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• Creative Technical Interview Questions for Developers

  - by John Shedletsky

  I do a good number of in-person technical interviews for new developers. I like to ask technical questions where I ask people to either code something up or develop an algorithm to solve a task. I feel my current repertoire is uninspired. In my opinion, the ideal interview question has these qualities: Multiple solutions, where some are obviously better than others, and some that involve subtle trade-offs (discussing tradeoffs is a good way to gauge someone's experience, in my opinion). Novelty - asking the "insert this element into a linked list" question is only good for weeding out people who never did their homework. Elegant - I like questions where the core problem isn't hidden in a lot of details. Everyone should be able to understand the problem, even if everyone can't solve it on the whiteboard. Elegant questions are difficult without involving undue amounts of "domain knowledge" or getting too narrow. Have you been on either side of an interview where someone (maybe you!) asked a particularly good programming or algorithms question?

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• question abouut string sort

  - by davit-datuashvili

  i have question from programming pearls problem is following show how to use lomuto's partitioning scheme to sort varying length bit strings in time proportional to the sum oof their length and algorithm is following each record in x[0..n-1] has an integer length and pointer to the array bit[0..length-1] code void bsort(l,u,depth){ if (l>=u) return; for (int i=l;i<u;i++) if (x[i].length<depth) swap(i,l++); m=l; if (x[i].bit[depth] ==0) swap(i,m++); bsort(l,m-1,depth+1); bsort(m,u,depth+1); please help me i need following things 1. how this algorith works 2.how implement in java?

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• Having trouble getting GET variables from url.

  - by mchl

  Hey everyone, I'm having trouble with this algorithm to extract the get variables from a url and print them each on a new line like: x=y z=hello etc. but instead it prints a seemingly random section of the url with no newlines to the file. There must be a logic error of some kind but i just can't spot it. for(i_m=0;i_m<len_m;i_m++) { if(var_m[i_m]=='&') { fwrite(var_m+offset_m, 1, amp_m, echo_out); fputc('\n',echo_out); offset_m+=amp_m; amp_m=0; } amp_m++; } any help appreciated.

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• Permutations with extra restrictions

  - by Full Decent

  I have a set of items, for example: {1,1,1,2,2,3,3,3}, and a restricting set of sets, for example {{3},{1,2},{1,2,3},{1,2,3},{1,2,3},{1,2,3},{2,3},{2,3}. I am looking for permutations of items, but the first element must be 3, and the second must be 1 or 2, etc. One such permutation that fits is: {3,1,1,1,2,2,3} Is there an algorithm to count all permutations for this problem in general? Is there a name for this type of problem? For illustration, I know how to solve this problem for certain types of "restricting sets". Set of items: {1,1,2,2,3}, Restrictions {{1,2},{1,2,3},{1,2,3},{1,2},{1,2}}. This is equal to 2!/(2-1)!/1! * 4!/2!/2!. Effectively permuting the 3 first, since it is the most restrictive and then permuting the remaining items where there is room.

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• Implementing "select distinct ... from ..." over a list of Python dictionaries

  - by daveslab

  Hi folks, Here is my problem: I have a list of Python dictionaries of identical form, that are meant to represent the rows of a table in a database, something like this: [ {'ID': 1, 'NAME': 'Joe', 'CLASS': '8th', ... }, {'ID': 1, 'NAME': 'Joe', 'CLASS': '11th', ... }, ...] I have already written a function to get the unique values for a particular field in this list of dictionaries, which was trivial. That function implements something like: select distinct NAME from ... However, I want to be able to get the list of multiple unique fields, similar to: select distinct NAME, CLASS from ... Which I am finding to be non-trivial. Is there an algorithm or Python included function to help me with this quandry? Before you suggest loading the CSV files into a SQLite table or something similar, that is not an option for the environment I'm in, and trust me, that was my first thought.

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• complexity of algorithms

  - by davit-datuashvili

  i have question what is complexity of this algorithm public class smax{ public static void main(String[]args){ int b[]=new int[11]; int a[]=new int[]{4,9,2,6,8,7,5}; for (int i=0;i int m=0; while (m int k=a[0]; for (int i=0;i k && b[a[i]]!=1){ b[a[i]]=1; } } m++; } for (int i=0;i for (int j=0;j //result=2 4 5 6 7 8 9 } } ?

