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  • Purely functional equivalent of weakhashmap?

    - by Jon Harrop
    Weak hash tables like Java's weak hash map use weak references to track the collection of unreachable keys by the garbage collector and remove bindings with that key from the collection. Weak hash tables are typically used to implement indirections from one vertex or edge in a graph to another because they allow the garbage collector to collect unreachable portions of the graph. Is there a purely functional equivalent of this data structure? If not, how might one be created? This seems like an interesting challenge. The internal implementation cannot be pure because it must collect (i.e. mutate) the data structure in order to remove unreachable parts but I believe it could present a pure interface to the user, who could never observe the impurities because they only affect portions of the data structure that the user can, by definition, no longer reach.

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  • Would this predicate work ? this came on my quiz to find the position of element X in a data structure called list

    - by M.K
    example: position(1,list(1,list(2,nil)),Z). Z = 1. position(3,list(1,list(2,list(3,nil)),Z). Z = 3. where Z is the position of element X in a data structure of a list in the above format Here was my solution: position(X,list(nil),0). %empty list position(X,list(X,T),1). %list with X as head or first element position(X,list(H,T),Z):- position(X,list(T,nil),Z1), %X is in tail of list (H,T) Z is Z1 + 1.

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  • [C++] Needed: A simple C++ container (stack, linked list) that is thread-safe for writing

    - by conradlee
    I am writing a multi-threaded program using OpenMP in C++. At one point my program forks into many threads, each of which need to add "jobs" to some container that keeps track of all added jobs. Each job can just be a pointer to some object. Basically, I just need the add pointers to some container from several threads at the same time. Is there a simple solution that performs well? After some googling, I found that STL containers are not thread-safe. Some stackoverflow threads address this question, but none form a consensus on a simple solution.

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  • How does including a .csv work in an enum?

    - by Tommy
    enum ID // IDs { ID_HEADER = 0, // ID 0 = headers #include "DATA.CSV" ID_LIMIT }; I inherited some code here..... Looking at "DATA.CSV" I see all the ID's used to populate the enum in column B, along with other data. My question: How does the enum know that it is using "column B" to retrieve it's members? There must be some other logic in the application yet I don't see it. What else should I look for? Thanks.

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  • Using unions to simplify casts

    - by Steven Lu
    I realize that what I am trying to do isn't safe. But I am just doing some testing and image processing so my focus here is on speed. Right now this code gives me the corresponding bytes for a 32-bit pixel value type. struct Pixel { unsigned char b,g,r,a; }; I wanted to check if I have a pixel that is under a certain value (e.g. r, g, b <= 0x10). I figured I wanted to just conditional-test the bit-and of the bits of the pixel with 0x00E0E0E0 (I could have wrong endianness here) to get the dark pixels. Rather than using this ugly mess (*((uint32_t*)&pixel)) to get the 32-bit unsigned int value, i figured there should be a way for me to set it up so I can just use pixel.i, while keeping the ability to reference the green byte using pixel.g. Can I do this? This won't work: struct Pixel { unsigned char b,g,r,a; }; union Pixel_u { Pixel p; uint32_t bits; }; I would need to edit my existing code to say pixel.p.g to get the green color byte. Same happens if I do this: union Pixel { unsigned char c[4]; uint32_t bits; }; This would work too but I still need to change everything to index into c, which is a bit ugly but I can make it work with a macro if i really needed to.

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  • Storing variables for app access

    - by Pavel
    Hi there everyone. I'm kinda noob to android so please bear with me. I'm currently developing app which uses tabs. My question now is: how to store values of variables so I can access them on the other tabs? I want to create something similar to sessions in PHP where I can save variables on one page and access on the other. Please can someone help me with this? There has to be an easy way to solve this. Thanks in advance!

