm working on coding the Pohlig-Hellman Algorithm but I am having problem understand the steps in the algorithm based on the definition of the algorithm. Going by the Wiki of the algorithm: http://en.wikipedia.org/wiki/Pohlig%E2%80%93Hellman_algorithm I know the first part 1) is to calculate the prime factor of p-1 - which is fine. Howeever, I am not sure what I need to do in steps 2) and 3). Can someone help with explaining this in plain english (i) - or pseudocode. I want to code the solution myself obviously but I cannot make any more progress unless i understand the algorithm. Note: I have done a lot of searching for this and I read S. Pohlig and M. Hellman (1978). "An Improved Algorithm for Computing Logarithms over GF(p) and its Cryptographic Significance but its still not really making sense to me. Thanks in advance

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• Why are primes important in cryptography?

  - by Michael Stum

  One thing that always strikes me as a non-cryptographer: Why is it so important to use Prime numbers? What makes them so special in cryptography? Does anyone have a simple short explanation? (I am aware that there are many primers and that Applied Cryptography is the Bible, but as said: I am not looking to implement my own cryptographic algorithm, and the stuff that I found just made my brain explode - no 10 pages of math formulas please :)) Thanks for all the answers. I've accepted the one that made the actual concept most clear to me.

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• Most optimized way to calculate modulus in C

  - by hasanatkazmi

  I have minimize cost of calculating modulus in C. say I have a number x and n is the number which will divide x when n == 65536 (which happens to be 2^16): mod = x % n (11 assembly instructions as produced by GCC) or mod = x & 0xffff which is equal to mod = x & 65535 (4 assembly instructions) so, GCC doesn't optimize it to this extent. In my case n is not x^(int) but is largest prime less than 2^16 which is 65521 as I showed for n == 2^16, bit-wise operations can optimize the computation. What bit-wise operations can I preform when n == 65521 to calculate modulus.

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• Selection of parameters in Diffie-Hellman

  - by allenzzzxd

  Hello, maybe it's not so proper to ask this question here... anyway, I'm trying to use the gmp library for the implementation of DH, but the problem here I got is: Once, when I was doing the tests to observe the output, although big values of prime and the private keys were selected: p was about more than 300 digits long in decimal a, b were about 100 digits long finally I got a shared secret key which was extremely small, perhaps smaller than 10^8 in decimal... This problem didn't show up many times, in fact, during all the observation, it appeared just once...but still, this was not so good at all. So I wonder if there are some methods which can avoid this... Thanx a lot

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• MAD method compression function

  - by Jacques

  I ran across the question below in an old exam. My answers just feels a bit short and inadequate. Any extra ideas I can look into or reasons I have overlooked would be great. Thanx Consider the MAD method compression function, mapping an object with hash code i to element [(3i + 7)mod9027]mod6000 of the 6000-element bucket array. Explain why this is a poor choice of compression function, and how it could be improved. I basically just say that the function could be improved by changing the value for p (or 9027) to an prime number and choosing an other constant for a (or 3) could also help.

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• What challenges are there in making an iPhone IDE for Windows/Linux?

  - by Moshe

  First of all, is this possible? If so: What challenges would I encounter in making an XCode imitation for iPhone/iPod development for Windows or Linux? I was thinking about using gcc as the actual compiler for the objecitve-c. Will that work? It doesn't need to compile to iPhone OS until it is to be tested on the device or submitted to the app store. Perhaps it will be easier to compile to the local OS format (Windows or Linux) until "prime-time".

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• Self updating application install with WIX?

  - by Brian ONeil

  I am writing an application that needs to be installed on a large number of desktops and also needs to update itself. We are looking at WIX for creating the installation. I have used ClickOnce and it is not a good solution for this install. WIX seems to fit, but there is no good process for auto update that I have found. I have looked at ClickThrough, but it doesn't seem ready for prime time yet. Does anyone have another good solution to use with WIX (or maybe another install program) to auto update an application install?

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• Interesting interview question. .Net

  - by rahul

  Coding Problem NumTrans There is an integer K. You are allowed to add to K any of its divisors not equal to 1and K. The same operation can be applied to the resulting number and so on. Notice that starting from the number 4, we can reach any composite number by applying several such operations. For example, the number 24 can be reached starting from 4 using 5 operations: 468121824 You will solve a more general problem. Given integers n and m, return the minimal number of the described operations necessary to transform n into m. Return -1 if m can't be obtained from n. Definition Method signature: int GetLeastCount (int n, int m) Constraints N will be between 4 and 100000, inclusive. M will be between N and 100000, inclusive. Examples 1) 4 576 Returns: 14 The shortest order of operations is: 468121827365481108162243324432576 2) 8748 83462 Returns: 10 The shortest order of operations is: 874813122196832624439366590497873283106834488346083462 3) 4 99991 Returns: -1 The number 99991 can't be obtained because it’s prime!