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• Print all ways to sum n integers so that they total a given sum.

  - by noghead

  Im trying to come up with an algorithm that will print out all possible ways to sum N integers so that they total a given value. Example. Print all ways to sum 4 integers so that they sum up to be 5. Result should be something like: 5 0 0 0 4 1 0 0 3 2 0 0 3 1 1 0 2 3 0 0 2 2 1 0 2 1 2 0 2 1 1 1 1 4 0 0 1 3 1 0 1 2 2 0 1 2 1 1 1 1 3 0 1 1 2 1 1 1 1 2

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• How to get it working in O(n)?

  - by evermean

  I came across an interview task/question that really got me thinking ... so here it goes: You have an array A[N] of N numbers. You have to compose an array Output[N] such that Output[i] will be equal to multiplication of all the elements of A[N] except A[i]. For example Output[0] will be multiplication of A[1] to A[N-1] and Output[1] will be multiplication of A[0] and from A[2] to A[N-1]. Solve it without division operator and in O(n). I really tried to come up with a solution but I always end up with a complexity of O(n^2). Perhaps the is anyone smarter than me who can tell me an algorithm that works in O(n) or at least give me a hint...

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• Delivery of JMS message before the transaction is committed

  - by ewernli

  Hi, I have a very simple scenario involving a database and a JMS in an application server (Glassfish). The scenario is dead simple: 1. an EJB inserts a row in the database and sends a message. 2. when the message is delivered with an MDB, the row is read and updated. The problem is that sometimes the message is delivered before the insert has been committed in the database. This is actually understandable if we consider the 2 phase commit protocol: 1. prepare JMS 2. prepare database 3. commit JMS 4. ( tiny little gap where message can be delivered before insert has been committed) 5. commit database I've discussed this problem with others, but the answer was always: "Strange, it should work out of the box". My questions are then: How could it work out-of-the box? My scenario sounds fairly simple, why isn't there more people with similar troubles? Am I doing something wrong? Is there a way to solve this issue correctly? Here are a bit more details about my understanding of the problem: This timing issue exist only if the participant are treated in this order. If the 2PC treats the participants in the reverse order (database first then message broker) that should be fine. The problem was randomly happening but completely reproducible. I found no way to control the order of the participants in the distributed transactions in the JTA, JCA and JPA specifications neither in the Glassfish documentation. We could assume they will be enlisted in the distributed transaction according to the order when they are used, but with an ORM such as JPA, it's difficult to know when the data are flushed and when the database connection is really used. Any idea?

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• nth ugly number

  - by Anil Katti

  Numbers whose only prime factors are 2, 3 or 5 are called ugly numbers. Example: 1, 2, 3, 4, 5, 6, 8, 9, 10, 12, 15, ... 1 can be considered as 2^0. I am working on finding nth ugly number. Note that these numbers are extremely sparsely distributed as n gets large. I wrote a trivial program that computes if a given number is ugly or not. For n 500 - it became super slow. I tried using memoization - observation: ugly_number * 2, ugly_number * 3, ugly_number * 5 are all ugly. Even with that it is slow. I tried using some properties of log - since that will reduce this problem from multiplication to addition - but, not much luck yet. Thought of sharing this with you all. Any interesting ideas? Using a concept similar to "Sieve of Eratosthenes" (thanks Anon) for (int i(2), uglyCount(0); ; i++) { if (i % 2 == 0) continue; if (i % 3 == 0) continue; if (i % 5 == 0) continue; uglyCount++; if (uglyCount == n - 1) break; } i is the nth ugly number. Even this is pretty slow. I am trying to find 1500th ugly number.