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  • How to represent a list of points in R

    - by Guido
    I am working with a large list of points (each point has three dimensions x,y,z). I am pretty new with R, so I would like to know what is the best way to represent that kind of information. As far as I know, an array allows me to represent any multidimensional data, so currently I am using: > points<-array( c(1,2,0,1,3,0,2,4,0,2,5,0,2,7,0,3,8,0), dim=c(3,6) ) > points [,1] [,2] [,3] [,4] [,5] [,6] [1,] 1 1 2 2 2 3 -- x dim [2,] 2 3 4 5 7 8 -- y dim [3,] 0 0 0 0 0 0 -- z dim The aim is to perform some computations to calculate the euclidean distance between two sets of points (any hint in this sense would also be highly appreciated)

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  • How to get results efficiently out of an Octree/Quadtree?

    - by Reveazure
    I am working on a piece of 3D software that has sometimes has to perform intersections between massive numbers of curves (sometimes ~100,000). The most natural way to do this is to do an N^2 bounding box check, and then those curves whose bounding boxes overlap get intersected. I heard good things about octrees, so I decided to try implementing one to see if I would get improved performance. Here's my design: Each octree node is implemented as a class with a list of subnodes and an ordered list of object indices. When an object is being added, it's added to the lowest node that entirely contains the object, or some of that node's children if the object doesn't fill all of the children. Now, what I want to do is retrieve all objects that share a tree node with a given object. To do this, I traverse all tree nodes, and if they contain the given index, I add all of their other indices to an ordered list. This is efficient because the indices within each node are already ordered, so finding out if each index is already in the list is fast. However, the list ends up having to be resized, and this takes up most of the time in the algorithm. So what I need is some kind of tree-like data structure that will allow me to efficiently add ordered data, and also be efficient in memory. Any suggestions?

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  • Clojure: I have many sorted maps and want to reduce in order all there values a super maps of keys -> vector

    - by Alex Foreman
    I have seen this but can't work out how to apply it (no pun intended) to my situation. I have a sorted list of maps like this: (note there can be more than two keys in the map) ({name1 3, name2 7}, {name1 35, name2 7}, {name1 0, name2 3}) What I am after is this data structure afterwards: ({:name1 [3,35,0]}, {:name2 [7,7,3]}) Ive been struggling with this for a while and cant seem to get anywhere near. Caveats: The data must stay sorted and I have N keywords not just two.

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  • NoSQL vs Relational Coding Styles

    - by Chris Henry
    When building objects that make use of data stored in a RDBMS, it's normally pretty clear what you're getting back, as dictated by the tables and columns being queried. However, when dealing with NoSQL, document-based systems, it's less clear what is being retrieved. What are common methods of keeping track of structure in which data is stored?

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  • Java Map question

    - by user552961
    I have one Map that contains some names and numbers Map<String,Integer> abc = new TreeMap<String,Integer>(); It works fine. I can put some values in it but when I call it in different class it gives me wrong order. For example: I putted abc.put("a",1); abc.put("b",5); abc.put("c",3); some time it returns the order (b,a,c) and some time (a,c,b). What is wrong with it? Is there any step that I am missing when I call this map?

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  • Strange code behaviour?

    - by goldenmean
    Hi, I have a C code in which i have a structure declaration which has an array of int[576] declared in it. For some reason, i had to remove this array from the structure, So i replaced this array with a pointer as int *ptr; declared some global array of same type, somewhere else in the code, and initialized this pointer by assigning the global array to this pointer. So i did not have to change the way i was accessing this array, from other parts of my code. But it works fine/gives desired output when i have the array declared in the structure, but it gives junk output when i declare it as a pointer in the structure and assign a global array to this pointer, as a part of the pointer initialization. All this code is being run on MS-VC 6.0/Windows setup/Intel-x86. I tried below things: 1)Suspected structure padding/alignment but could not get any leads? If at all structure alignment could be a culprit how can i proceed to narrow it down and confirm it? 2) I have made sure that in both cases the array is initialized to some default values, say 0 before its first use, and its not being used before initialization. 3)I tried using global array as well as malloc based memory for this newly declared array. Same result, junk output. Am i missing something? How can i zero down the problem. Any pointers would be helpful. Thanks, -AD.

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  • Flexible array members in C - bad?

