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• How do LL(*) parsers work?

  - by freezer878

  I cannot find any complete description about LL(*) parser, such as ANTLR, on Internet. I'm wondering what is the difference between an LL(k) parser and an LL(*) one and why they can't support left-recusrive grammars despite their flexibility.

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• Tracking/Counting Word Frequency

  - by Joel Martinez

  I'd like to get some community consensus on a good design to be able to store and query word frequency counts. I'm building an application in which I have to parse text inputs and store how many times a word has appeared (over time). So given the following inputs: "To Kill a Mocking Bird" "Mocking a piano player" Would store the following values: Word Count ------------- To 1 Kill 1 A 2 Mocking 2 Bird 1 Piano 1 Player 1 And later be able to quickly query for the count value of a given arbitrary word. My current plan is to simply store the words and counts in a database, and rely on caching word count values ... But I suspect that I won't get enough cache hits to make this a viable solution long term. Can anyone suggest algorithms, or data structures, or any other idea that might make this a well-performing solution?

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• Given a few strings, how many strings can be lexicographically least by modifying the alphabet?

  - by Jackson W

  Number of strings can be huge as in 30000. Given N strings, output which ones can be lexicographically least after modifying the english alphabet. e.g. acdbe...... for example if the strings were: omm moo mom ommnom "mom" is already lexicographically least with the original english alphabet. we can make the word "omm" least by switching "m" and "o" in the alphabet ("abcdefghijklonmpqrstuvwxyz"). the other ones you cant make lexicographically last, no matter what you do. any fast way to do this? I have no ways to approach this except try every single possible alphabet

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• in-place permutation of a array follows this rule

  - by Mgccl

  Suppose there is an array, we want to find everything in the odd index, and move it to the end. Everything in the even index move it to the beginning. The relative order of all odd index items and all even index items are preserved. Suppose the values of the array, a[i] = i, n is even. Then we have. 0,1,2,3,4,5,...,n-1 after the operation 0,2,4,6,...,n-2,1,3,5,7,...,n-1 Can this be done in-place and in O(n) time?

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• Determining the order of a list of numbers (possibly without sorting)

  - by Victor Liu

  I have an array of unique integers (e.g. val[i]), in arbitrary order, and I would like to populate another array (ord[i]) with the the sorted indexes of the integers. In other words, val[ord[i]] is in sorted order for increasing i. Right now, I just fill in ord with 0, ..., N, then sort it based on the value array, but I am wondering if we can be more efficient about it since ord is not populated to begin with. This is more of a question out of curiousity; I don't really care about the extra overhead from having to prepopulate a list and then sort it (it's small, I use insertion sort). This may be a silly question with an obvious answer, but I couldn't find anything online.

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• Just for fun (C# and C++)...time yourself [closed]

  - by Ted

  Possible Duplicate: What is your solution to the FizzBuzz problem? OK guys this is just for fun, no flamming allowed ! I was reading the following http://www.codinghorror.com/blog/2007/02/why-cant-programmers-program.html and couldn't believe the following sentence... " I've also seen self-proclaimed senior programmers take more than 10-15 minutes to write a solution." For those that can't be bothered to read the article, the background is this: ....I set out to develop questions that can identify this kind of developer and came up with a class of questions I call "FizzBuzz Questions" named after a game children often play (or are made to play) in schools in the UK. An example of a Fizz-Buzz question is the following: Write a program that prints the numbers from 1 to 100. But for multiples of three print "Fizz" instead of the number and for the multiples of five print "Buzz". For numbers which are multiples of both three and five print "FizzBuzz". SO I decided to test myself. I took 5 minutes in C++ and 3mins in c#! So just for fun try it and post your timings + language used! P.S NO UNIT TESTS REQUIRED, NO OUTSOURCING ALLOWED, SWITCH OFF RESHARPER! :-) P.S. If you'd like to post your source then feel free

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• An O(1) Sort ~~~

  - by FlySwat

  Before you stone me for being a heretic, There is a sort that proclaims to be O(1), called "Bead Sort" (http://en.wikipedia.org/wiki/Bead_sort) , however that is pure theory, when actually applied I found that it was actually O(N * M), which is pretty pathetic. That said, Lets list out some of the fastest sorts, and their best case and worst case speed in Big O notation. ~~ FlySwat ~~

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• What are the advantages of a rebase over a merge in git?

