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Is anybody aware of a concurrent version of Tarjan's SCCs algorithm, Kosaraju's algorithm or any other fast, O(|V| + |E|) algorithm for finding SCCs? Neither of those algorithms seem to be very hard to multithread, but I'd be happy for somebody else to have done this job.
What I'm trying to handle here is an 8 GB directed graph, which I keep in RAM using a big AWS instance, and I'd like to make a good use of all 16 cores.
That's possibly the best paper I have found so far, I'll have a go at the implementation. http://domino.research.ibm.com/library/cyberdig.nsf/1e4115aea78b6e7c85256b360066f0d4/d8e3597a4172437b8525709f006e42b0?OpenDocument
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I am looking for an optimized 4-connectivity or 8-connectivity Connected Component Labelling source code in MATLAB or C++. I saw many implementation of connected component labelling (4-connectivity) in MATLAB.
One of the implementations that works faster is the recursive implementation explained here: http://www.mathworks.com/matlabcentral/fileexchange/38010-connected-component-labeling-like-bwlabel
MATLAB has a built-in bwlabeln or bwlabel, which is far more optimized. They claim to use union-find method from two-pass algorithm described in Sedgewick's Algorithms in C, Addison-Wesley. However, it is hard to find any source code of it. Does anyone have idea about it? An optimized code is really needed.
You can indeed work by scanning the image in scanline order and when you meet a component seed-fill it.
You will find two efficient (and very similar) algorithms in Graphics GEMS 1:
A SEED FILL ALGORITHM, Paul S. Heckbert
FILLING A REGION IN A FRAME BUFFER, Ken Fishkin
and with a little effort some implementations. (The papers give Pascal-like code which is easy to translate.)
They run in linear time, use an explicit stack and don't require union-find.
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I was using geeksforgeeks C++ implementation of Prim's Algorithm but I am getting Time Limit Exceeded in few testcases. So is there any faster implementation? Here is implementation I am currently using. To make question general, which is fastest implementation of Prim Algo in C++?
This implementation of Prim is very inefficient. On each iteration it looks for the smallest edge doing an iteration over all edges. If you use a binary heap for instance you will have an implementation that is asymptotically faster(m*log(n) vs m*n for your current implementation). Also you are supposed to write your own code not to use something someone's already written for you.
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I am in a quest of understanding majority of important algorithm that SO community has used in read world applciations. I know a ready list can be extracted from wiki page. But, i am interested only those algorithm or problem that community has faced either in their projects or asked in interviews. Few lines on that algorithm will also be helpful.
I am looking beyond the generic algorithm D&C, DP, Greedy...
If you are interesting about optimization problems which can be used any computer applications such as network and socket programming these could be useful for you;
NearbyNeighbour
Munkres
Hungarian
BruteForce
Min&Max Finding Algorithms
Ant and Bee Colonies Algorithms
General Genetic Algorithm etc.
I totally advice you to search all aboves but genetic algorithms and ant colonies algorithm are asked many interviewers.
I hope that helps.
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I've been working on a data mining project lately, and it confuses me a lot that alternating decision tree seems to have more accuracy than WEKA built-in j48 algorithm. I don't have much idea about how these two algorithms are implemented, I hope someone can explain this from the algorithm point of view. Thanks a lot.
I don't have much idea about how these two algorithms are implemented
No one can explain to you why one can perform better than the other if you don't even understand the starting points. Learn about C4.5 and then learn about ADTrees.
Otherwise this would be an exercise in trying to teach you two algorithms in a single giant post - which is futile.
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Imagine that you have ropes which are 5 meters long. And you want to cut the rope in some certain lengths(30 cm,73 cm) for some certain times. I want to write a program that minimize the total length of the excessed robe and tells you how you should cut every rope. But, I don't know where to start and use what algorithm. Can you give me some reference? Thank you in advance.
What you are looking for is so called Cutting stock problem.
Start by looking at this Wikipedia article and follow Suggested readings. I remember we had this as a part of some course back at the university (although I can't remember which one), so you could have a look at coursera.
Seems like homework, but I can still point you in the right direction. What you have on hand is an example of dynamic programming. From what I can understand from your question, you have a sub-case of the ever popular knapsack problem. Which is in essence an optimization problem of using the space on hand most efficiently, thus reducing the waste. Tweak it a bit to your own needs and you should be able to manage to get the solution for your problem.