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• Reverse regular expressions to generate data

  - by Anton Gogolev

  In one of the StackOverflow Podcasts (the one where guys were discussing data generation for testing DBs -- either #11 or #12), Jeff mentioned something like "reverse regular expressions", which are used exactly for that purpose: given a regex, produce a string which will eventually match said regex. What is the correct term for this whole concept? Is this a well-known concept?

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• Javascript Number Random Scrambler

  - by stjowa

  Hi, I need a Javascript random number scrambler for my website. Seems simple, but I can not figure out how to do it. Can anyone help me out? I have the following array of numbers: 1 2 3 4 5 6 7 8 9 I would like to be able to have these numbers scrambled randomly. Like the following: 3 6 4 2 9 5 1 8 7 or 4 1 7 3 5 9 2 6 8 So, specifically, I would like a function that takes in an array of numbers (1 - n) and then returns that same array of numbers - scrambled randomly with different calls to the function. Maybe a noob function, but can't seem to figure it out. Thanks!

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• Time Complexities of recursive algorithms

  - by Peter

  Whenever I see a recursive solution, or I write recursive code for a problem, it is really difficult for me to figure out the time complexity, in most of the cases I just say its exponential? How is it exponential actually? How people say it is 2^n, when it is n!, when it is n^n or n^k. I have some questions in mind, let say find all permutations of a string (O(n!)) find all sequences which sum up to k in an array (exponential, how exactly do I calculate). Find all subsets of size k whose sum is 0 (will k come somewhere in complexity , it should come right?). Can any1 help me how to calculate the exact complexity of such questions, I am able to wrote code for them , but its hard understanding the exact time complexity.

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• Simple/Basic steganography algorithms and methods

  - by tomp

  What are the basic and simpliest steganography algorithms and methods? I mean the steganography applied to images. How does simple program that hides data to images work? How does the program recognize the encrypted message in image without the source image? What are the main techniques used?

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• Efficiently selecting a set of random elements from a linked list

  - by Matt Sheppard

  Say I have a linked list of numbers of length N. N is very large and I don’t know in advance the exact value of N. How can I most efficiently write a function that will return k completely random numbers from the list?

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• Writing a program to check if a graph is bipartite

  - by John Graveston

  Hi. I need to write a program that check if a graph is bipartite. I have read through wikipedia articles about graph coloring and bipartite graph. These two article suggest methods to test bipartiteness like BFS search, but I cannot write a program implementing these methods. Any help would be appreciated!

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• Why DFS and not BFS for finding cycle in graphs.

  - by badcompany

  Predominantly DFS is used to find a cycle in graphs and not BFS. Any reasons? Both can find if a node has already been visited while traversing the tree/graph.

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• Load balancing and shedulling algorithms .NET

  - by Lukas Šalkauskas

  Hello there, so here is my problem: I have several different configuarion servers. I have different calculations (jobs), I can predict how long approx. each job will take to be caclulated. Also I have priorities. My question is how to keep all machines loaded 99-100% and shedule the jobs in the best way. Each machine can do several calculations at the time. Jobs are pushed to the machine. Central machine knows current load of each machine. Also I would like to to assign some king of machine learning here, because I will know statistics of each job (started, finished, cpu load etc.). How to distribute jobs(calculations) in the best possible way, also keep in mind priority. Any suggestions ? Ideas ? Algorithms ?

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• Data structure behind T9 type of dictionary

  - by anony

  How does a T9 dictionary work? What is the data structure behind it. If we type '4663' we get 'good' when we press down button we get 'gone' then 'home' etc...

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• Methodologies or algorithms for filling in missing data

  - by tbone

  I am dealing with datasets with missing data and need to be able to fill forward, backward, and gaps. So, for example, if I have data from Jan 1, 2000 to Dec 31, 2010, and some days are missing, when a user requests a timespan that begins before, ends after, or encompasses the missing data points, I need to "fill in" these missing values. Is there a proper term to refer to this concept of filling in data? Imputation is one term, don't know if it is "the" term for it though. I presume there are multiple algorithms & methodologies for filling in missing data (use last measured, using median/average/moving average, etc between 2 known numbers, etc. Anyone know the proper term for this problem, any online resources on this topic, or ideally links to open source implementations of some algorithms (C# preferably, but any language would be useful)

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• Technologies required for connectedTv

  - by harigm

  Can anyone help me suggesting which technologies will be useful to implement a ConnectedTv. In ConnectedTv where the Internet is configured in TV, while watching TV, users can browse the TV.

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• C++ priority queue structure used ?

  - by John Retallack

  While searching for some functions in C++ STL documentation I read that push and pop for priority queues needs constant time. "Constant (in the priority_queue). Although notice that push_heap operates in logarithmic time." My question is what kind of data structure is used to mantain a priority queue with O(1) for push and pop ?

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• crossing edges in the travelling salesman problem

  - by bob

  Does there exist a travelling salesman problem where the optimal solution has edges that cross?

