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• Most Frequent 3 page sequence in a weblog

  - by Sundararajan S

  Given a web log which consists of fields 'User ' 'Page url'. We have to find out the most frequent 3-page sequence that users takes. There is a time stamp. and it is not guaranteed that the single user access will be logged sequentially it could be like user1 Page1 user2 Pagex user1 Page2 User10 Pagex user1 Page 3 her User1s page sequence is page1- page2- page 3

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• 'Fixed' for loop - what is more efficient?

  - by pimvdb

  I'm creating a tic-tac-toe game, and one of the functions has to iterate through each of the 9 fields (tic-tac-toe is played on a 3x3 grid). I was wondering what is more efficient (which one is perhaps faster, or what is the preferred way of scripting in such situation) - using two for nested loops like this: for(var i=0; i<3; i++) { for(var j=0; j<3; j++) { checkField(i, j); } } or hard-coding it like this: checkField(0, 0); checkField(0, 1); checkField(0, 2); checkField(1, 0); checkField(1, 1); checkField(1, 2); checkField(2, 0); checkField(2, 1); checkField(2, 2); As there are only 9 combinations, it would be perhaps overkill to use two nested for loops, but then again this is clearer to read. The for loop, however, will increment variables and check whether i and j are smaller than 3 every time as well. In this example, the time saving at least might be negligible, but what is the preferred way of coding in this case? Thanks.

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• Moving a unit precisely along a path in x,y coordinates

  - by Adam Eberbach

  I am playing around with a strategy game where squads move around a map. Each turn a certain amount of movement is allocated to a squad and if the squad has a destination the points are applied each turn until the destination is reached. Actual distance is used so if a squad moves one position in the x or y direction it uses one point, but moving diagonally takes ~1.4 points. The squad maintains actual position as float which is then rounded to allow drawing the position on the map. The path is described by touching the squad and dragging to the end position then lifting the pen or finger. (I'm doing this on an iPhone now but Android/Qt/Windows Mobile would work the same) As the pointer moves x, y points are recorded so that the squad gains a list of intermediate destinations on the way to the final destination. I'm finding that the destinations are not evenly spaced but can be further apart depending on the speed of the pointer movement. Following the path is important because obstacles or terrain matter in this game. I'm not trying to remake Flight Control but that's a similar mechanic. Here's what I've been doing, but it just seems too complicated (pseudocode): getDestination() { - self.nextDestination = remove_from_array(destinations) - self.gradient = delta y to destination / delta x to destination - self.angle = atan(self.gradient) - self.cosAngle = cos(self.angle) - self.sinAngle = sin(self.angle) } move() { - get movement allocation for this turn - if self.nextDestination not valid - - getNextDestination() - while(nextDestination valid) && (movement allocation remains) { - - find xStep and yStep using movement allocation and sinAngle/cosAngle calculated for current self.nextDestination - - if current position + xStep crosses the destination - - - find x movement remaining after self.nextDestination reached - - - calculate remaining direct path movement allocation (xStep remaining / cosAngle) - - - make self.position equal to self.nextDestination - - else - - - apply xStep and yStep to current position - } - round squad's float coordinates to integer screen coordinates - draw squad image on map } That's simplified of course, stuff like sign needs to be tweaked to ensure movement is in the right direction. If trig is the best way to do it then lookup tables can be used or maybe it doesn't matter on modern devices like it used to. Suggestions for a better way to do it? an update - iPhone has zero issues with trig and tracking tens of positions and tracks implemented as described above and it draws in floats anyway. The Bresenham method is more efficient, trig is more precise. If I was to use integer Bresenham I would want to multiply by ten or so to maintain a little more positional accuracy to benefit collisions/terrain detection.

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• A graph problem

  - by copperhead

  I am struggling to solve the following problem http://uva.onlinejudge.org/external/1/193.html However Im not able to get a fast solution. And as seen by the times of others, there should be a solution of maximum n^2 complexity http://uva.onlinejudge.org/index.php?option=com_onlinejudge&Itemid=8&category=3&page=show_problem&problemid=129&page=problem_stats Can I get some help?