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• Non recursive way to position a genogram in 2D points for x axis. Descendant are below

  - by Nassign

  I currently was tasked to make a genogram for a family consisting of siblings, parents with aunts and uncles with grandparents and greatgrandparents for only blood relatives. My current algorithm is using recursion. but I am wondering how to do it in non recursive way to make it more efficient. it is programmed in c# using graphics to draw on a bitmap. Current algorithm for calculating x position, the y position is by getting the generation number. public void StartCalculatePosition() { // Search the start node (The only node with targetFlg set to true) Person start = null; foreach (Person p in PersonDic.Values) { if (start == null) start = p; if (p.Targetflg) { start = p; break; } } CalcPositionRecurse(start); // Normalize the position (shift all values to positive value) // Get the minimum value (must be negative) // Then offset the position of all marriage and person with that to make it start from zero float minPosition = float.MaxValue; foreach (Person p in PersonDic.Values) { if (minPosition > p.Position) { minPosition = p.Position; } } if (minPosition < 0) { foreach (Person p in PersonDic.Values) { p.Position -= minPosition; } foreach (Marriage m in MarriageList) { m.ParentsPosition -= minPosition; m.ChildrenPosition -= minPosition; } } } /// <summary> /// Calculate position of genogram using recursion /// </summary> /// <param name="psn"></param> private void CalcPositionRecurse(Person psn) { // End the recursion if (psn.BirthMarriage == null || psn.BirthMarriage.Parents.Count == 0) { psn.Position = 0.0f; if (psn.BirthMarriage != null) { psn.BirthMarriage.ParentsPosition = 0.0f; psn.BirthMarriage.ChildrenPosition = 0.0f; } CalculateSiblingPosition(psn); return; } // Left recurse if (psn.Father != null) { CalcPositionRecurse(psn.Father); } // Right recurse if (psn.Mother != null) { CalcPositionRecurse(psn.Mother); } // Merge Position if (psn.Father != null && psn.Mother != null) { AdjustConflict(psn.Father, psn.Mother); // Position person in center of parent psn.Position = (psn.Father.Position + psn.Mother.Position) / 2; psn.BirthMarriage.ParentsPosition = psn.Position; psn.BirthMarriage.ChildrenPosition = psn.Position; } else { // Single mom or single dad if (psn.Father != null) { psn.Position = psn.Father.Position; psn.BirthMarriage.ParentsPosition = psn.Position; psn.BirthMarriage.ChildrenPosition = psn.Position; } else if (psn.Mother != null) { psn.Position = psn.Mother.Position; psn.BirthMarriage.ParentsPosition = psn.Position; psn.BirthMarriage.ChildrenPosition = psn.Position; } else { // Should not happen, checking in start of function } } // Arrange the siblings base on my position (left younger, right older) CalculateSiblingPosition(psn); } private float GetRightBoundaryAncestor(Person psn) { float rPos = psn.Position; // Get the rightmost position among siblings foreach (Person sibling in psn.Siblings) { if (sibling.Position > rPos) { rPos = sibling.Position; } } if (psn.Father != null) { float rFatherPos = GetRightBoundaryAncestor(psn.Father); if (rFatherPos > rPos) { rPos = rFatherPos; } } if (psn.Mother != null) { float rMotherPos = GetRightBoundaryAncestor(psn.Mother); if (rMotherPos > rPos) { rPos = rMotherPos; } } return rPos; } private float GetLeftBoundaryAncestor(Person psn) { float rPos = psn.Position; // Get the rightmost position among siblings foreach (Person sibling in psn.Siblings) { if (sibling.Position < rPos) { rPos = sibling.Position; } } if (psn.Father != null) { float rFatherPos = GetLeftBoundaryAncestor(psn.Father); if (rFatherPos < rPos) { rPos = rFatherPos; } } if (psn.Mother != null) { float rMotherPos = GetLeftBoundaryAncestor(psn.Mother); if (rMotherPos < rPos) { rPos = rMotherPos; } } return rPos; } /// <summary> /// Check if two parent group has conflict and compensate on the conflict /// </summary> /// <param name="leftGroup"></param> /// <param name="rightGroup"></param> public void AdjustConflict(Person leftGroup, Person rightGroup) { float leftMax = GetRightBoundaryAncestor(leftGroup); leftMax += 0.5f; float rightMin = GetLeftBoundaryAncestor(rightGroup); rightMin -= 0.5f; float diff = leftMax - rightMin; if (diff > 0.0f) { float moveHalf = Math.Abs(diff) / 2; RecurseMoveAncestor(leftGroup, 0 - moveHalf); RecurseMoveAncestor(rightGroup, moveHalf); } } /// <summary> /// Recursively move a person and all his/her ancestor /// </summary> /// <param name="psn"></param> /// <param name="moveUnit"></param> public void RecurseMoveAncestor(Person psn, float moveUnit) { psn.Position += moveUnit; foreach (Person siblings in psn.Siblings) { if (siblings.Id != psn.Id) { siblings.Position += moveUnit; } } if (psn.BirthMarriage != null) { psn.BirthMarriage.ChildrenPosition += moveUnit; psn.BirthMarriage.ParentsPosition += moveUnit; } if (psn.Father != null) { RecurseMoveAncestor(psn.Father, moveUnit); } if (psn.Mother != null) { RecurseMoveAncestor(psn.Mother, moveUnit); } } /// <summary> /// Calculate the position of the siblings /// </summary> /// <param name="psn"></param> /// <param name="anchor"></param> public void CalculateSiblingPosition(Person psn) { if (psn.Siblings.Count == 0) { return; } List<Person> sibling = psn.Siblings; int argidx; for (argidx = 0; argidx < sibling.Count; argidx++) { if (sibling[argidx].Id == psn.Id) { break; } } // Compute position for each brother that is younger that person int idx; for (idx = argidx - 1; idx >= 0; idx--) { sibling[idx].Position = sibling[idx + 1].Position - 1; } for (idx = argidx + 1; idx < sibling.Count; idx++) { sibling[idx].Position = sibling[idx - 1].Position + 1; } }