  - by eSKay

  In this article, the author explains rebasing with this diagram: Rebase: If you have not yet published your branch, or have clearly communicated that others should not base their work on it, you have an alternative. You can rebase your branch, where instead of merging, your commit is replaced by another commit with a different parent, and your branch is moved there. while a normal merge would have looked like this: So, if you rebase, you are just losing a history state (which would be garbage collected sometime in the future). So, why would someone want to do a rebase at all? What am I missing here?

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• Determine coordinates of rotated rectangle

  - by MathieuK

  I'm creating an utility application that should detect and report the coordinates of the corners of a transparent rectangle (alpha=0) within an image. So far, I've set up a system with Javascript + Canvas that displays the image and starts a floodfill-like operation when I click inside the transparent rectangle in the image. It correctly determines the bounding box of the floodfill operation and as such can provide me the correct coordinates. Here's my implementation so-far: http://www.scriptorama.nl/image/ (works in recent Firefox / Safari ). However, the bounding box approach breaks down then the transparent rectangle is rotated (CW or CCW) as the resulting bounding box no longer properly represents the proper width and height. I've tried to come up with a few alternatives to detect to corners, but have not been able to think up a proper solution. So, does anyone have any suggestions on how I might approach this so I can properly detect the coordinates of 4 corners of the rotated rectangle?

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• Get `n` random values between 2 numbers having average `x`

  - by Somnath Muluk

  I want to get n random numbers(e.g n=16)(whole numbers) between 1 to 5(including both) so that average is x. x can be any value between (1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5). I am using PHP. e.g. Suppose I have average x= 3. Then required 16 whole numbers between 1 to 5(including both). like (1,5,3,3,3,3,2,4,2,4,1,5,1,5,3,3) Update: if x=3.5 means average of 16 numbers should be between 3.5 to 4. and if x=4 means average of 16 numbers should be between 4 to 4.5 and if x=5 means all numbers are 5

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• Finding patterns in Puzzle games.

  - by José Joel.

  I was wondering, which are the most commonly used algorithms applied to finding patterns in puzzle games conformed by grids of cells. I know that depends of many factors, like the kind of patterns You want to detect, or the rules of the game...but I wanted to know which are the most commonly used algorithms in that kind of problems... For example, games like columns, bejeweled, even tetris. I also want to know if detecting patterns by "brute force" ( like , scanning all the grid trying to find three adyacent cells of the same color ) is significantly worst that using particular algorithms in very small grids, like 4 X 4 for example ( and again, I know that depends of the kind of game and rules ...) Which structures are commonly used in this kind of games ?

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• Effective way to calculate a similarity percentage between data sets

  - by UltraVi01

  I am currently working with User objects -- each of which have many Goal objects. The Goal objects are not User specific, that is, Users can share the same Goal. I am attempting to fashion a way to calculate a "similarity percentage" between two Users... (i.e., taking into account how many Goals they share as well as how many Goals they do not share) Does anyone have experience with this type of situation? I am using Grails with Mysql if that is helpful. Thanks

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• A quick way to map unordered list of longs to buffer location ?

  - by alhazen

  I have a large number of points (indexed by long) that are processed by multiple threads and I'm using a buffer to hold the output results in order. As the number of points processed is huge, what would be an efficient way to map the indexes of the points to the corresponding ordered position in the buffer ? Example: long bufferIndex bufferIndex index (if BufferSize = 2) (if BufferSize = 4) ---------------------------------------------- 2938 0 0 2939 1 1 2941 1 3 2940 0 2 Thanks.

