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• Pointer mysteriously moves

  - by Armen Ablak

  Hi, I have this code for Node rotation and in a line which is marked something happens and I don't really know what and why :). //Test case 30 \ 16 / 29 RotationRight(node->mParent); //call template<class T> void SplayTree<T>::RotationRight(SplayNode<T> *&node) const { SplayNode<T> *left = node->mLeft; SplayNode<T> *parent = node->mParent; node->mLeft = left->mRight; if(left->HasRight()) left->mRight->mParent = node; left->mRight = node; //node in this line points to 0x00445198 {30} left->mParent = node->mParent; //and in this line it points to 0x00444fb8 {16} (node, not node->mParent) node->mParent = left; node = left; } Well, left-mParent points to node also, so I basically do node = node-mParent. The problem is I can't find a work around - how to unpin in from node and change it's pointing address without changing it's.

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• more efficient version of this?

  - by john connor

  i have this thingy here : function numOfPackets(bufferSize, packetSize) { if (bufferSize <= 0 || packetSize > bufferSize) return 0; if (packetSize < 0) throw Error(); var out = 0; for(;;){ out++; bufferSize = bufferSize - packetSize; if( packetSize > bufferSize ) break; } return out; } which i run at often , can u give me more efficent variant of it?

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• Help to learn Image Search algorithm

  - by R Manu

  I am a beginner in image processing. I want to write an application in C++ or in C# for Searching an image in a list of images Searching for a particular feature (for e.g. face) in a list of images. Can anybody suggest where should I start from? What all should I learn before doing this? Where can I find the correct information regarding this?

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• What can be parameters other than time and space while analyzing certain algorithms?

  - by sul4bh

  Dear all: I was interested to know about parameters other than space and time during analysing the effectiveness of an algorithms. For example, we can focus on the effective trap function while developing encryption algorithms. What other things can you think of ?

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• How to read arbitrary number of values using std::copy?

  - by Miro Kropacek

  Hi, I'm trying to code opposite action to this: std::ostream outs; // properly initialized of course std::set<int> my_set; // ditto outs << my_set.size(); std::copy( my_set.begin(), my_set.end(), std::ostream_iterator<int>( outs ) ); it should be something like this: std::istream ins; std::set<int>::size_type size; ins >> size; std::copy( std::istream_iterator<int>( ins ), std::istream_iterator<int>( ins ) ???, std::inserter( my_set, my_set.end() ) ); But I'm stuck with the 'end' iterator -- input interators can't use std::advance and neither I can use two streams with the same source... Is there any elegant way how to solve this? Of course I can use for loop, but maybe there's something nicer :)

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• Minimize the sequence by putting appropriate operations ' DP'

  - by Vikas

  Given a sequence,say, 222 We have to put a '+' or '* ' between each adjacent pair. '* ' has higher precedence over '+' We have to o/p the string whose evaluation leads to minimum value. O/p must be lexicographically smallest if there are more than one. inp:222 o/p: 2*2+2 Explaination: 2+2+2=6 2+2*2=6 2*2+2=6 of this 3rd is lexicographically smallest. I was wondering how to construct a DP solution for this.

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• GWT Calendrical Calculations

  - by Kyle Hayes

  We have a GWT application that needs to display various holidays. Is there a library available to do these calendrical calculations? If not, we'll have to do our own that we can ingest a set of rules to. Cheers

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• Efficient determination of which strings in an array are substrings of the others?

  - by byte

  In C#, Say you have an array of strings, which contain only characters '0' and '1': string[] input = { "0101", "101", "11", "010101011" }; And you'd like to build a function: public void IdentifySubstrings(string[] input) { ... } That will produce the following: "0101 is a substring of 010101011" "101 is a substring of 0101" "101 is a substring of 010101011" "11 is a substring of 010101011" And you are NOT able to use built-in string functionality (such as String.Substring). How would one efficiently solve this problem? Of course you could plow through it via brute force, but it just feels like there ought to be a way to accomplish it with a tree (since the only values are 0's and 1's, it feels like a binary tree ought to fit somehow). I've read a little bit about things like suffix trees, but I'm uncertain if that's the right path to be going down. Any efficient solutions you can think of?