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• How can I tweak this A* search pathfinding algorithm to handle different terrain movement values?

  - by user422318

  I'm creating a 2D map-based action game with similar interaction design as Diablo II. In other words, the player clicks around a map to move their player. I just finished player movement and am moving on to pathfinding. In the game, enemies should charge the player's character. There are also five different terrain types that give different movement bonuses. I want the AI to take advantage of these terrain bonuses as they try to reach the player. I was told to check out the A* search algorithm (http://en.wikipedia.org/wiki/A*_search_algorithm). I'm doing this game in HTML5 and JavaScript, and found a version in JavaScript: http://www.briangrinstead.com/blog/astar-search-algorithm-in-javascript I'm trying to figure out how to tweak it though. Below are my ideas about what I need to change. What else do I need to worry about? When I create a graph, I will need to initialize the 2D array I pass in passed on with a traversal of a map that corresponds to the different terrain types. in graph.js: "GraphNodeType" definition needs to be modified to handle the 5 terrain types. There will be no walls. in astar.js: The g and h scoring will need to be modified. How should I do this? in astar.js: isWall() should probably be removed. My game doesn't have walls. in astar.js: I'm not sure what this is. I think it indicates a node that isn't valid to be processed. When would this happen, though? At a high level, how do I change this algorithm from "oh, is there a wall there?" to "will this terrain get me to the player faster than the terrain around me?" Because of time, I'm also debating reusing my Bresenham algorithm for the enemies. Unfortunately, the different terrain movement bonuses won't be used by the AI, which will make the game suck. :/ I'd really like to have this in for the prototype, but I'm not a developer by trade nor am I a computer scientist. :D If you know of any code that does what I'm looking for, please share! Sanity check tips for this are also appreciated.

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• Applying the Knuth-Plass algorithm (or something better?) to read two books with different length and amount of chapters in parallel

