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• C++ typedef for partial templates

  - by Gokul

  Hi, i need to do a typedef like this. template< class A, class B, class C > class X { }; template< class B, class C > typedef X< std::vector<B>, B, C > Y; I just found that it is not supported in C++. Can someone advise me on how to achieve the same through alternative means? Thanks, Gokul.

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• struct with template variables in c++

  - by monkeyking

  I'm playing around with template, so I'm not trying to reinvent the std::vector, I'm trying to get a grasp of templateting in c++. Can I do the following template <typename T> typedef struct{ size_t x; T *ary; }array; What I'm trying to do is a basic templated version of typedef struct{ size_t x; int *ary; }iArray; It looks like its working if I use a class instead of struct, so is it not possible with typedef structs? thanks

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• [MATH]Project an axis to a plane

  - by Robert

  how do i project the X axis to a plane if the plane is defined by three points? see pic here: http://129.25.16.135:2080/v6.5/books/usb/graphics/iconventions-local-surfaces.png The default local 1-direction is the projection of the global x-axis onto the surface. If the global x-axis is within 0.1° of being normal to the surface, the local 1-direction is the projection of the global z-axis onto the surface. let's suppose that these points are: t1 = [-0.362879991531372, 0.357021987438202, -0.373737007379532] t2 = [-0.383525013923645, 0.371621012687683, -0.383549988269806] t3 = [-0.383534014225006, 0.34629300236702, -0.38544899225235] is the vector [0.78280971952246, -0.0307519963686645, 0.411184845614438] correct answer in this case? i've calculated the angle between the surf and x (1,0,0) is ~28°. and can you give the entire procedure for the calculation, because i'm just puzzled.

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• C++: need indexed set

  - by user231536

  I need an indexed associative container that operates as follows: initially empty, size=0. when I add a new element to it, it places it at index [size], very similar to a vector's push_back. It increments the size and returns the index of the newly added element. if the element already exists, it returns the index where it occurs. Set seems the ideal data structure for this but I don't see any thing like getting an index from a find operation. Find on a set returns an iterator to the element. Will taking the difference with set.begin() be the correct thing to do in this situation?

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• algorithms that destruct and copy_construct

  - by FredOverflow

  I am currently building my own toy vector for fun, and I was wondering if there is something like the following in the current or next standard or in Boost? template<class T> void destruct(T* begin, T* end) { while (begin != end) { begin -> ~T(); ++begin; } } template<class T> T* copy_construct(T* begin, T* end, T* dst) { while (begin != end) { new(dst) T(*begin); ++begin; ++dst; } return dst; }

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• Error occurs while using SPADE method in R

  - by Yuwon Lee

  I'm currently mining sequence patterns using SPADE algorithm in R. SPADE is included in "arulesSequence" package of R. I'm running R on my CentOS 6.3 64bit. For an exercise, I've tried an example presented in http://en.wikibooks.org/wiki/Data_Mining_Algorithms_In_R/Sequence_Mining/SPADE When I tried to do "cspade(x, parameter = list(support = 0.4), control = list(verbose = TRUE))" R says: parameter specification: support : 0.4 maxsize : 10 maxlen : 10 algorithmic control: bfstype : FALSE verbose : TRUE summary : FALSE preprocessing ... 1 partition(s), 0 MB [0.096s] mining transactions ... 0 MB [0.066s] reading sequences ...Error in asMethod(object) : 's' is not an integer vector When I try to run SPADE on my Window 7 32bit, it runs well without any error. Does anybody know why such errors occur?

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• How would you write this Clojure snippet in Ruby and/or Haskell?

  - by dnolen

  I was working on a Rails template and was trying to write a bit of code that allows me to populate a table or multiple columns of ul tags "top-to-bottom" and "left-to-right" across however many columns I specify. I'm just getting the hang of Ruby so I couldn't figure this out. I'm also curious about an idiomatic Haskell version for this useful snippet. Improvements to Clojure version appreciated: (defn table [xs & {:keys [cols direction] :or {cols 1 direction 'right}}] (into [] (condp = direction 'down (let [c (count xs) q (int (/ c cols)) n (if (> (mod c q) 0) (inc q) q)] (apply map vector (partition n n (repeat nil) xs))) 'right (map vec (partition cols cols (repeat nil) xs))))) With this bit of code I can then do the following: (table (range 10) :cols 3) Printed out this would look like so: 0 1 2 3 4 5 6 7 8 9 And the trickier one: (table (range 10) :cols 3 :direction 'down) Looks like so: 0 4 8 1 5 9 2 6 3 7

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• MATLAB pdist function

  - by James

  Hi, I am using the pdist command to find the distance between x and y coordinates stored in a matrix. X = [100 100; 0 100; 100 0; 500 400; 300 600;]; D = pdist(X,'euclidean') Which returns a 15 element vector. : [0.734979755525412 3.40039811339820 2.93175207511321 1.83879677592575 2.40127440268306 2.75251513299386 2.21488402640753 1.10610649500317 1.81674017301699 0.903207751535635 1.99116952754924 1.05069952386082 1.24122819418333 1.08583377275532 1.38729428638035] Is there a way to associate these distances with the coordinates they were derived from, i.e. store them in a matrix with the general row form: [Length xcoordinate1 ycoordinate1 xcoordinate2 ycoordinate2] Where there is a row for each length found? Thanks in advance

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• What is most efficient way of setting row to zeros for a sparce scipy matrix?