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• Intersection() and Except() is too slow with large collections of custom objects

  - by Theo

  I am importing data from another database. My process is importing data from a remote DB into a List<DataModel> named remoteData and also importing data from the local DB into a List<DataModel> named localData. I am then using LINQ to create a list of records that are different so that I can update the local DB to match the data pulled from remote DB. Like this: var outdatedData = this.localData.Intersect(this.remoteData, new OutdatedDataComparer()).ToList(); I am then using LINQ to create a list of records that no longer exist in remoteData, but do exist in localData, so that I delete them from local database. Like this: var oldData = this.localData.Except(this.remoteData, new MatchingDataComparer()).ToList(); I am then using LINQ to do the opposite of the above to add the new data to the local database. Like this: var newData = this.remoteData.Except(this.localData, new MatchingDataComparer()).ToList(); Each collection imports about 70k records, and each of the 3 LINQ operation take between 5 - 10 minutes to complete. How can I make this faster? Here is the object the collections are using: internal class DataModel { public string Key1{ get; set; } public string Key2{ get; set; } public string Value1{ get; set; } public string Value2{ get; set; } public byte? Value3{ get; set; } } The comparer used to check for outdated records: class OutdatedDataComparer : IEqualityComparer<DataModel> { public bool Equals(DataModel x, DataModel y) { var e = string.Equals(x.Key1, y.Key1) && string.Equals(x.Key2, y.Key2) && ( !string.Equals(x.Value1, y.Value1) || !string.Equals(x.Value2, y.Value2) || x.Value3 != y.Value3 ); return e; } public int GetHashCode(DataModel obj) { return 0; } } The comparer used to find old and new records: internal class MatchingDataComparer : IEqualityComparer<DataModel> { public bool Equals(DataModel x, DataModel y) { return string.Equals(x.Key1, y.Key1) && string.Equals(x.Key2, y.Key2); } public int GetHashCode(DataModel obj) { return 0; } }

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• finding the maximum in series

  - by peril brain

  I need to know a code that will automatically:- search a specific word in excel notes it row or column number (depends on data arrangement) searches numerical type values in the respective row or column with that numeric value(suppose a[7][0]or a[0][7]) it compares all other values of respective row or column(ie. a[i][0] or a[0][i]) sets that value to the highest value only if IT HAS GOT NO FORMULA FOR DERIVATION i know most of coding but at a few places i got myself stuck... i'm writing a part of my program upto which i know: using System; using System.Collections.Generic; using System.Linq; using System.Text; using System.IO; using System.Threading; using Microsoft.Office.Interop; using Excel = Microsoft.Office.Interop.Excel; Excel.Application oExcelApp; namespace a{ class b{ static void main(){ try { oExcelApp = (Excel.Application)System.Runtime.InteropServices.Marshal.GetActiveObject("Excel.Application"); ; if(oExcelApp.ActiveWorkbook != null) {Excel.Workbook xlwkbook = (Excel.Workbook)oExcelApp.ActiveWorkbook; Excel.Worksheet ws = (Excel.Worksheet)xlwkbook.ActiveSheet; Excel.Range rn; rn = ws.Cells.Find("maximum", Type.Missing, Excel.XlFindLookIn.xlValues, Excel.XlLookAt.xlPart,Excel.XlSearchOrder.xlByRows, Excel.XlSearchDirection.xlNext, false, Type.Missing, Type.Missing); }}} now ahead of this i only know tat i have to use cell.value2 ,cell.hasformula methods..... & no more idea can any one help me with this..

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• Why is Dictionary.First() so slow?

  - by Rotsor

  Not a real question because I already found out the answer, but still interesting thing. I always thought that hash table is the fastest associative container if you hash properly. However, the following code is terribly slow. It executes only about 1 million iterations and takes more than 2 minutes of time on a Core 2 CPU. The code does the following: it maintains the collection todo of items it needs to process. At each iteration it takes an item from this collection (doesn't matter which item), deletes it, processes it if it wasn't processed (possibly adding more items to process), and repeats this until there are no items to process. The culprit seems to be the Dictionary.Keys.First() operation. The question is why is it slow? Stopwatch watch = new Stopwatch(); watch.Start(); HashSet<int> processed = new HashSet<int>(); Dictionary<int, int> todo = new Dictionary<int, int>(); todo.Add(1, 1); int iterations = 0; int limit = 500000; while (todo.Count > 0) { iterations++; var key = todo.Keys.First(); var value = todo[key]; todo.Remove(key); if (!processed.Contains(key)) { processed.Add(key); // process item here if (key < limit) { todo[key + 13] = value + 1; todo[key + 7] = value + 1; } // doesn't matter much how } } Console.WriteLine("Iterations: {0}; Time: {1}.", iterations, watch.Elapsed); This results in: Iterations: 923007; Time: 00:02:09.8414388. Simply changing Dictionary to SortedDictionary yields: Iterations: 499976; Time: 00:00:00.4451514. 300 times faster while having only 2 times less iterations. The same happens in java. Used HashMap instead of Dictionary and keySet().iterator().next() instead of Keys.First().