  - by user147133

  I have a Bible reading plan that covers the whole Bible in 180 days. For the most of the time, I read 5 chapters in the Old Testament and 1 or 2 (1.5) chapters in the New Testament each day. The problem is that some chapters are longer than others (for example Psalm 119 which is 7 times longer than a average chapter in the Bible), and the plan I'm following doesn't take that in count. I end up with some days having a lot more to read than others. I thought I could use programming to make myself a better plan. I have a datastructure with a list of all chapters in the bible and their length in number of lines. (I found that the number of lines is the best criteria, but it could have been number of verses or number of words as well) I then started to think about this problem as a line wrap problem. Think of a chapter like a word, a day like a line and the whole plan as a paragraph. The "length" of a word (a chapter) is the number of lines in that chapter. I could then generate the best possible reading plan by applying a simplified Knuth-Plass algorithm to find the best breakpoints. This works well if I want to read the Bible from beginning to end. But I want to read a little from the new testament each day in parallel with the old testament. Of course I can run the Knuth-Plass algorithm on the Old Testament first, then on the New Testament and get two separate plans. But those plans merged is not a optimal plan. Worst-case days (days with extra much reading) in the New Testament plan will randomly occur on the same days as the worst-case days in the Old Testament. Since the New Testament have about 180*1.5 chapters, the plan is generally to read one chapter the first day, two the second, one the third etc... And I would like the plan for the Old Testament to compensate for this alternating length. So I will need a new and better algorithm, or I will have to use the Knuth-Plass algorithm in a way that I've not figured out. I think this could be a interesting and challenging nut for people interested in algorithms, so therefore I wanted to see if any of you have a good solution in mind.

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• faster implementation of sum ( for Codility test )

  - by Oscar Reyes

  How can the following simple implementation of sum be faster? private long sum( int [] a, int begin, int end ) { if( a == null ) { return 0; } long r = 0; for( int i = begin ; i < end ; i++ ) { r+= a[i]; } return r; } EDIT Background is in order. Reading latest entry on coding horror, I came to this site: http://codility.com which has this interesting programming test. Anyway, I got 60 out of 100 in my submission, and basically ( I think ) is because this implementation of sum, because those parts where I failed are the performance parts. I'm getting TIME_OUT_ERROR's So, I was wondering if an optimization in the algorithm is possible. So, no built in functions or assembly would be allowed. This my be done in C, C++, C#, Java or pretty much in any other. EDIT As usual, mmyers was right. I did profile the code and I saw most of the time was spent on that function, but I didn't understand why. So what I did was to throw away my implementation and start with a new one. This time I've got an optimal solution [ according to San Jacinto O(n) -see comments to MSN below - ] This time I've got 81% on Codility which I think is good enough. The problem is that I didn't take the 30 mins. but around 2 hrs. but I guess that leaves me still as a good programmer, for I could work on the problem until I found an optimal solution: Here's my result. I never understood what is those "combinations of..." nor how to test "extreme_first"

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• Getting the submatrix with maximum sum?

  - by guirgis

  With the help of the Algorithmist and Larry and a modification of Kadane's Algorithm, here is my solution: int dim = matrix.length; //computing the vertical prefix sum for columns int[][] ps = new int[dim][dim]; for (int i = 0; i < dim; i++) { for (int j = 0; j < dim; j++) { if (j == 0) { ps[j][i] = matrix[j][i]; } else { ps[j][i] = matrix[j][i] + ps[j - 1][i]; } } } int maxSoFar = 0; int min , subMatrix; //iterate over the possible combinations applying Kadane's Alg. //int toplefti =0, topleftj=0, bottomrighti=0, bottomrightj=0; for (int i = 0; i < dim; i++) { for (int j = i; j < dim; j++) { min = 0; subMatrix = 0; for (int k = 0; k < dim; k++) { if (i == 0) { subMatrix += ps[j][k]; } else { subMatrix += ps[j][k] - ps[i-1][k]; } if(subMatrix < min){ min = subMatrix; } if((subMatrix - min) > maxSoFar){ maxSoFar = subMatrix - min; } } } } The only problem left is to determine the submatrix elements, i mean the top left and the bottom right corners. I managed to do this in one dimensional case. Any suggestions?

