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• Is there a website to look up common, already written functions?

  - by pinnacler

  I'm sitting here writing a function that I'm positive has been written before, somewhere on earth. It's just too common to have not been attempted, and I'm wondering why I can't just go to a website and search for a function that I can then copy and paste into my project in 2 seconds, instead of wasting my day reinventing the wheel. Sure there are certain libraries you can use, but where do you find these libraries and when they are absent, is there a site like I'm describing? Possibly a wiki of some type that contains free code that anybody can edit and improve? Edit: I can code things fine, I just don't know HOW to do them. So for example, right now, I'm trying to localize a robot/car/point in space. I KNOW there is a way to do it, just based off of range and distance. Triangulation and Trilateration. How to code that is a different story. A site that could have psuedo code, step by step how to do that would be ridiculously helpful. It would also ensure the optimal solution since everybody can edit it. I'm also writing in Matlab, which I hate because it's quirky, adding to my desire for creating a website like I describe.

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• Finding unreachable sections of a 2D map

  - by dada

  I don't want you to solve this problem for me, i just want to ask for some ideas. This is the input below, and it represents a map. The 'x' represents land, and the dots - water. So with the 'x' you can represent 'islands' on the map. xxx.x...xxxxx xxxx....x...x ........x.x.x ..xxxxx.x...x ..x...x.xxx.x ..x.x.x...x.. ..x...x...xxx ...xxxxxx.... x............ As you can see, there are some islands which are closed, i.e. if some boat is inside its territory, it won't be able to get out, for ex: ..xxxxx. ..x...x. ..x.x.x. ..x...x. ..xxxxx. And there are some open islands which is possible to get out of them, ex: .xxxxx .x...x .x.x.x .xxx.x The problem is this: For a given NxM map like those above, calculate howm any of the islands are open, and how many are closed. I repeat: I don't want you to solve it, just need some sugestions, ideas for solving. thanks

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• Reverse factorial

  - by dada

  Well, we all know that if N is given it's easy to calculate N!. But what about reversing? N! is given and you are about to find N - Is that possible ? I'm curious.

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• KD-Trees and missing values (vector comparison)

  - by labratmatt

  I have a system that stores vectors and allows a user to find the n most similar vectors to the user's query vector. That is, a user submits a vector (I call it a query vector) and my system spits out "here are the n most similar vectors." I generate the similar vectors using a KD-Tree and everything works well, but I want to do more. I want to present a list of the n most similar vectors even if the user doesn't submit a complete vector (a vector with missing values). That is, if a user submits a vector with three dimensions, I still want to find the n nearest vectors (stored vectors are of 11 dimensions) I have stored. I have a couple of obvious solutions, but I'm not sure either one seem very good: Create multiple KD-Trees each built using the most popular subset of dimensions a user will search for. That is, if a user submits a query vector of thee dimensions, x, y, z, I match that query to my already built KD-Tree which only contains vectors of three dimensions, x, y, z. Ignore KD-Trees when a user submits a query vector with missing values and compare the query vector to the vectors (stored in a table in a DB) one by one using something like a dot product. This has to be a common problem, any suggestions? Thanks for the help.

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• Reduce number of points in line

  - by culebrón

  I'm searching for algorithms to reduce the LOD of polylines, lines (looped or not) of nodes. In simple words, I want to take hi-resolution coastline data and be able to reduce its LOD hundred- or thousandfold to render it in small-scale. I found polygon reduction algorithms (but they require triangles) and Laplacian smoothing, but that doesn't seem exactly what I need.

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• Deletion procedure for a Binary Search Tree

  - by Metz

  Consider the deletion procedure on a BST, when the node to delete has two children. Let's say i always replace it with the node holding the minimum key in its right subtree. The question is: is this procedure commutative? That is, deleting x and then y has the same result than deleting first y and then x? I think the answer is no, but i can't find a counterexample, nor figure out any valid reasoning. EDIT: Maybe i've got to be clearer. Consider the transplant(node x, node y) procedure: it replace x with y (and its subtree). So, if i want to delete a node (say x) which has two children i replace it with the node holding the minimum key in its right subtree: y = minimum(x.right) transplant(y, y.right) // extracts the minimum (it doesn't have left child) y.right = x.right y.left = x.left transplant(x,y) The question was how to prove the procedure above is not commutative.

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• Best way to Fingerprint and Verify html structure.

  - by Lukas Šalkauskas

  Hello there, I just want to know what is your opinion about how to fingerprint/verify html/links structure. The problem I want to solve is: fingerprint for example 10 different sites, html pages. And after some time I want to have possibility to verify them, so is, if site has been changed, links changed, verification fails, othervise verification success. My base Idea is to analyze link structure by splitting it in some way, doing some kind of tree, and from that tree generate some kind of code. But I'm still in brainstorm stage, where I need to discuss this with someone, and know other ideas. So any ideas, algos, and suggestions would be usefull.