    - by Lionel
    I recently read that using flexible array members in C was poor software engineering practice. However, that statement was not backed by any argument. Is this an accepted fact? (Flexible array members are a C feature introduced in C99 whereby one can declare the last element to be an array of unspecified size. For example: ) struct header { size_t len; unsigned char data[]; };

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  • Data Structure / Hash Function to link Sets of Ints to Value

    - by Gaminic
    Given n integer id's, I wish to link all possible sets of up to k id's to a constant value. What I'm looking for is a way to translate sets (e.g. {1, 5}, {1, 3, 5} and {1, 2, 3, 4, 5, 6, 7}) to unique values. Guarantees: n < 100 and k < 10 (again: set sizes will range in [1, k]). The order of id's doesn't matter: {1, 5} == {5, 1}. All combinations are possible, but some may be excluded. All sets and values are constant and made only once. No deletes or inserts, no value updates. Once generated, the only operations taking place will be look-ups. Look-ups will be frequent and one-directional (given set, look up value). There is no need to sort (or otherwise organize) the values. Additionally, it would be nice (but not obligatory) if "neighboring" sets (drop one id, add one id, swap one id, etc) are easy to reach, as well as "all sets that include at least this set". Any ideas?

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  • how to order string logically

    - by just_name
    Q: I have the following case : set of letters (grades) A,A+,A-,B,B+,B- stored as strings in the database i wanna to order these grades logically from the small one to the big one ,, but this not what happen in real.. because these are strings the order is: A,A+,A- i wanna ASC A-,A,A+ DESC A+,A,A- i bind those grades in drop down list and i wanna these grades with this logical order in it.. is there any idea how to do something like this..

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  • Vectors or Java arrays for Tetris?

    - by StackedCrooked
    I'm trying to create a Tetris-like game with Clojure and I'm having some trouble deciding the data structure for the playing field. I want to define the playing field as a mutable grid. The individual blocks are also grids, but don't need to be mutable. My first attempt was to define a grid as a vector of vectors. For example an S-block looks like this: :s-block { :grids [ [ [ 0 1 1 ] [ 1 1 0 ] ] [ [ 1 0 ] [ 1 1 ] [ 0 1 ] ] ] } But that turns out to be rather tricky for simple things like iterating and painting (see the code below). For making the grid mutable my initial idea was to make each row a reference. But then I couldn't really figure out how to change the value of a specific cell in a row. One option would have been to create each individual cell a ref instead of each row. But that feels like an unclean approach. I'm considering using Java arrays now. Clojure's aget and aset functions will probably turn out to be much simpler. However before digging myself in a deeper mess I want to ask ideas/insights. How would you recommend implementing a mutable 2d grid? Feel free to share alternative approaches as well. Source code current state: Tetris.clj (rev452)

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  • Ordered Data Structure that allows to efficiently remove duplicate items

    - by devoured elysium
    I need a data structure that Must be ordered (adding elements a, b and c to an empty structure, will make them be at positions 0, 1 and 2). Allows to add repeated items. This is, I can have a list with a, b, c, a, b. Allows removing all ocurrences of a given item (if I do something like delete(1), it will delete all ocurrences of 1 in the structure). I can't really pick what the best data structure could be in here. I thought at first about something like a List(the problem is having an O(n) operation when removing items), but maybe I'm missing something? What about trees/heaps? Hashtables/maps? I'll have to assume I'll do as much adding as removing with this data structure. Thanks

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  • Efficiently fill resultset in object model

    - by Zeeshan
    Hi, I have an object model whose structure is Dashboard  List of panels     List of containers       List of widgets If i get whole dashboard, with panels + containers + widgets, from Database then multiple I/O requires I want to get it in one I/O .For this i prepared a query which gives me this resultset. DASHBOARDID   PANELID  CONTAINERID  WIDGETID 13                          11                    5              2 13                          11                    5              3 13                          11                    6              4 13                          11                    6              5 13                          12                    7              6 13                          12                    7              7 13                          12                    8              8 13                          12                    8              9 Using list datastructure this model is able to be filled but it takes time i want to efficiently fill this resultset in above object model. is there nay way ?