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• Python threading question (Working with a method that blocks forever)

  - by Nix

  I am trying to wrap a thread around some receiving logic in python. Basically we have an app, that will have a thread in the background polling for messages, the problem I ran into is that piece that actually pulls the messages waits forever for a message. Making it impossible to terminate... I ended up wrapping the pull in another thread, but I wanted to make sure there wasn't a better way to do it. Original code: class Manager: def __init__(self): receiver = MessageReceiver() receiver.start() #do other stuff... class MessageReceiver(Thread): receiver = Receiver() def __init__(self): Thread.__init__(self) def run(self): #stop is a flag that i use to stop the thread... while(not stopped ): #can never stop because pull below blocks message = receiver.pull() print "Message" + message What I refectored to: class Manager: def __init__(self): receiver = MessageReceiver() receiver.start() class MessageReceiver(Thread): receiver = Receiver() def __init__(self): Thread.__init__(self) def run(self): pullThread = PullThread(self.receiver) pullThread.start() #stop is a flag that i use to stop the thread... while(not stopped and pullThread.last_message ==None): pass message = pullThread.last_message print "Message" + message class PullThread(Thread): last_message = None def __init__(self, receiver): Thread.__init(self, target=get_message, args=(receiver)) def get_message(self, receiver): self.last_message = None self.last_message = receiver.pull() return self.last_message I know the obvious locking issues exist, but is this the appropriate way to control a receive thread that waits forever for a message? One thing I did notice was this thing eats 100% cpu while waiting for a message... **If you need to see the stopping logic please let me know and I will post.

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• R implementation of sorting by reversals

  - by user1357015

  I was wondering if there is an implementation in R where it sorts a permutation of n numbers into the original 1...n sequence and provides the number of reversals needed. Eg an implementation of the "sorting by reversals" or "sorting by translocation" as outlined in this ppt. Specifically, I have a permutation of a sequence of n elements, pi(n), and I want to figure out how close it is to the original sequence. The number of reversals seems a good metric. Thanks!

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• Length of Encrypted String

  - by Agnel Kurian

  I need to create a database column which will store a string encrypted using Triple DES. How do I determine the length of the encrypted string column? (Answers for algorithms other than Triple DES are also welcome.)

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• Finding users near other user

  - by Bunny Rabbit

  what algorithms should I explore to implement a feature which lets a user find other user located near him , the latitude and the longitudes of all the user are known in advance and are fixed [not dynamic]. Also i believe that there should be a better way to store such data then simply storing the lat , long of the user against his user id in a database.What are the efficient ways to handle this ?

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• Generate all the ways to intersperse a list of lists, keeping each list in order.

  - by dreeves

  Given a list of lists like this [[1,2,3],[a,b,c,d],[x,y]] generate all permutations of the flattened list, [1,2,3,a,b,c,d,x,y], such that the elements of each sublist occur in the same order. For example, this one is okay [a,1,b,2,x,y,3,c,d] but this one is not [y,1,2,3,a,b,c,x,d] because y must occur after x, that being how x and y are ordered in the original sublist. I believe the number of such lists is determined by the multinomial coefficient. I.e., if there are k sublists, n_i is the length of the ith sublist, and n is the sum of the n_i's then the number of such permutations is n!/(n_i! * ... * n_k!). The question is how to generate those sublists. Pseudocode is great. An actual implementation in your language of choice is even better!

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• question about permutation problem

  - by davit-datuashvili

  i have posted similar problem here http://stackoverflow.com/questions/2920315/permutation-of-array but i want following we know that with length n there is n! possible permutation from which one such that all element are in order they are in sorted variant so i want break permutation when array is in order and print result but something is wrong i think that problem is repeated of permutation here is my code import java.util.*; public class permut{ public static Random r=new Random(); public static void display(int a[],int n){ for (int i=0;i<n;i++){ System.out.println(a[i]); } } public static void Permut(int a[],int n){ int j=0; int k=0; while (j<fact(n)){ int s=r.nextInt(n); for (int i=0;i<n;i++){ k=a[i]; a[i]=a[s]; a[s]=k; } j++; if (sorted(a,n)) display(a,n); break; } } public static void main(String[]args){ int a[]=new int[]{3,4,1,2}; int n=a.length; Permut(a,n); } public static int fact(int n){ if (n==0 || (n==1) ) return 1; return n*fact(n-1); } public static boolean sorted(int a[],int n ){ boolean flag=false; for (int i=0;i<n-1;i++){ if (a[i]<a[i+1]){ flag=true; } else{ flag=false; } } return flag; } } can anybody help me? result is nothing