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• Problem implementing sorting algorithm in C with an array of structs

  - by dilog

  Well here is my little problem, first my code: struct alumn { char name[100]; char lastname[100]; int par; int nota; }; typedef struct alumn alumn; int bubble(alumn **arr, int length) { int i,j; alumn *temp; for (i=0; i<=length-2; i++) { for (j=i+1; j<=length-1;j++) { if ((*arr)[i].nota > (*arr)[j].nota) { temp = arr[i]; arr[i] = arr[j]; arr[j] = temp; } } } } int main(int argc, char **argv) { alumn *alumns; ... here goes some other code ... bubble(&alumns,totalAlumns); return 0; } My problem is that this algorith is not sorting anything. I'm having a hard time doing the swap, i tried everything but nothing works :( . Any help??? struct alumn { char name[100]; char lastname[100]; int par; int nota; }; typedef struct alumn alumn; int bubble(alumn **arr, int length) { int i,j; alumn *temp; for (i=0; i<=length-2; i++) { for (j=i+1; j<=length-1;j++) { if ((*arr)[i].nota > (*arr)[j].nota) { temp = arr[i]; arr[i] = arr[j]; arr[j] = temp; } } } } int main(int argc, char **argv) { alumn *alumns; ... here goes some other code ... bubble(&alumns,totalAlumns); return 0; } My problem is that this algorith is not sorting anything. I'm having a hard time doing the swap, i tried everything but nothing works :( . Any help???

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• VBA compare and sort strings with quirky characters

  - by Smandoli

  I am comparing text values from two DAO recordsets in MS Access. I sort on the text field, then go through both recordsets comparing the values from each. The sets are substantially different and while they're mostly alpha-numeric, spaces and symbols like hyphens and periods are very common. My program depends on predictable sorting and fool-proof comparing. But unfortunately, the sort will rank two values differently than the comparison function. StrComp is the obvious first choice: varResult = StrComp(Val_1, Val_2) RFA-300 14.9044 RFA300 14-2044 But for the two pairs above, StrComp returns a different value than one would expect based on the sort. Including vbTextCompare or vbBinaryCompare affects StrComp's result, but not so as to solve the problem. Note the values must always be compared as strings. Of course I make sure that "14-2044" and "14.9044" aren't evaluated as -2030 and ~15. That's not the cause of my problem. I learned API-based functions are more reliable for quirky texts, so I tried these: varResult = CompareString(LOCALE_SYSTEM_DEFAULT, _ SORT_STRINGSORT, strVal_2, -1, strVal_1, -1) varResult = CompareString(LOCALE_SYSTEM_DEFAULT, _ NORM_IGNOREWIDTH, strVal_2, -1, strVal_1, -1) The first one returns the opposite of StrComp. The second one returns the same as StrComp. But neither yields a result that is consistent with the sort order. (NORM_IGNOREWIDTH is probably not relevant, but I needed a place-holder substitute and it looked as good as any.) UPDATE: This is a complete rewrite of the original post, deleting all the info about why I really need this -- just take my word for it and enjoy the brevity.

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• open source smooth particle hydrodynamics

  - by user325181

  Anyone know of any open source libraries for particle based large scale smooth particle hydrodynamics. I am looking for a easier way of simulating large scale planetary body impacts with rotation. I was also wondering if you had any ideas on how to visualize the output from said simulation. I have tried using IBM graphviz, but it is very difficult to work with. Any pointers would be appreciated. Thanks!

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• curious ill conditioned numerical problem

  - by aaa

  hello. somebody today showed me this curious ill conditioned problem (apparently pretty famous), which looks relatively simple ƒ = (333.75 - a^2)b^6 + a^2 (11a^2 b^2 - 121b^4 - 2) + 5.5b^8 + a/(2^b) where a = 77617 and b = 33096 can you determine correct answer?

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• Find a missing 32bit integer among a unsorted array containing at most 4 billion ints

  - by pierr

  Hi, This is the problem described in Programming pearls. I can not understand binary search method descrbied by the author. Can any one helps to elaborate? Thanks. EDIT: I can understand binary search in general. I just can not understand how to apply binary search in this special case. How to decide the missing number is in or not in some range so that we can choose another. English is not my native language, that is one reason I can not understand the author well. So, use plain english please:) EDIT: Thank you all for your great answer and comments ! The most important lesson I leant from solving this question is Binary search applies not only on sorted array!