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• Active Record/ORM vs Normal Forms?

  - by Arsenal

  Hello, I've been playing around with Active Record a bit, and I have noticed that A.C./ORM always uses the following database model when creating a one-to-one relationship Person id | country_id | name | ... Country id | tld | name | ... No I wondered, isn't this a violiation of the third Normal Form? This clearly states "Every non-prime attribute is non-transitively dependent on every key of the table". Well this country_id isn't dependent of personid is it? So is this wrong or am I just not getting the point?

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• Matlab code help. Backward Euler method.

  - by m0s

  Here is the matlab/freemat code I got to solve ODE numerically using backward Euler method. However the results are inconsistent with my textbook results, and sometimes even ridiculously inconsistent. Please point out what is wrong with the code. I have already asked this question on mathoverflow.com no help there, hope someone here can help. function [x,y]=backEuler(f,xinit,yinit,xfinal,h) %f - this is your y prime %xinit - initial X %yinit - initial Y %xfinal - final X %h - step size n=(xfinal-xinit)/h; %Calculate steps %Inititialize arrays... %1st elements take xinit and yinit corespondigly, the rest fill with 0s x=[xinit zeros(1,n)]; y=[yinit zeros(1,n)]; %Numeric routine for i=1:n x(i+1)=x(i)+h; ynew=y(i)+h*(f(x(i),y(i))); y(i+1)=y(i)+h*f(x(i+1),ynew); end end

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• Floating Point Arithmetic - Modulo Operator on Double Type

  - by CrimsonX

  So I'm trying to figure out why the modulo operator is returning such a large unusual value. If I have the code: double result = 1.0d % 0.1d; it will give a result of 0.09999999999999995. I would expect a value of 0 Note this problem doesn't exist using the dividing operator - double result = 1.0d / 0.1d; will give a result of 10.0, meaning that the remainder should be 0. Let me be clear: I'm not surprised that an error exists, I'm surprised that the error is so darn large compared to the numbers at play. 0.0999 ~= 0.1 and 0.1 is on the same order of magnitude as 0.1d and only one order of magnitude away from 1.0d. Its not like you can compare it to a double.epsilon, or say "its equal if its < 0.00001 difference". I've read up on this topic on StackOverflow, in the following posts one two three, amongst others. Can anyone suggest explain why this error is so large? Any any suggestions to avoid running into the problems in the future (I know I could use decimal instead but I'm concerned about the performance of that).

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• java: relationship of the Runnable and Thread interfaces

  - by Karl Patrick

  I realize that the method run() must be declared because its declared in the Runnable interface. But my question comes when this class runs how is the Thread object allowed if there is no import call to a particular package? how does runnable know anything about Thread or its methods? does the Runnable interface extend the Thread class? Obviously I don't understand interfaces very well. thanks in advance. class PrimeFinder implements Runnable{ public long target; public long prime; public boolean finished = false; public Thread runner; PrimeFinder(long inTarget){ target = inTarget; if(runner == null){ runner = new Thread(this); runner.start() } } public void run(){ } }

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• Recursive function causing a stack overflow

  - by dbyrne

  I am trying to write a simple sieve function to calculate prime numbers in clojure. I've seen this question about writing an efficient sieve function, but I am not to that point yet. Right now I am just trying to write a very simple (and slow) sieve. Here is what I have come up with: (defn sieve [potentials primes] (if-let [p (first potentials)] (recur (filter #(not= (mod % p) 0) potentials) (conj primes p)) primes)) For small ranges it works fine, but causes a stack overflow for large ranges: user=> (sieve (range 2 30) []) [2 3 5 7 11 13 17 19 23 29] user=> (sieve (range 2 15000) []) java.lang.StackOverflowError (NO_SOURCE_FILE:0) I thought that by using recur this would be a non-stack-consuming looping construct? What am I missing?

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• Places to start for system programmer transitioning to web programming

  - by Sean Ochoa

  So here's where I'm coming from: My background is in C#, C++, VB Script, php, javascript, PowerShell, T-SQL, and VB 6. I have some experience with python, and a brief introduction to Ruby On Rails. At work, we're transitioning to a web based UI in the next year or so, but in asp.net & SilverLight. I would like to, if possible, learn more open source web technologies on the side. And, hopefully, in a year and a half or so, I would like to transition to a more open source web technology position. I found that I do really like python, but I'm open to pretty much anything. And yes, I do know Linux (ubuntu and gentoo), as well. And, here's my question: What technologies, frameworks, IDEs, or systems should I be highly proficient in to become a prime candidate for a position doing web application development using non-Microsoft technologies?

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• Is there a high-level gestures library for iPhone development?