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• Ray-box Intersection Theory

  - by Myx

  Hello: I wish to determine the intersection point between a ray and a box. The box is defined by its min 3D coordinate and max 3D coordinate and the ray is defined by its origin and the direction to which it points. Currently, I am forming a plane for each face of the box and I'm intersecting the ray with the plane. If the ray intersects the plane, then I check whether or not the intersection point is actually on the surface of the box. If so, I check whether it is the closest intersection for this ray and I return the closest intersection. The way I check whether the plane-intersection point is on the box surface itself is through a function bool PointOnBoxFace(R3Point point, R3Point corner1, R3Point corner2) { double min_x = min(corner1.X(), corner2.X()); double max_x = max(corner1.X(), corner2.X()); double min_y = min(corner1.Y(), corner2.Y()); double max_y = max(corner1.Y(), corner2.Y()); double min_z = min(corner1.Z(), corner2.Z()); double max_z = max(corner1.Z(), corner2.Z()); if(point.X() >= min_x && point.X() <= max_x && point.Y() >= min_y && point.Y() <= max_y && point.Z() >= min_z && point.Z() <= max_z) return true; return false; } where corner1 is one corner of the rectangle for that box face and corner2 is the opposite corner. My implementation works most of the time but sometimes it gives me the wrong intersection. I was wondering if the way I'm checking whether the intersection point is on the box is correct or if I should use some other algorithm. Thanks.

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• How should I Test a Genetic Algorithm

  - by James Brooks

  I have made a quite few genetic algorithms; they work (they find a reasonable solution quickly). But I have now discovered TDD. Is there a way to write a genetic algorithm (which relies heavily on random numbers) in a TDD way? To pose the question more generally, How do you test a non-deterministic method/function. Here is what I have thought of: Use a specific seed. Which wont help if I make a mistake in the code in the first place but will help finding bugs when refactoring. Use a known list of numbers. Similar to the above but I could follow the code through by hand (which would be very tedious). Use a constant number. At least I know what to expect. It would be good to ensure that a dice always reads 6 when RandomFloat(0,1) always returns 1. Try to move as much of the non-deterministic code out of the GA as possible. which seems silly as that is the core of it's purpose. Links to very good books on testing would be appreciated too.

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• How do you construct an array suitable for numpy sorting?

  - by Alex

  I need to sort two arrays simultaneously, or rather I need to sort one of the arrays and bring the corresponding element of its associated array with it as I sort. That is if the array is [(5, 33), (4, 44), (3, 55)] and I sort by the first axis (labeled below dtype='alpha') then I want: [(3.0, 55.0) (4.0, 44.0) (5.0, 33.0)]. These are really big data sets and I need to sort first ( for nlog(n) speed ) before I do some other operations. I don't know how to merge my two separate arrays though in the proper manner to get the sort algorithm working. I think my problem is rather simple. I tried three different methods: import numpy x=numpy.asarray([5,4,3]) y=numpy.asarray([33,44,55]) dtype=[('alpha',float), ('beta',float)] values=numpy.array([(x),(y)]) values=numpy.rollaxis(values,1) #values = numpy.array(values, dtype=dtype) #a=numpy.array(values,dtype=dtype) #q=numpy.sort(a,order='alpha') print "Try 1:\n", values values=numpy.empty((len(x),2)) for n in range (len(x)): values[n][0]=y[n] values[n][1]=x[n] print "Try 2:\n", values #values = numpy.array(values, dtype=dtype) #a=numpy.array(values,dtype=dtype) #q=numpy.sort(a,order='alpha') ### values = [(x[0], y[0]), (x[1],y[1]) , (x[2],y[2])] print "Try 3:\n", values values = numpy.array(values, dtype=dtype) a=numpy.array(values,dtype=dtype) q=numpy.sort(a,order='alpha') print "Result:\n",q I commented out the first and second trys because they create errors, I knew the third one would work because that was mirroring what I saw when I was RTFM. Given the arrays x and y (which are very large, just examples shown) how do I construct the array (called values) that can be called by numpy.sort properly? *** Zip works great, thanks. Bonus question: How can I later unzip the sorted data into two arrays again?

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• How does one convert 16-bit RGB565 to 24-bit RGB888?