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• All words in a trie data-structure

  - by John Smith

  I'm trying to put all words in a trie in a string, a word is detonated by the eow field being true for a certain character in the trie data structure, hence a trie can could have letters than lead up to no word, for ex "abc" is in the trie but "c"'s eow field is false so "abc" is not a word Here is my Data structure struct Trie { bool eow; //when a Trie field isWord = true, hence there is a word char letter; Trie *letters[27]; }; and here is my attemped print-all function, basically trying to return all words in one string seperated by spaces for words string printAll( string word, Trie& data) { if (data.eow == 1) return word + " "; for (int i = 0; i < 26; i++) { if (data.letters[i] != NULL) printAll( word + data.letters[i]->letter, *(data.letters[i])); } return ""; } Its not outputting what i want, any suggestions?

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• Pruning: When to Stop?

  - by cam

  When does pruning stop being efficient in a depth-first search? I've been working on an efficient method to solve the N-Queens problem and I'm looking at pruning for the first time. I've implemented it for the first two rows, but when does it stop being efficient? How far should I prune to?

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• Refresh decorator

  - by Morgoth

  I'm trying to write a decorator that 'refreshes' after being called, but where the refreshing only occurs once after the last function exits. Here is an example: @auto_refresh def a(): print "In a" @auto_refresh def b(): print "In b" a() If a() is called, I want the refresh function to be run after exiting a(). If b() is called, I want the refresh function to be run after exiting b(), but not after a() when called by b(). Here is an example of a class that does this: class auto_refresh(object): def __init__(self, f): print "Initializing decorator" self.f = f def __call__(self, *args, **kwargs): print "Before function" if 'refresh' in kwargs: refresh = kwargs.pop('refresh') else: refresh = False self.f(*args, **kwargs) print "After function" if refresh: print "Refreshing" With this decorator, if I run b() print '---' b(refresh=True) print '---' b(refresh=False) I get the following output: Initializing decorator Initializing decorator Before function In b Before function In a After function After function --- Before function In b Before function In a After function After function Refreshing --- Before function In b Before function In a After function After function So when written this way, not specifying the refresh argument means that refresh is defaulted to False. Can anyone think of a way to change this so that refresh is True when not specified? Changing the refresh = False to refresh = True in the decorator does not work: Initializing decorator Initializing decorator Before function In b Before function In a After function Refreshing After function Refreshing --- Before function In b Before function In a After function Refreshing After function Refreshing --- Before function In b Before function In a After function Refreshing After function because refresh then gets called multiple times in the first and second case, and once in the last case (when it should be once in the first and second case, and not in the last case).

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• Zoom image to pixel level

  - by zaf

  For an art project, one of the things I'll be doing is zooming in on an image to a particular pixel. I've been rubbing my chin and would love some advice on how to proceed. Here are the input parameters: Screen: sw - screen width sh - screen height Image: iw - image width ih - image height Pixel: px - x position of pixel in image py - y position of pixel in image Zoom: zf - zoom factor (0.0 to 1.0) Background colour: bc - background colour to use when screen and image aspect ratios are different Outputs: The zoomed image (no anti-aliasing) The screen position/dimensions of the pixel we are zooming to. When zf is 0 the image must fit the screen with correct aspect ratio. When zf is 1 the selected pixel fits the screen with correct aspect ratio. One idea I had was to use something like povray and move the camera towards a big image texture or some library (e.g. pygame) to do the zooming. Anyone think of something more clever with simple pseudo code? To keep it more simple you can make the image and screen have the same aspect ratio. I can live with that. I'll update with more info as its required.

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• How to find nth element from the end of a singly linked list?

  - by Codenotguru

  The following function is trying to find the nth to last element of a singly linked list. For example: If the elements are 8->10->5->7->2->1->5->4->10->10 then the result is 7th to last node is 7. Can anybody help me on how this code is working or is there a better and simpler approach? LinkedListNode nthToLast(LinkedListNode head, int n) { if (head == null || n < 1) { return null; } LinkedListNode p1 = head; LinkedListNode p2 = head; for (int j = 0; j < n - 1; ++j) { // skip n-1 steps ahead if (p2 == null) { return null; // not found since list size < n } p2 = p2.next; } while (p2.next != null) { p1 = p1.next; p2 = p2.next; } return p1; }

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• Accenture interview question - Find the only unpaired element in the array

  - by karank

  You have been given an array of size 2n+1 that have n pair of integers(can be +ve, -ve or 0) and one unpaired element. How would you find the unpaired element. Pair means duplicate. So (3,3) is a pair and (3,-3) is not a pair.

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• MATLAB: Reading floating point numbers and strings from a file

  - by xsound

  I am using the following functions for writing and reading 4098 floating point numbers in MATLAB: Writing: fid = fopen(completepath, 'w'); fprintf(fid, '%1.30f\r\n', y) Reading: data = textread(completepath, '%f', 4098); where y contains 4098 numbers. I now want to write and read 3 strings at the end of this data. How do I read two different datatypes? Please help me. Thanks in advance.