  - by Alex Reinking

  I'm trying to convert the following MATLAB code to Python and am having trouble finding a solution that works in any reasonable amount of time. M = diag(sum(a)) - a; where = vertcat(in, out); M(where,:) = 0; M(where,where) = 1; Here, a is a sparse matrix and where is a vector (as are in/out). The solution I have using Python is: M = scipy.sparse.diags([degs], [0]) - A where = numpy.hstack((inVs, outVs)).astype(int) M = scipy.sparse.lil_matrix(M) M[where, :] = 0 # This is the slowest line M[where, where] = 1 M = scipy.sparse.csc_matrix(M) But since A is 334863x334863, this takes like three minutes. If anyone has any suggestions on how to make this faster, please contribute them! For comparison, MATLAB does this same step imperceptibly fast. Thanks!

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• specyfic syntax question

  - by bua

  Hi there, Is it possible to create template to the initialization like: template <typename C> typename C::value_type fooFunction(C& c) {...}; std::vector<string> vec_instance; fooFunction(cont<0>(vec_instance)); fooFunction(cont<1>(vec_instance)); In general i'm interested is it possible to specify template using integer (ie. 0) instead of true type name. And how to achieve above?

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• inputMismatchException Java reading doubles from plain text file

  - by user939287

  Using double variable = inputFile.nextDouble(); Gives the mismatch error and I can't figure out why... Anyone know what's up? The input file is just a bunch of doubles like 5.0... Okay here is the code snippet String fileName; Scanner scanner = new Scanner(System.in); System.out.println("\nEnter file name that contains the matrix and vector: "); fileName = scanner.nextLine(); Scanner inputFile = new Scanner(fileName); double a1 = inputFile.nextDouble(); the input file is a plain text document .txt in this format 5.0 4.0 -3.0 4.0 2.0 5.0 6.0 5.0 -2.0 -13.0 4.0 12.0 I don't understand why it wouldn't take those as doubles... As far as what its expecting the format of the file to be... I suppose binary? isn't that the default? I didn't specify in the code...

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• [C++] Why am I not getting the same values?

  - by xbonez

  I am using Bode's formuala to calculate distance of nth planet from sun dist = (4 + 3*(2^(n-2)))/10 If I calculate the distance this way, I get the right values: dist[2] = ((4 + 3*1)/10.0) ; dist[3] = ((4 + 3*2)/10.0) ; dist[4] = ((4 + 3*4)/10.0) ; But doing it this way, gives me incorrect values: vector double> dist(5); for (unsigned int i = 2; i < 5; i++) { dist[i] = ((4 + 3*(2^(3-2)))/10.0) ; } ` Why so?

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• C++ - Efficient container for large amounts of searchable data?

  - by Francisco P.

  Hello, everybody! I am implementing a text-based version of Scrabble for a College project. My dictionary is quite large, weighing in at around 400.000 words (std::string). Searching for a valid word will suck, big time, in terms of efficiency if I go for a vector<string> ( O(n) ). Are there any good alternatives? Keep in mind, I'm enrolled in freshman year. Nothing TOO complex! Thanks for your time! Francisco

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• C++ - How to call a member function for an inherited object.

  - by Francisco P.

  Hello! I have a few classes (heat, gas, contact, pressure) inheriting from a main one (sensor). I have a need to store them in a vector<Sensor *> (part of the specification). At some point in time, I need to call a function that indiscriminately stores those Sensor *. (also part of the specification, not open for discussion) Something like this: for(size_t i = 0; i < Sensors.size(); ++i) Sensors[i]->storeSensor(os) //os is an ofstream kind of object, passed onwards by reference Where and how shall storeSensor be defined? Is there any simple way to do this or will I need to disregard the specification? Mind you, I'm a beginner! Thanks for your time!

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• What is wrong with my loop?

  - by user3966541

  I have the following loop and don't understand why it only runs once: std::vector<sf::RectangleShape> shapes; const int res_width = 640; const int res_height = 480; for (int x = 0; x < res_width / 50; x += 50) { for (int y = 0; y < res_height / 50; y += 50) { sf::RectangleShape shape(sf::Vector2f(50, 50)); shape.setPosition(x * 50, y * 50); sf::Color color = (x % 2 == 0) ? sf::Color::Green : sf::Color::Red; shape.setFillColor(sf::Color::Green); shapes.push_back(shape); } }

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• lambda+for_each+delete on STL containers

  - by rubenvb

  I'm trying to get a simple delete every pointer in my vector/list/... function written with an ultra cool lambda function. Mind you, I don't know c**p about those things :) template <typename T> void delete_clear(T const& cont) { for_each(T.begin(), T.end(), [](???){ ???->delete() } ); T.clear(); } I have no clue what to fill in for the ???'s. Any help is greatly appreciated!