  - by n8gray

  The iPhone platform has a number of common gesture idioms. For example, there are taps, pinches, and swipes, each with varying number of fingers. But when you're developing an app, it's up to you to implement these things based on low-level information about the number and locations of touches. It seems like this is a prime candidate for a library. You would register a delegate, set some parameters like multi-tap interval and swipe threshold, and get calls like swipeStarted/Ended, pinchStarted/Ended, multiTap, etc. Does such a library exist?

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• How do you get "in the zone"?

  - by Wayne Werner

  Hi, I've just started my first real programming job and am pleased to discover that this is exactly what I want to do for the rest of my life. When it comes round to ~1 hour before it's time to go home and I think "Man, do I have to go home already?" I'd say that's A Good Thing(tm). One thing I've discovered though is that it takes a little while for my brain to get "in gear" or "in the Zone", so I'm curious what other folks do to get programming at their prime. My current flow is when I get here I visit SO and look at the interesting problems - I find it helps get my brain moving. After 20-30 minutes I start looking at my code/specs/etc to decide what I want/need to work on first. So how do you get started?

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• Learn Prolog Now! DCG Practice Example

  - by Timothy

  I have been progressing through Learn Prolog Now! as self-study and am now learning about Definite Clause Grammars. I am having some difficulty with one of the Practical Session's tasks. The task reads: The formal language anb2mc2mdn consists of all strings of the following form: an unbroken block of as followed by an unbroken block of bs followed by an unbroken block of cs followed by an unbroken block of ds, such that the a and d blocks are exactly the same length, and the c and d blocks are also exactly the same length and furthermore consist of an even number of cs and ds respectively. For example, ε, abbccd, and aaabbbbccccddd all belong to anb2mc2mdn. Write a DCG that generates this language. I am able to write rules that generate andn, b2mc2m, and even anb2m and c2mndn... but I can't seem to join all these rules into anb2mc2mdn. The following are my rules that can generate andn and b2mc2m. s1 --> []. s1 --> a,s1,d. a --> [a]. d --> [d]. s2 --> []. s2 --> c,c,s2,d,d. c --> [c]. d --> [d]. Is anb2mc2mdn really a CFG, and is it possible to write a DCG using only what was taught in the lesson (no additional arguments or code, etc)? If so, can anyone offer me some guidance how I can join these so that I can solve the given task?

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• smallest perimiter rectangle with given integer area and integer sides

  - by remuladgryta

  Given an integer area A, how can one find integer sides w and h of a rectangle such that w*h = A and w+h is as small as possible? I'd rather the algorithm be simple than efficient (although within reasonable efficiency). What would be the best way to accomplish this? Finding out the prime factors of A, then combining them in some way that tries to balance w and h? Finding the two squares with integer sides with areas closest to A and then somehow interpolating between them? Any other method i'm not thinking of?

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• FastCGI C++ program and missing SCRIPT_NAME

  - by Simone Margaritelli

  Hi guys, i'm studying the fastcgi developement kit because i'm writing a new scripting language and i'd like to write a fastcgi version of the interpreter to run scripts as webpages. I'm using this example from the sdk, located at /srv/http/bin/echocpp So, in my httpd.conf file, i have the following lines : FastCgiServer /srv/http/bin/echocpp -idle-timeout 120 -processes 4 ScriptAlias / /srv/http/bin/echocpp/ ... ... AddHandler fastcgi-script hy Where 'hy' is the extensions of my scripts. Then, when i try browse, for instance http://localhost/~evilsocket/prime.hy I see all the environment variables as expected from the echo-cpp.cpp programm, except for the SCRIPT_NAME that is empty. Is there something i'm missing out of this? How am i supposed to obtain the full path of the script to run it inside my fastcgi version of the interpreter? Thanks

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• Can someone explain this java interface to me please?

  - by Karl Patrick

  I realize that the method run must be declared because its declared in the runnable interface. But my question comes when this class runs how is the Thread object allowed if there is no import call to a particular package? how does runnable know anything about Thread or its methods? does the runnable interface extend the thread class? Obviously i dont understand interfaces very well. thanks in advance. class PrimeFinder implements Runnable{ public long target; public long prime; public boolean finished = false; public Thread runner; PrimeFinder(long inTarget){ target = inTarget; if(runner == null){ runner = new Thread(this); runner.start() } } public void run(){ } }

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• Modulus PHP Problem

  - by Eli

  I have a problem, I am trying to calculate what the lowest prime is of a number but I do not understand the result that PHP is giving me. If I have this number $number = 600851475143; Then I modulus it: $primes = array( 2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47, 53, 59, 61, 67, 71, 73, 79, 83, 89, 97); foreach($primes as $key=>$value) { if($number % $value == 0 ) {echo $value; break; } } Why is it that $value = 3? If $value = 3, that means that 600851475143 / 3 should be an integer, but its not. So I do not understand why that if() evaluates to true?

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