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• Need Urgent Help! Find a repeated numbers out of 3 boxes

  - by james1

  Hi guys, I really need help with finding a repeated number out of 3 boxes. Let's say that i have 3 boxes, each box contain 10 piece of numbered paper (1 - 10) but there is a number the same in all 3 boxes eg: box1 has number 4 and box2 has number 4 and box3 also has number 4. How to find that repeated number in java with an efficient/fastest way possible? Thanks in advance!

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• Random access gzip stream

  - by jkff

  I'd like to be able to do random access into a gzipped file. I can afford to do some preprocessing on it (say, build some kind of index), provided that the result of the preprocessing is much smaller than the file itself. Any advice? My thoughts were: Hack on an existing gzip implementation and serialize its decompressor state every, say, 1 megabyte of compressed data. Then to do random access, deserialize the decompressor state and read from the megabyte boundary. This seems hard, especially since I'm working with Java and I couldn't find a pure-java gzip implementation :( Re-compress the file in chunks of 1Mb and do same as above. This has the disadvantage of doubling the required disk space. Write a simple parser of the gzip format that doesn't do any decompressing and only detects and indexes block boundaries (if there even are any blocks: I haven't yet read the gzip format description)

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• red black tree balancing?

  - by Anirudh Kaki

  i am working to generate tango tree, where i need to check whether every sub tree in tango is balanced or not. if its not balanced i need to make it balance? I trying so hard to make entire RB-tree balance but i not getting any proper logic so can any one help me out?? here i am adding code to check how to find my tree is balanced are not but when its not balanced how can i make it balance. static boolean verifyProperty5(rbnode n) { int left = 0, right = 0; if (n != null) { bh++; left = blackHeight(n.left, 0); right = blackHeight(n.right, 0); } if (left == right) { System.out.println("black height is :: " + bh); return true; } else { System.out.println("in balance"); return false; } } public static int blackHeight(rbnode root, int len) { bh = 0; blackHeight(root, path1, len); return bh; } private static void blackHeight(rbnode root, int path1[], int len) { if (root == null) return; if (root.color == "black"){ root.black_count = root.parent.black_count+1; } else{ root.black_count = root.parent.black_count; } if ((root.left == null) && (root.right == null)) { bh = root.black_count; } blackHeight(root.left, path1, len); blackHeight(root.right, path1, len); }

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• Minimum cost strongly connected digraph

  - by Kazoom

  I have a digraph which is strongly connected (i.e. there is a path from i to j and j to i for each pair of nodes (i, j) in the graph G). I wish to find a strongly connected graph out of this graph such that the sum of all edges is the least. To put it differently, I need to get rid of edges in such a way that after removing them, the graph will still be strongly connected and of least cost for the sum of edges. I think it's an NP hard problem. I'm looking for an optimal solution, not approximation, for a small set of data like 20 nodes. Edit A more general description: Given a grap G(V,E) find a graph G'(V,E') such that if there exists a path from v1 to v2 in G than there also exists a path between v1 and v2 in G' and sum of each ei in E' is the least possible. so its similar to finding a minimum equivalent graph, only here we want to minimize the sum of edge weights rather than sum of edges. Edit: My approach so far: I thought of solving it using TSP with multiple visits, but it is not correct. My goal here is to cover each city but using a minimum cost path. So, it's more like the cover set problem, I guess, but I'm not exactly sure. I'm required to cover each and every city using paths whose total cost is minimum, so visiting already visited paths multiple times does not add to the cost.

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• calculating parameters for defining subsections of quadratic bezier curves

  - by jedierikb

  I have a quadratic bezier curve described as (startX, startY) to (anchorX, anchorY) and using a control point(controlX, controlY). I have two questions: (1) I want to determine y points on that curve based on an x point. (2) Then, given two intermediary points on that bezier curve, I want to know the 6 parameters needed to create that mini-bezier curve.

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• Find three numbers appeared only once

  - by shilk

  In a sequence of length n, where n=2k+3, that is there are k unique numbers appeared twice and three numbers appeared only once. The question is: how to find the three unique numbers that appeared only once? for example, in sequence 1 1 2 6 3 6 5 7 7 the three unique numbers are 2 3 5. Note: 3<=n<1e6 and the number will range from 1 to 2e9 Memory limits: 1000KB , this implies that we can't store the whole sequence. Method I have tried(Memory limit exceed): I initialize a tree, and when read in one number I try to remove it from the tree, if the remove returns false(not found), I add it to the tree. Finally, the tree has the three numbers. It works, but is Memory limit exceed. I know how to find one or two such number(s) using bit manipulation. So I wonder if we can find three using the same method(or some method similar)? Method to find one/two number(s) appeared only once: If there is one number appeared only once, we can apply XOR to the sequence to find it. If there are two, we can first apply XOR to the sequence, then separate the sequence into 2 parts by one bit of the result that is 1, and again apply XOR to the 2 parts, and we will find the answer.

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