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• Fast dot product for a very special case

  - by psihodelia

  Given a vector X of size L, where every scalar element of X is from a binary set {0,1}, it is to find a dot product z=dot(X,Y) if vector Y of size L consists of the integer-valued elements. I suggest, there must exist a very fast way to do it. Let's say we have L=4; X[L]={1, 0, 0, 1}; Y[L]={-4, 2, 1, 0} and we have to find z=X[0]*Y[0] + X[1]*Y[1] + X[2]*Y[2] + X[3]*Y[3] (which in this case will give us -4). It is obvious that X can be represented using binary digits, e.g. an integer type int32 for L=32. Then, all what we have to do is to find a dot product of this integer with an array of 32 integers. Do you have any idea or suggestions how to do it very fast?

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• Using Ruby, what is the most efficient way to check if any key in a hash matches any values within a

  - by Jason

  I want to compare the keys in a hash of parameters against an array of elements for a match. For example: params = {"key1", "key2", "key3"} params_to_match = ["key2","key3"] I could do this, but I'm sure there is a much more elegant way to acheive the same result params.each_key{|key| if params_to_match.include?(key.to_s) return end }

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• Find numbers that equals a sum in an array

  - by valli-R

  I want to find the first set of integers in an array X that the sum equals a given number N. For example: X = {5, 13, 24, 9, 3, 3} N = 28 Solution = {13, 9, 3, 3} Here what I have so far : WARNING, I know it uses global and it is bad,that's not the point of the question. <?php function s($index = 0, $total = 0, $solution = '') { global $numbers; global $sum; echo $index; if($total == 28) { echo '<br/>'.$solution.' = '.$sum.'<br/>'; } elseif($index < count($numbers) && $total != 28) { s($index + 1, $total, $solution); s($index + 1, $total + $numbers[$index], $solution.' '.$numbers[$index]); } } $numbers = array(5, 13, 24, 9, 3, 3); $sum = 28; s(); ?> I don't get how I can stop the process when it finds the solution.. I know I am not far from good solution.. Thanks in advance

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• K-Means Algorithm and java code

  - by Thandar

  Hi all, I need to calculate for grouping objects according to their size. I got k-means algorithms in java which calculate mostly for classifying according to their two or more features and the results are not satisfy for me.I only want to calculate for grouping objects based on one feature.Pseudocode or code would be helpful, too. Thanks u all for helping.

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• Count subset of binary pattern ..

  - by mr.bio

  Hi there . I have a A=set of strings and a B=seperate string. I want to count the number of occurences in from B in A. Example : A: 10001 10011 11000 10010 10101 B: 10001 result would be 3.(10001 is a subset of 10001,10011,10101) So i need a function that takes a set and string and returns an int. int myfunc(set<string> , string){ int result; // My Brain is melting return result ; }

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• How to compute palindrome from a stream of characters in sub-linear space/time?

  - by wrick

  I don't even know if a solution exists or not. Here is the problem in detail. You are a program that is accepting an infinitely long stream of characters (for simplicity you can assume characters are either 1 or 0). At any point, I can stop the stream (let's say after N characters were passed through) and ask you if the string received so far is a palindrome or not. How can you do this using less sub-linear space and/or time.

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• longest common subsequence

  - by davit-datuashvili

  i have following code public class LCS1 { public static String lcs(String a,String b) { String x; String y; int alen=a.length(); int blen=b.length(); if (alen==0 || blen==0) { return ""; } else if (a.charAt(alen-1)==b.charAt(blen-1)) { return lcs(a.substring(0,alen-1),b.substring(0,blen-1)); } else { x=lcs(a,b.substring(0,blen-1)); y=lcs(a.substring(0,alen-1),b); } return (x.length()>y.length()) ? x : y; } public static void main(String[]args){ String a="computer"; String b="houseboat"; System.out.println(lcs(a,b)); } } it should return "out" but returns nothing what is problem?

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• Is there a good radixsort-implementation for floats in C#

  - by CommuSoft

  I have a datastructure with a field of the float-type. A collection of these structures needs to be sorted by the value of the float. Is there a radix-sort implementation for this. If there isn't, is there a fast way to access the exponent, the sign and the mantissa. Because if you sort the floats first on mantissa, exponent, and on exponent the last time. You sort floats in O(n).

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• Operations on bytes in C#

  - by Hooch

  Hello. I'm writing application to control LEDS on LPT. I have everything working except this. This is one small function. I have sth like that: I want to build function that will take two argument and return one number: In actual code those binary numers will be in hex. I put them there like that so that it's easier for you to visualize it. Example1: arg1 = 1100 1100 arg2 = 1001 0001 retu = 0100 1100 Example2: arg1 = 1111 1111 arg2 = 0001 0010 retu = 1110 1101 Example3: arg1 = 1111 0000 arg2 = 0010 0010 retu = 1101 0000 I have no idea how this function should look like. I want it to be as fast as possible. I'll call this function 200 times per second.