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• Managed code (C#) vs Matlabs and C++ for speed

  - by dotnetdev

  Hi, I am about to start developing an edge detection system (once I've read through a couple of books, which I'm doing so at good speed), but one thing I am wondering is the speed of an app like Matlabs (which can compile code to C++) vs AFORGE.NET for edge detecton. Is unmanaged code generally faster? Thanks

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• How do I modify an attribute across all rows in a table?

  - by prgmatic

  Hi folks, My apologies for asking such a novice question but, I need help building a script using either PHP or directly in MySQL that can do the following: Take the values of a column in a table (text) Change them into capitalized words (from "this is a title" to "This Is A Title") Replace the old values (uncapitalized) with the new values (capitalized). Thanks for the help and support.

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• Take the intersection of an arbitrary number of lists in python

  - by thepandaatemyface

  Suppose I have a list of lists of elements which are all the same (i'll use ints in this example) [range(100)[::4], range(100)[::3], range(100)[::2], range(100)[::1]] What would be a nice and/or efficient way to take the intersection of these lists (so you would get every element that is in each of the lists)? For the example that would be: [0, 12, 24, 36, 48, 60, 72, 84, 96]

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• Python - How to find a correlation between two vectors ?

  - by psihodelia

  Given two vectors X and Y I have to find their correlation, i.e. their linear dependence/independence. Both vectors have equal dimension. A resulted answer should be a floating point number from [-1.0 .. 1.0]. Example: X=[-1, 2, 0] Y=[ 4, 2, -0.3] Find y=cor(X,Y) such that y belongs to [-1.0 .. 1.0]. It should be a simple construction involving a list-comprehension. No external library is allowed. UPDATE: ok, if dot product is enough, then here is my solution: nX = 1/(sum([x*x for x in X]) ** 0.5) nY = 1/(sum([y*y for y in Y]) ** 0.5) cor = sum([(x*nX)*(y*nY) for x,y in zip(X,Y) ]) right?

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• C: Why some string.h algorithms work slower than simple hand-made ones?

  - by MInner

  For example, strlen() takes about 3*O(n). Why 3? I've wrote a really simple strlen-like function - it works much faster. int string_len(char s[],int &l) { for(l=0;s[l];l++); return l; } Well, some of my friends says, nearly all of string.h algorithms are slower than they should be.

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• What is the data structure behind google suggest?

  - by anony

  if we have some search terms with frequency, how do I suggest the top 5 frequently searched words? The data structure used must be easily updatable.(for eg: if I want to update frequency of a term or add a new term etc...)

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• Is there a standard pattern for scanning a job table executing some actions?

  - by Howiecamp

  (I realize that my title is poor. If after reading the question you have an improvement in mind, please either edit it or tell me and I'll change it.) I have the relatively common scenario of a job table which has 1 row for some thing that needs to be done. For example, it could be a list of emails to be sent. The table looks something like this: ID Completed TimeCompleted anything else... ---- --------- ------------- ---------------- 1 No blabla 2 No blabla 3 Yes 01:04:22 ... I'm looking either for a standard practice/pattern (or code - C#/SQL Server preferred) for periodically "scanning" (I use the term "scanning" very loosely) this table, finding the not-completed items, doing the action and then marking them completed once done successfully. In addition to the basic process for accomplishing the above, I'm considering the following requirements: I'd like some means of "scaling linearly", e.g. running multiple "worker processes" simultaneously or threading or whatever. (Just a specific technical thought - I'm assuming that as a result of this requirement, I need some method of marking an item as "in progress" to avoid attempting the action multiple times.) Each item in the table should only be executed once. Some other thoughts: I'm not particularly concerned with the implementation being done in the database (e.g. in T-SQL or PL/SQL code) vs. some external program code (e.g. a standalone executable or some action triggered by a web page) which is executed against the database Whether the "doing the action" part is done synchronously or asynchronously is not something I'm considering as part of this question.

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• Encoding arbitrary data into numbers?

  - by Pekka

  Is there a common method to encode and decode arbitrary data so the encoded end result consists of numbers only - like base64_encode but without the letters? Fictitious example: $encoded = numbers_encode("Mary had a little lamb"); echo $encoded; // outputs e.g. 122384337422394237423 (fictitious result) $decoded = numbers_decode("122384337422394237423"); echo $decoded; // outputs "Mary had a little lamb"

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• how to find distinct digit set numbers over a range of integers?