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• How to find the largest power of 2 less than the given number

  - by nazar_art

  I need to find the largest power of 2 less than the given number. And I stuck and can't find any solution. Code: public class MathPow { public int largestPowerOf2 (int n) { int res = 2; while (res < n) { res =(int)Math.pow(res, 2); } return res; } } This doesn't work correctly. Testing output: Arguments Actual Expected ------------------------- 9 16 8 100 256 64 1000 65536 512 64 256 32 How to solve this issue?

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• Collision free hash function for a specific data structure

  - by Max

  Is it possible to create collision free hash function for a data structure with specific properties. The datastructure is int[][][] It contains no duplicates The range of integers that are contained in it is defined. Let's say it's 0..1000, the maximal integer is definitely not greater than 10000. Big problem is that this hash function should also be very fast. Is there a way to create such a hash function? Maybe at run time depending on the integer range?

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• Optimizing / simplifying a path

  - by user146780

  Say I have a path with 150 nodes / verticies. How could I simplify if so that for example a straight line with 3 verticies, would remove the middle one since it does nothing to add to the path. Also how could I avoid destroying sharp corners? And how could I remove tiny variations and have smooth curves remaining. Thanks

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• Automatic images translation to 3d model

  - by farrakhov-bulat

  I'm quite interested in automatic images translation to 3d models. Not really for commercial product, but from the point of possible academic research and implementation. What I'd like to achieve is almost transparent for user process of transformation series of images (fewer is better) to 3d model which might be shown in flash/silverlight/javafx or similar. Consider online furniture store with 3d models of all items in stock. Kinda cool to have ability to see the product in 3d before purchasing it. I managed to find a few pieces of software, like insight3d, but it couldn't be used in my case I guess. So, are there any similar projects or tips for me? If it would require to write that piece of software - I'd really love to dig into research on this field.

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• How does pattern matching work behind the scenes in F#?

  - by kryptic

  Hello Everyone, I am completely new to F# (and functional programming in general) but I see pattern matching used everywhere in sample code. I am wondering for example how pattern matching actually works? For example, I imagine it working the same as a for loop in other languages and checking for matches on each item in a collection. This is probably far from correct, how does it actually work behind the scenes?

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• Do encryption algorithms provide really unique results?

  - by Mikulas Dite

  I was wondering whether md5, sha1 and anothers return unique values. For example, sha1() for test returns a94a8fe5ccb19ba61c4c0873d391e987982fbbd3, which is 40 characters long. So, sha1 for strings larger than 40 chars must be the same (of course it's scrambled, because the given input may contain whitespaces and special chars etc.). Due to this, when we are storing users' passwords, they can enter either their original password or some super-long one, which nobody knows. Is this right, or do these hash algorithms provide really unique results - I'm quite sure it's hardly possible.

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• How to calculate the state of a graph?

  - by zcb

  Given a graph G=(V,E), each node i is associated with 'Ci' number of objects. At each step, for every node i, the Ci objects will be taken away by the neighbors of i equally. After K steps, output the number of objects of the top five nodes which has the most objects. Some Constrains: |V|<10^5, |E|<2*10^5, K<10^7, Ci<1000 My current idea is: represent the transformation in each step with a matrix. This problem is converted to the calculation of the power of matrix. But this solution is much too slow considering |V| can be 10^5. Is there any faster way to do it?

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• Are there any well-known algorithms or computer models that computer scientists use to predict FIFA

  - by Khnle

  Occasionally I read news articles that mention about some computer models that computer scientists use to predict winners of some sporting events or the odds for betting which I think there must be a mathematical model behind it. I never bothered to think twice even though I am a "pseudo computer scientist" myself. With the 2010 FIFA World Cup just underway, and since I am also a "pseudo football/soccer player" myself, I just started to wonder about these calculations algorithms. For example, I know one factor is determining the strength of opponents, so that a win against a strong opponent can count more than a win against a weak opponent. But it now kind of gets in a circular loop, or at least how does one determine the strength of a team in the first place, before that team can be considered strong or weak? If it's based on a historical data then there's no way that could be accurate, because those players of the past are no longer on the fields so their impact is none (except maybe if they become coaches like Maradona) Anyway, long question short, if you're happen to be working in this field or have some knowledge, please shed some lights.

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