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• C++ Questions about vectors

  - by xbonez

  Hey guys, I have a CS exam tomorrow. Just want to get a few questions cleared up. Thanks a lot, and I really appreciate the help. Que 1. What are parallel vectors? Vectors of the same length that contain data that is meant to be processed together Vectors that are all of the same data type Vectors that are of the same length Any vector of data type parallel Que 2. Arrays are faster and more efficient than vectors. True False Que 3. Arrays can be a return type of a function call. True False Que 4. Vectors can be a return type of a function call. True False

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• Why is the destructor of the class called twice ?

  - by dicaprio

  Apologies if the question sounds silly, I was following experts in SO and trying some examples myself, and this is one of them. I did try the search option but didn't find an answer for this kind. class A { public: A(){cout<<"A Contruction"<<endl;} ~A(){cout<<"A destruction"<<endl;} }; int main() { vector<A> t; t.push_back(A()); // After this line, when the scope of the object is lost. } Why is the destructor of the class called twice ?

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• what to use in place of std::map::emplace?

  - by kfmfe04

  For containers such as std::map< std::string, std::unique_ptr< Foo >>, it looks like emplace() has yet to be implemented in stdc++ as of gcc 4.7.2. Unfortunately, I can't store Foo directly by value as it is an abstract super-class. As a simple, but inefficient, place-holder, I've just been using std::map< std::string, Foo* > in conjunction with a std::vector< std::unique_ptr< Foo >> for garbage collection. Do you have a interim solution that is more efficient and more easily replaced once emplace() is available?

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• C++: Copying to dereferenced pointer...

  - by bbb

  Hi. I currently have a weird problem with a program segfaulting but im not able to spot the error. I think the problem boils down to this. struct S {int a; vector<sometype> b;} S s1; // fill stuff into a and b S* s2 = new S(); *s2 = s1; Could it be that the final copying is illegal in some way? Im really confused right now... Thanks

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• Why only random-access-iterator implements operator+ in C++?

  - by xopht

  I'd like get far next value for STL list iterator but it doesn't implement operator+, vector has it though. Why and how can I get the value where I want? I think I can do that if I call operator++ several times, but isn't that a little bit dirty? What I want to do is the following: list<int> l; ...omitted... list<int>::iterator itr = l.begin() + 3; // but, list iterator does not have // operator+ What is the best solution for what I want?

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• partial string matching - R

  - by DonDyck

  I need to write a query in R to match partial string in column names. I am looking for something similar to LIKE operator in SQL. For e.g, if I know beginning, middle or end part of the string I would write the query in format: LIKE 'beginning%middle%' in SQL and it would return matching strings. In pmatch or grep it seems I can only specify 'beginning' , 'end' and not the order. Is there any similar function in R that I am looking for? For example, say I am looking in the vector: y<- c("I am looking for a dog", "looking for a new dog", "a dog", "I am just looking") Lets say I want to write a query which picks "looking for a new dog" and I know start of the string is "looking" and end of string is "dog". If I do a grep("dog",y) it will return 1,2,3. Is there any way I can specify beginning and end in grep?

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• C++ setting up "flags"

  - by sub

  Example: enum Flags { A, B, C, D }; class MyClass { std::string data; int foo; // Flags theFlags; (???) } How can I achieve that it is possible to set any number of the "flags" A,B,C and D in the enum above in an instance of MyClass? My goal would be something like this: if ( MyClassInst.IsFlagSet( A ) ) // ... MyClassInst.SetFlag( A ); //... Do I have to use some array or vector? If yes, how? Are enums a good idea in this case?

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• Clojure: find repetition

  - by demi

  Let we have a list of integers: 1, 2, 5, 13, 6, 5, 7 and I want to find the first number has a duplicate before it and return a vector of two indices, In my sample, it's 5 at [2, 5]. What I did so far is loop, but can I do it more elegant, short way? (defn get-cycle [xs] (loop [[x & xs_rest] xs, indices {}, i 0] (if (nil? x) [0 i] ; Sequence is over before we found a duplicate. (if-let [x_index (indices x)] [x_index i] (recur xs_rest (assoc indices x i) (inc i)))))) No need to return number itself, because I can get it by index and, second, it may be not always there.

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• C++: String and unions

  - by sub

  I'm having a (design) problem: I'm building an interpreter and I need some storage for variables. There are basically two types of content a variable here can have: string or int. I'm using a simple class for the variables, all variables are then stored in a vector. However, as a variable can hold a number or a string, I don't want C++ to allocate both and consume memory for no reason. That's why I wanted to use unions: union { string StringValue; int IntValue; } However, strings don't work with unions. Is there any workaround so no memory gets eaten for no reason?

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