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• How to determine item at a index in pattern

  - by el.gringogrande

  I have the following elements in a list/array a1,a2,a3 and these elements are used to build another list in a predictable pattern example a1,a1,a2,a2,a3,a3,a1,a1,a2,a2,a3,a3... The pattern may change but I will always know how many times each element repeats and all elements repeats the same number of times. And the elements always show up in the same order. so another pattern might be a1,a1,a1,a2,a2,a2,a3,a3,a3,a1,a1,a1,a2,a2,a2,a3,a3,a3... or a1,a2,a3,a1,a2,a3 it will never be a2,a2,a1,a1,a3,a3... or a1,a2,a3,a2,a3,a1 etc How I determine what element is at any index in the list?

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• parsing of mathematical expressions

  - by gcc

  (in c90) (linux) input: sqrt(2 - sin(3*A/B)^2.5) + 0.5*(C*~(D) + 3.11 +B) a b /*there are values for a,b,c,d */ c d input: cos(2 - asin(3*A/B)^2.5) +cos(0.5*(C*~(D)) + 3.11 +B) a b /*there are values for a,b,c,d */ c d input: sqrt(2 - sin(3*A/B)^2.5)/(0.5*(C*~(D)) + sin(3.11) +ln(B)) /*max lenght of formula is 250 characters*/ a b /*there are values for a,b,c,d */ c /*each variable with set of floating numbers*/ d As you can see infix formula in the input depends on user. My program will take a formula and n-tuples value. Then it calculate the results for each value of a,b,c and d. If you wonder I am saying ;outcome of program is graph. /sometimes,I think i will take input and store in string. then another idea is arise " I should store formula in the struct" but i don't know how I can construct the code on the base of structure./ really, I don't know way how to store the formula in program code so that I can do my job. can you show me? /* a,b,c,d is letters cos,sin,sqrt,ln is function*/

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• How to best match two strings?

  - by Sebastian Hoitz

  Hi, do you know any good algorithms that match two strings and then return a percentage in how many percent those two strings match? And are there some, that work with databases too?

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• Setting last N bits in an array

  - by Martin

  I'm sure this is fairly simple, however I have a major mental block on it, so I need a little help here! I have an array of 5 integers, the array is already filled with some data. I want to set the last N bits of the array to be random noise. [int][int][int][int][int] set last 40 bits [unchanged][unchanged][unchanged][24 bits of old data followed 8 bits of randomness][all random] This is largely language agnostic, but I'm working in C# so bonus points for answers in C#

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• What Software Engineering Areas should be stressed upon while Interviewing Candidate for Fulltime So

  - by Rachel

  Hi, This question is somewhat related to other posts which I found on Stackoverflow but not exactly and so am prompted to ask about it. I know we must ask for Data-Structures and Algorithms but what specific data-structures or Algorithms or other CS Concepts should be asked while interviewing Sr. Software Engineering Fulltime Position as compared with Software Engineering Position. Thanks.

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• How to do "map chunks", like terraria or minecraft maps?

  - by O'poil

  Due to performance issues, I have to cut my maps into chunks. I manage the maps in this way: listMap[x][y] = new Tile (x,y); I tried in vain to cut this list for several "chunk" to avoid loading all the map because the fps are not very high with large map. And yet, when I update or Draw I do it with a little tile range. Here is how I proceed: foreach (List<Tile> list in listMap) { foreach (Tile leTile in list) { if ((leTile.Position.X < screenWidth + hero.Pos.X) && (leTile.Position.X > hero.Pos.X - tileSize) && (leTile.Position.Y < screenHeight + hero.Pos.Y) && (leTile.Position.Y > hero.Pos.Y - tileSize) ) { leTile.Draw(spriteBatch, gameTime); } } } (and the same thing, for the update method). So I try to learn with games like minecraft or terraria, and any two manages maps much larger than mine, without the slightest drop of fps. And apparently, they load "chunks". What I would like to understand is how to cut my list in Chunk, and how to display depending on the position of my character. I try many things without success. Thank you in advance for putting me on the right track! Ps : Again, sorry for my English :'( Pps : I'm not an experimented developer ;)

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• What is the best way to find the digit at n position in a decimal number?