  - by jleedev

  I’ve got my hands on a 16-bit rgb565 image (specifically, an Android framebuffer dump), and I would like to convert it to 24-bit rgb888 for viewing on a normal monitor. The question is, how does one convert a 5- or 6-bit channel to 8 bits? The obvious answer is to shift it. I started out by writing this: uint16_t buf; while (read(0, &buf, sizeof buf)) { unsigned char red = (buf & 0xf800) >> 11; unsigned char green = (buf & 0x07c0) >> 5; unsigned char blue = buf & 0x003f; putchar(red << 3); putchar(green << 2); putchar(blue << 3); } However, this doesn’t have one property I would like, which is for 0xffff to map to 0xffffff, instead of 0xf8fcf8. I need to expand the value in some way, but I’m not sure how that should work. The Android SDK comes with a tool called ddms (Dalvik Debug Monitor) that takes screen captures. As far as I can tell from reading the code, it implements the same logic; yet its screenshots are coming out different, and white is mapping to white. Here’s the raw framebuffer, the smart conversion by ddms, and the dumb conversion by the above algorithm. (By the way, this conversion is implemented in ffmpeg, but it’s just performing the dumb conversion listed above, leaving the LSBs at all zero.) I guess I have two questions: What’s the most sensible way to convert rgb565 to rgb888? How is DDMS converting its screenshots?

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• Big problem with Dijkstra algorithm in a linked list graph implementation

  - by Nazgulled

  Hi, I have my graph implemented with linked lists, for both vertices and edges and that is becoming an issue for the Dijkstra algorithm. As I said on a previous question, I'm converting this code that uses an adjacency matrix to work with my graph implementation. The problem is that when I find the minimum value I get an array index. This index would have match the vertex index if the graph vertexes were stored in an array instead. And the access to the vertex would be constant. I don't have time to change my graph implementation, but I do have an hash table, indexed by a unique number (but one that does not start at 0, it's like 100090000) which is the problem I'm having. Whenever I need, I use the modulo operator to get a number between 0 and the total number of vertices. This works fine for when I need an array index from the number, but when I need the number from the array index (to access the calculated minimum distance vertex in constant time), not so much. I tried to search for how to inverse the modulo operation, like, 100090000 mod 18000 = 10000 and, 10000 invmod 18000 = 100090000 but couldn't find a way to do it. My next alternative is to build some sort of reference array where, in the example above, arr[10000] = 100090000. That would fix the problem, but would require to loop the whole graph one more time. Do I have any better/easier solution with my current graph implementation?

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• Raytracing (LoS) on 3D hex-like tile maps

  - by herenvardo

  Greetings, I'm working on a game project that uses a 3D variant of hexagonal tile maps. Tiles are actually cubes, not hexes, but are laid out just like hexes (because a square can be turned to a cube to extrapolate from 2D to 3D, but there is no 3D version of a hex). Rather than a verbose description, here goes an example of a 4x4x4 map: (I have highlighted an arbitrary tile (green) and its adjacent tiles (yellow) to help describe how the whole thing is supposed to work; but the adjacency functions are not the issue, that's already solved.) I have a struct type to represent tiles, and maps are represented as a 3D array of tiles (wrapped in a Map class to add some utility methods, but that's not very relevant). Each tile is supposed to represent a perfectly cubic space, and they are all exactly the same size. Also, the offset between adjacent "rows" is exactly half the size of a tile. That's enough context; my question is: Given the coordinates of two points A and B, how can I generate a list of the tiles (or, rather, their coordinates) that a straight line between A and B would cross? That would later be used for a variety of purposes, such as determining Line-of-sight, charge path legality, and so on. BTW, this may be useful: my maps use the (0,0,0) as a reference position. The 'jagging' of the map can be defined as offsetting each tile ((y+z) mod 2) * tileSize/2.0 to the right from the position it'd have on a "sane" cartesian system. For the non-jagged rows, that yields 0; for rows where (y+z) mod 2 is 1, it yields 0.5 tiles. I'm working on C#4 targeting the .Net Framework 4.0; but I don't really need specific code, just the algorithm to solve the weird geometric/mathematical problem. I have been trying for several days to solve this at no avail; and trying to draw the whole thing on paper to "visualize" it didn't help either :( . Thanks in advance for any answer

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• Python: speed up removal of every n-th element from list.