  - by evil.coder

  Suppose i have a unsigned integer, call it low and one another call it high such that highlow. The problem is to find distinct digit set numbers over this range. For example, suppose low is 1 and high is 20 then the answer is 20, because all the numbers in this range are of distinct digit sets. If suppose low is 1 and high is 21, then the answer is 20, because 12 and 21 have same digit set i.e.1, 2. I am not looking for a bruteforce algo., if anyone has a better solution then a usual bruteforce approach, please tell..

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• Typecasting a floating value or using the math.h floor* functions?

  - by nobody

  Hi, I am coding up an implementation of Interpolation Search in C. The question is actually rather simple, I need to use the floating operations to do linear interpolation to find the correct index which will eventually be an integer result. In particular my probe index is: t = i + floor((((k-low)/(high-low)) * (j-i))); where, i,j,k,t are unsigned ints, and high,low are doubles. Would this be equivalent to: t = i + (unsigned int)(((k-low)/(high-low)) * (j-i)); Is there any reason I would actually want to use math.h floor* functions over just a simple (int) typecast?

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• Storing a bucket of numbers in an efficient data structure

  - by BlitzKrieg

  I have a buckets of numbers e.g. - 1 to 4, 5 to 15, 16 to 21, 22 to 34,.... I have roughly 600,000 such buckets. The range of numbers that fall in each of the bucket varies. I need to store these buckets in a suitable data structure so that the lookups for a number is as fast as possible. So my question is what is the suitable data structure and a sorting mechanism for this type of problem. Thanks in advance

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• Solving simultaneous equations

  - by Milo

  Here is my problem: Given x, y, z and ratio where z is known and ratio is known and is a float representing a relative value, I need to find x and y. I know that: x / y == ratio y - x == z What I'm trying to do is make my own scroll pane and I'm figuring out the scrollbar parameters. So for example, If the scrollbar must be able to scroll 100 values (z) and the thumb must consume 80% of the bar (ratio = 0.8) then x would be 400 and y would be 500. Thanks

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• Clamping a vector to a minimum and maximum?

  - by user146780

  I came accross this: t = Clamp(t/d, 0, 1) but I'm not sure how to perform this operation on a vector. What are the steps to clamp a vector if one was writing their own vector implementation? Thanks clamp clamping a vector to a minimum and a maximum ex: pc = # the point you are coloring now p0 = # start point p1 = # end point v = p1 - p0 d = Length(v) v = Normalize(v) # or Scale(v, 1/d) v0 = pc - p0 t = Dot(v0, v) t = Clamp(t/d, 0, 1) color = (start_color * t) + (end_color * (1 - t))

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• Base X string encoding

  - by Paul Stone

  I'm looking for a routine that will encode a string (stream of bytes) into an arbitrary base/alphabet (like base64 encoding but I get to choose the alphabet). I've seen a few routines that do base X encoding for a number, but not for a string.

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• What is image hashing used for?

  - by seanbrant

  I hear this term sometimes and am wondering what it is used for?

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• Byte-Pairing for data compression

  - by user1669533

  Question about Byte-Pairing for data compression. If byte pairing converts two byte values to a single byte value, splitting the file in half, then taking a gig file and recusing it 16 times shrinks it to 62,500,000. My question is, is byte-pairing really efficient? Is the creation of a 5,000,000 iteration loop, to be conservative, efficient? I would like some feed back on and some incisive opinions please. Dave, what I read was: "The US patent office no longer grants patents on perpetual motion machines, but has recently granted at least two patents on a mathematically impossible process: compression of truly random data." I was not inferring the Patent Office was actually considering what I am inquiring about. I was merely commenting on the notion of a "mathematically impossible process." If someone has, in some way created a method of having a "single" data byte as a placeholder of 8 individual bytes of data, that would be a consideration for a patent. Now, about the mathematically impossibility of an 8 to 1 compression method, it is not so much a mathematically impossibility, but a series of rules and conditions that can be created. As long as there is the rule of 8 or 16 bit representation of storing data on a medium, there are ways to manipulate data that mirrors current methods, or creation by a new way of thinking.