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  • What is a good java data structure for storing nested items (like cities in states)?

    - by anotherAlan
    I'm just getting started in Java and am looking for advice on a good way to store nested sets of data. For example, I'm interested in storing city population data that can be accessed by looking up the city in a given state. (Note: eventually, other data will be stored with each city as well, this is just the first attempt at getting started.) The current approach I'm using is to have a StateList Object which contains a HashMap that stores State Objects via a string key (i.e. HashMap<String, State>). Each State Object contains its own HashMap of City Objects keyed off the city name (i.e. HashMap<String, City>). A cut down version of what I've come up with looks like this: // TestPopulation.java public class TestPopulation { public static void main(String [] args) { // build the stateList Object StateList sl = new StateList(); // get a test state State stateAl = sl.getState("AL"); // make sure it's there. if(stateAl != null) { // add a city stateAl.addCity("Abbeville"); // now grab the city City cityAbbevilleAl = stateAl.getCity("Abbeville"); cityAbbevilleAl.setPopulation(2987); System.out.print("The city has a pop of: "); System.out.println(Integer.toString(cityAbbevilleAl.getPopulation())); } // otherwise, print an error else { System.out.println("That was an invalid state"); } } } // StateList.java import java.util.*; public class StateList { // define hash map to hold the states private HashMap<String, State> theStates = new HashMap<String, State>(); // setup constructor that loads the states public StateList() { String[] stateCodes = {"AL","AK","AZ","AR","CA","CO"}; // etc... for (String s : stateCodes) { State newState = new State(s); theStates.put(s, newState); } } // define method for getting a state public State getState(String stateCode) { if(theStates.containsKey(stateCode)) { return theStates.get(stateCode); } else { return null; } } } // State.java import java.util.*; public class State { // Setup the state code String stateCode; // HashMap for cities HashMap<String, City> cities = new HashMap<String, City>(); // define the constructor public State(String newStateCode) { System.out.println("Creating State: " + newStateCode); stateCode = newStateCode; } // define the method for adding a city public void addCity(String newCityName) { City newCityObj = new City(newCityName); cities.put(newCityName, newCityObj); } // define the method for getting a city public City getCity(String cityName) { if(cities.containsKey(cityName)) { return cities.get(cityName); } else { return null; } } } // City.java public class City { // Define the instance vars String cityName; int cityPop; // setup the constructor public City(String newCityName) { cityName = newCityName; System.out.println("Created City: " + newCityName); } public void setPopulation(int newPop) { cityPop = newPop; } public int getPopulation() { return cityPop; } } This is working for me, but I'm wondering if there are gotchas that I haven't run into, or if there are alternate/better ways to do the same thing. (P.S. I know that I need to add some more error checking in, but right now, I'm focused on trying to figure out a good data structure.) (NOTE: Edited to change setPop() and getPop() to setPopulation() and getPopulation() respectively to avoid confucsion)

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  • Possible ways of representing data in memory (.net)

    - by This is it
    Hi What are the possible ways of representing data in memory in .Net (or in general)? It would be great if data could be sorted and looked up by key (or multiple keys). We are thinking to use collections, arrays, list of collections/arrays. One object would be in several collections (one sorted asc, other desc, etc.). Maybe this is not a good idea, and we would like to hear some other possible solutions. Thank you

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  • Small questions on data structure

    - by John Graveston
    Hi, I'm trying to search the parent of a node with Kruskal's algorithm. My program works just fine, but I think I have heard of a method to improve the speed of the algorithm by reconstructing the tree while searching for the parent node and connecting it to the parent node. I'm pretty sure that I've heard of this somewhere, maybe in a lecture. Can anyone refresh my memory? And also, given a number of arrays, when searching for the minimum and the maximum value from a certain section of an array, what is the name of the tree that can calculate the minimum/maximum value from the array by making a binary tree that has the minimum/maximum value of each array in O(log N)?

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