  - by ChristopheD

  I'm trying to solve this programming riddle and althought the solution (see code below) works correct, it is too slow for succesful submission. Any pointers as how to make this run faster? (removal of every n-th element from a list)? Or suggestions for a better algorithm to calculate the same; seems I can't think of anything else then brute-force for now... Basically the task at hand is: GIVEN: L = [2,3,4,5,6,7,8,9,10,11,........] 1. Take the first remaining item in list L (in the general case 'n'). Move it to the 'lucky number list'. Then drop every 'n-th' item from the list. 2. Repeat 1 TASK: Calculate the n-th number from the 'lucky number list' ( 1 <= n <= 3000) My current code (it calculates the 3000 first lucky numbers in about a second on my machine - but unfortunately too slow): """ SPOJ Problem Set (classical) 1798. Assistance Required URL: http://www.spoj.pl/problems/ASSIST/ """ sieve = range(3, 33900, 2) luckynumbers = [2] while True: wanted_n = input() if wanted_n == 0: break while len(luckynumbers) < wanted_n: item = sieve[0] luckynumbers.append(item) items_to_delete = set(sieve[::item]) sieve = filter(lambda x: x not in items_to_delete, sieve) print luckynumbers[wanted_n-1]

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• Average of two strings in alphabetical/lexicographical order

  - by Bemmu

  Suppose you take the strings 'a' and 'z' and list all the strings that come between them in alphabetical order: ['a','b','c' ... 'x','y','z']. Take the midpoint of this list and you find 'm'. So this is kind of like taking an average of those two strings. You could extend it to strings with more than one character, for example the midpoint between 'aa' and 'zz' would be found in the middle of the list ['aa', 'ab', 'ac' ... 'zx', 'zy', 'zz']. Might there be a Python method somewhere that does this? If not, even knowing the name of the algorithm would help. I began making my own routine that simply goes through both strings and finds midpoint of the first differing letter, which seemed to work great in that 'aa' and 'az' midpoint was 'am', but then it fails on 'cat', 'doggie' midpoint which it thinks is 'c'. I tried Googling for "binary search string midpoint" etc. but without knowing the name of what I am trying to do here I had little luck.

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• sample java code for approximate string matching or boyer-moore extended for approximate string matc

  - by Dolphin

  Hi I need to find 1.mismatch(incorrectly played notes), 2.insertion(additional played), & 3.deletion (missed notes), in a music piece (e.g. note pitches [string values] stored in a table) against a reference music piece. This is either possible through exact string matching algorithms or dynamic programming/ approximate string matching algos. However I realised that approximate string matching is more appropriate for my problem due to identifying mismatch, insertion, deletion of notes. Or an extended version of Boyer-moore to support approx. string matching. Is there any link for sample java code I can try out approximate string matching? I find complex explanations and equations - but I hope I could do well with some sample code and simple explanations. Or can I find any sample java code on boyer-moore extended for approx. string matching? I understand the boyer-moore concept, but having troubles with adjusting it to support approx. string matching (i.e. to support mismatch, insertion, deletion). Also what is the most efficient approx. string matching algorithm (like boyer-moore in exact string matching algo)? Greatly appreciate any insight/ suggestions. Many thanks in advance

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• Average of two strings in alphabetical order

  - by Bemmu

  Suppose you take the strings 'a' and 'z' and list all the strings that come between them in alphabetical order: ['a','b','c' ... 'x','y','z']. Take the midpoint of this list and you find 'm'. So this is kind of like taking an average of those two strings. You could extend it to strings with more than one character, for example the midpoint between 'aa' and 'zz' would be found in the middle of the list ['aa', 'ab', 'ac' ... 'zx', 'zy', 'zz']. Might there be a Python method somewhere that does this? If not, even knowing the name of the algorithm would help. I began making my own routine that simply goes through both strings and finds midpoint of the first differing letter, which seemed to work great in that 'aa' and 'az' midpoint was 'am', but then it fails on 'cat', 'doggie' midpoint which it thinks is 'c'. Rather than invent a method I thought it better to ask. I tried Googling for "binary search string midpoint" etc. but without knowing the name of what I am trying to do here I had little luck.

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