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• Whats the best data-structure for storing 2-tuple (a, b) which support adding, deleting tuples and c

  - by bhups

  Hi So here is my problem. I want to store 2-tuple (key, val) and want to perform following operations: - keys are strings and values are Integers - multiple keys can have same value - adding new tuples - updating any key with new value (any new value or updated value is greater than the previous one, like timestamps) - fetching all the keys with values less than or greater than given value - deleting tuples. Hash seems to be the obvious choice for updating the key's value but then lookups via values will be going to take longer (O(n)). The other option is balanced binary search tree with key and value switched. So now lookups via values will be fast (O(lg(n))) but updating a key will take (O(n)). So is there any data-structure which can be used to address these issues? Thanks.

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• How to select number of lines from large text files?

  - by MiNdFrEaK

  I was wondering how to select number of lines from a certain text file. As an example: I have a text file containing the following lines: branch 27 : rect id 23400 rect: -115.475609 -115.474907 31.393650 31.411301 branch 28 : rect id 23398 rect: -115.474907 -115.472282 31.411301 31.417351 branch 29 : rect id 23396 rect: -115.472282 -115.468033 31.417351 31.427151 branch 30 : rect id 23394 rect: -115.468033 -115.458733 31.427151 31.438181 Non-Leaf Node: level=1 count=31 address=53 branch 0 : rect id 42 rect: -115.768539 -106.251556 31.425039 31.717550 branch 1 : rect id 50 rect: -109.559479 -106.009361 31.296721 31.775299 branch 2 : rect id 51 rect: -110.937401 -106.226143 31.285870 31.771971 branch 3 : rect id 54 rect: -109.584412 -106.069092 31.285240 31.775230 branch 4 : rect id 56 rect: -109.570961 -106.000954 31.296721 31.780769 branch 5 : rect id 58 rect: -115.806213 -106.366188 31.400450 31.687519 branch 6 : rect id 59 rect: -113.173859 -106.244057 31.297440 31.627750 branch 7 : rect id 60 rect: -115.811478 -106.278252 31.400450 31.679470 branch 8 : rect id 61 rect: -109.953888 -106.020111 31.325319 31.775270 branch 9 : rect id 64 rect: -113.070969 -106.015968 31.331841 31.704750 branch 10 : rect id 68 rect: -113.065689 -107.034576 31.326300 31.770809 branch 11 : rect id 71 rect: -112.333344 -106.059860 31.284081 31.662920 branch 12 : rect id 73 rect: -115.071083 -106.309677 31.267879 31.466850 branch 13 : rect id 74 rect: -116.094414 -106.286308 31.236290 31.424770 branch 14 : rect id 75 rect: -115.423264 -106.286308 31.229691 31.415510 branch 15 : rect id 76 rect: -116.111656 -106.313110 31.259390 31.478300 branch 16 : rect id 77 rect: -116.247467 -106.309677 31.240231 31.451799 branch 17 : rect id 78 rect: -116.170792 -106.094543 31.156429 31.391781 branch 18 : rect id 79 rect: -116.225723 -106.292709 31.239960 31.442850 branch 19 : rect id 80 rect: -116.268013 -105.769913 31.157240 31.378111 branch 20 : rect id 82 rect: -116.215424 -105.827202 31.198441 31.383421 branch 21 : rect id 83 rect: -116.095734 -105.826439 31.197460 31.373819 branch 22 : rect id 84 rect: -115.423264 -105.815018 31.182640 31.368891 branch 23 : rect id 85 rect: -116.221527 -105.776512 31.160931 31.389830 branch 24 : rect id 86 rect: -116.203369 -106.473831 31.168350 31.367611 branch 25 : rect id 87 rect: -115.727631 -106.501587 31.189100 31.395941 branch 26 : rect id 88 rect: -116.237289 -105.790756 31.164780 31.358959 branch 27 : rect id 89 rect: -115.791344 -105.990044 31.072620 31.349529 branch 28 : rect id 90 rect: -115.736847 -106.495079 31.187969 31.376900 branch 29 : rect id 91 rect: -115.721710 -106.000130 31.160351 31.354601 branch 30 : rect id 92 rect: -115.792236 -106.000793 31.166620 31.378811 Leaf Node: level=0 count=21 address=42 branch 0 : rect id 18312 rect: -106.412270 -106.401367 31.704750 31.717550 branch 1 : rect id 18288 rect: -106.278252 -106.253387 31.520321 31.548361 I just want those lines which are in between Non-Leaf Node level=1 to Leaf Node Level=0 and also there are a lot of segments like this and I need them all.

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• True random number generator

  - by goldenmean

  Sorry for this not being a "real" question, but Sometime back i remember seeing a post here about randomizing a randomizer randomly to generate truly random numbers, not just pseudo random. I dont see it if i search for it. Does anybody know about that article?

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