C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  two-dimentional array with non constant variables          C++Talk.NET Forum Index -> C++ language (comp.lang.c++) View previous topic :: View next topic   Author Message Al Guest Posted: Thu Oct 27, 2005 12:59 pm    Post subject: two-dimentional array with non constant variables I'd like to declare (in a Matrix class) a two-dimentional array with user-defined coordinates. The constructor is: Matrix(int c, int l): col(c), lin(l), a(new float[col][lin]) {} the compiler says 'lin' should be a constant, but I want it to be defined from the user, so what should I do? and how must 'a' be declared?? help me please Back to top Kai-Uwe Bux Guest Posted: Thu Oct 27, 2005 1:18 pm    Post subject: Re: two-dimentional array with non constant variables Al wrote: Quote: I'd like to declare (in a Matrix class) a two-dimentional array with user-defined coordinates. The constructor is: Matrix(int c, int l): col(c), lin(l), a(new float[col][lin]) {} the compiler says 'lin' should be a constant, but I want it to be defined from the user, so what should I do? and how must 'a' be declared?? help me please a) Do not roll your own matrix code unless you absolutely have to. Numerical analysis is hard. Use a library instead. There are many good linear algebra libraries out there. Google is your friend. b) Do not use float for numerical computations unless you know with certainty that the precision is good enough for your needs. Beware that linear algebra algorithms can go mad on precision. c) If you absolutely need to run your own matrix class, do not do the memory management by yourself. Simply do template <typename ArithmeticType> class Matrix std::size_t row_size; std::size_t col_size; std::vector< ArithmeticType > data; Matrix ( std::size_t rows, std::size_t cols ) : row_size ( rows ) , col_size ( cols ) , data ( rows*cols ) {} ArithmeticType & operator() ( std::size_t row, std::size_t col ) { // check for bounds here if you please ... return( this->data[row*this->cols_size + col ] ); } ArithmeticType const & operator() ( std::size_t row, std::size_t col ) const { // check for bounds here if you please ... return( this->data[ row*this->cols_size + col ] ); } }; You might also want to have a look into std::valarray and the numeric header. d) Adhere to established math conventions: matrices have rows and columns, and rows go first. Make that second nature, and bugs will go away. e) Again: do not roll your own code. Best Kai-Uwe Bux Back to top mlimber Guest Posted: Thu Oct 27, 2005 1:19 pm    Post subject: Re: two-dimentional array with non constant variables Al wrote: Quote: I'd like to declare (in a Matrix class) a two-dimentional array with user-defined coordinates. The constructor is: Matrix(int c, int l): col(c), lin(l), a(new float[col][lin]) {} the compiler says 'lin' should be a constant, but I want it to be defined from the user, so what should I do? and how must 'a' be declared?? help me please Please consult the FAQ: http://www.parashift.com/c++-faq-lite/freestore-mgmt.html#faq-16.16 But, I'd suggest you use a vector< vector if possible. It can make life simpler in many ways. Cheers! --M Back to top Al Guest Posted: Thu Oct 27, 2005 7:10 pm    Post subject: Re: two-dimentional array with non constant variables you know... just newbie training... thx Back to top Old Wolf Guest Posted: Thu Oct 27, 2005 10:48 pm    Post subject: Re: two-dimentional array with non constant variables Al wrote: Quote: I'd like to declare (in a Matrix class) a two-dimentional array with user-defined coordinates. The constructor is: Matrix(int c, int l): col(c), lin(l), a(new float[col][lin]) {} the compiler says 'lin' should be a constant, but I want it to be defined from the user, so what should I do? and how must 'a' be declared?? You've struck right to the heart of the problem. 'a' must be declared as: float (*a)[lin]; But C++ requires that the array dimension is a compile-time constant. So it is not possible to have this array as you would like. You could either make 'a' a one-dimension array and add a function for accessing it as if it were 2-D; or you could use a vector of vectors. Back to top Axter Guest Posted: Fri Oct 28, 2005 5:57 am    Post subject: Re: two-dimentional array with non constant variables Al wrote: Quote: I'd like to declare (in a Matrix class) a two-dimentional array with user-defined coordinates. The constructor is: Matrix(int c, int l): col(c), lin(l), a(new float[col][lin]) {} the compiler says 'lin' should be a constant, but I want it to be defined from the user, so what should I do? and how must 'a' be declared?? help me please Check out the following link for an example: http://code.axter.com/dynamic_2d_array.h However, I recommend you use a vector of vector. Example: int col = 123; int row = 456; vector<vector My2dArray(col, vector<int>(row)); You can reference both the above vector code and the dynamic_2d_array class using double index ([][]) My2dArray[0][0] = 99; Back to top Al Guest Posted: Fri Oct 28, 2005 9:45 pm    Post subject: Re: two-dimentional array with non constant variables thx I've never used vectors of vectors before, but it's a good reason to learn it! How do you use 'typename' ?? (from Kai-Uwe Bux: template<typename ArithmeticType>) Back to top Al Guest Posted: Fri Oct 28, 2005 10:40 pm    Post subject: Re: two-dimentional array with non constant variables what should I do to get the size of the second dimension of the vector, without using it as a parameter? void getVector(vector<vector& v) { for(int i = 0;i<v.size();i++) { for(int j = 0;j< /*here*/ ;j++) { cout <<"v["< cin >>v[i][j]; } } } Back to top Mark P Guest Posted: Sat Oct 29, 2005 12:05 am    Post subject: Re: two-dimentional array with non constant variables Al wrote: Quote: what should I do to get the size of the second dimension of the vector, without using it as a parameter? v[i].size() Quote: void getVector(vector<vector& v) { for(int i = 0;i<v.size();i++) { for(int j = 0;j< /*here*/ ;j++) { cout <<"v["< cin >>v[i][j]; } } } Back to top Display posts from previous: All Posts1 Day7 Days2 Weeks1 Month3 Months6 Months1 Year Oldest FirstNewest First         C++Talk.NET Forum Index -> C++ language (comp.lang.c++) All times are GMT Page 1 of 1   Jump to: Select a forum C/C++----------------C LanguageC Language (Moderated)C++ Language (Moderated)C++ language (comp.lang.c++)C++ language, library and standards Non-English C/C++ Newsgroups----------------C++ (German)C++ (French)  You cannot post new topics in this forum You cannot reply to topics in this forum You cannot edit your posts in this forum You cannot delete your posts in this forum You cannot vote in polls in this forum Powered by phpBB © 2001, 2006 phpBB Group SEO toolkit © 2004-2006 webmedic.