  - by Elijah

  Background I'm working on a symmetric rounding class and I find that I'm stuck with regards to how to best find the number at position x that I will be rounding. I'm sure there is an efficient mathematical way to find the single digit and return it without having to resort to string parsing. Problem Suppose, I have the following (C#) psuedo-code: var position = 3; var value = 102.43587m; // I want this no ? (that is 5) protected static int FindNDigit(decimal value, int position) { // This snippet is what I am searching for } Also, it is worth noting that if my value is a whole number, I will need to return a zero for the result of FindNDigit. Does anyone have any hints on how I should approach this problem? Is this something that is blaringly obvious that I'm missing?

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• Meshing of Point Clouds from a 3Dlaser scanner

  - by iceman

  Is there any package/software which can do Meshing of Point Clouds in real time? What is the data structure used to represent 3D Point Clouds ?

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• Is this linear search implementation actually useful?

  - by Helper Method

  In Matters Computational I found this interesting linear search implementation (it's actually my Java implementation ;-)): public static int linearSearch(int[] a, int key) { int high = a.length - 1; int tmp = a[high]; // put a sentinel at the end of the array a[high] = key; int i = 0; while (a[i] != key) { i++; } // restore original value a[high] = tmp; if (i == high && key != tmp) { return NOT_CONTAINED; } return i; } It basically uses a sentinel, which is the searched for value, so that you always find the value and don't have to check for array boundaries. The last element is stored in a temp variable, and then the sentinel is placed at the last position. When the value is found (remember, it is always found due to the sentinel), the original element is restored and the index is checked if it represents the last index and is unequal to the searched for value. If that's the case, -1 (NOT_CONTAINED) is returned, otherwise the index. While I found this implementation really clever, I wonder if it is actually useful. For small arrays, it seems to be always slower, and for large arrays it only seems to be faster when the value is not found. Any ideas?

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• Sort string based upon the count of characters Options

  - by prp

  Sample Data : input : "abcdacdc" Output : "cadb" here we have to sort strings in order of count of characters. If the count is same for characters. maintain the original order of the characters from input string. my approach: i have used array of 26 for maintaining occurrence of all characters and sort it then print it.But while doing so i am not able to maintain order in case if two characters have same count. please suggest any improvement or any other algo.

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• Dynamic Programming resources in C?

  - by EsotericMe

  Hi everyone, I'll be writing the online Google test tomorrow as a fresher. Apparently, they definitely ask one problem on Dynamic Programming? Does anyone know of a good resource for collection of DP problems in C along with solutions? I know what DP is & have used it on an occasion or twice. However I feel to crack a DP problem in test, prior practice of typical problems will make it easier to approach. Any good resources or problem sets with solutions in C will be highly appreciated. Thanks.

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• generic binary Search in c#

  - by Pro_Zeck

  Below is my Generic Binary Search it works ok with the intgers type array it finds all the elements in it . But the Problem Arises when i use a string array to find any string data. It runs ok for the first index and last index elements but i cant find the middle elements. Stringarray = new string[] { "b", "a", "ab", "abc", "c" }; public static void BinarySearch<T>(T[] array, T searchFor, Comparer<T> comparer) { int high, low, mid; high = array.Length - 1; low = 0; if (array[0].Equals(searchFor)) Console.WriteLine("Value {0} Found At Index {1}",array[0],0); else if (array[high].Equals(searchFor)) Console.WriteLine("Value {0} Found At Index {1}", array[high], high); else { while (low <= high) { mid = (high + low) / 2; if (comparer.Compare(array[mid], searchFor) == 0) { Console.WriteLine("Value {0} Found At Index {1}", array[mid], mid); break; } else { if (comparer.Compare(searchFor, array[mid]) > 0) high = mid + 1; else low = mid + 1; } } if (low > high) { Console.WriteLine("Value Not Found In the Collection"); } } }

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