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• How to generate a number in arbitrary range using random()={0..1} preserving uniformness and density?

  - by psihodelia

  Generate a random number in range [x..y] where x and y are any arbitrary floating point numbers. Use function random(), which returns a random floating point number in range [0..1] from P uniformly distributed numbers (call it "density"). Uniform distribution must be preserved and P must be scaled as well. I think, there is no easy solution for such problem. To simplify it a bit, I ask you how to generate a number in interval [-0.5 .. 0.5], then in [0 .. 2], then in [-2 .. 0], preserving uniformness and density? Thus, for [0 .. 2] it must generate a random number from P*2 uniformly distributed numbers. The obvious simple solution random() * (x - y) + y will generate not all possible numbers because of the lower density for all abs(x-y)>1.0 cases. Many possible values will be missed. Remember, that random() returns only a number from P possible numbers. Then, if you multiply such number by Q, it will give you only one of P possible values, scaled by Q, but you have to scale density P by Q as well.

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• Pecking order of pigeons?

  - by sc_ray

  I was going though problems on graph theory posted by Prof. Ericksson from my alma-mater and came across this rather unique question about pigeons and their innate tendency to form pecking orders. The question goes as follows: Whenever groups of pigeons gather, they instinctively establish a pecking order. For any pair of pigeons, one pigeon always pecks the other, driving it away from food or potential mates. The same pair of pigeons always chooses the same pecking order, even after years of separation, no matter what other pigeons are around. Surprisingly, the overall pecking order can contain cycles—for example, pigeon A pecks pigeon B, which pecks pigeon C, which pecks pigeon A. Prove that any finite set of pigeons can be arranged in a row from left to right so that every pigeon pecks the pigeon immediately to its left. Since this is a question on Graph theory, the first things that crossed my mind that is this just asking for a topological sort of a graphs of relationships(relationships being the pecking order). What made this a little more complex was the fact that there can be cyclic relationships between the pigeons. If we have a cyclic dependency as follows: A-B-C-A where A pecks on B,B pecks on C and C goes back and pecks on A If we represent it in the way suggested by the problem, we have something as follows: C B A But the above given row ordering does not factor in the pecking order between C and A. I had another idea of solving it by mathematical induction where the base case is for two pigeons arranged according to their pecking order, assuming the pecking order arrangement is valid for n pigeons and then proving it to be true for n+1 pigeons. I am not sure if I am going down the wrong track here. Some insights into how I should be analyzing this problem will be helpful. Thanks

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• Retrieving the top 100 numbers from one hundred million of numbers

  - by didxga

  One of my friend been asked with a question, Retrieving the max top 100 numbers from one hundred million of numbers, in a recent job interview. Do you have any idea to come up with an efficient way to solve it? regards!

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• All Permutations of a string when corresponding characters are not in the same place

  - by r20rock

  I need all possible permutations of a given string such that no character should remain at the same place as in the input string. Eg. : for input "ask" Output: all possible permutaions like "ksa", "kas"... such that 'a' is not in the 1st position , 's' is not in the 2nd positions and so on... in any permutation. I only need the count of such possible permutations I can do this by generating all permutations and filtering them but I need a very efficient way of doing this. All characters in the string are "UNIQUE" Preferred language C++.

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• Genetic programming in c++, library suggestions?

  - by shuttle87

  I'm looking to add some genetic algorithms to an Operations research project I have been involved in. Currently we have a program that aids in optimizing some scheduling and we want to add in some heuristics in the form of genetic algorithms. Are there any good libraries for generic genetic programming/algorithms in c++? Or would you recommend I just code my own? I should add that while I am not new to c++ I am fairly new to doing this sort of mathematical optimization work in c++ as the group I worked with previously had tended to use a proprietary optimization package. We have a fitness function that is fairly computationally intensive to evaluate and we have a cluster to run this on so parallelized code is highly desirable. So is c++ a good language for this? If not please recommend some other ones as I am willing to learn another language if it makes life easier. thanks!

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