C Matrix Class

Question

In C  if I wanted to create a matrix struct  I would use   struct matrix     int col  row    double data 1      I want the matrix entries stored                      right after this struct     Then I can allocate it with  matrix  allocate matrix int row  int col      matrix  m   malloc sizeof matrix    sizeof double     row   col - 1      m- gt row   row  m- gt col   col    return m      Now do I do the equiv in C     EDIT   I want to know the cannonical way to implement a matrix class in C

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You could do it that way  The only difference is you d need to cast the result from malloc   Rather  you would use a vector  either as a 1D array with computed indexing or an embedded vector   The former matches your code better    For example   template  lt typename T gt     often  they are templates struct matrix          should probably be hidden away  and the class would        provide  at  and  operator    for access     int col  row      std  vector lt T gt  data       matrix int columns  int rows        col columns   row rows        data col   row             matrix m 4  4   m data 1   1   4                 Or   template  lt typename T gt  struct matrix       int col  row      std  vector lt std  vector lt T gt   gt  data       matrix int columns  int rows        col columns   row rows        data col  std  vector row             matrix m 4  4   m data 1  1                 But these are only examples  You d want to make a full-fledged class  if you want more advice on that  edit your question and clarify you d like to know the canonical way of implementing matrix classes   There are pre-existing matrix classes  My favorite is that from boost  UBLAS

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The library called  Matrix supports so many features including mathematics operations  dumping and logging features  associative containers  multiple dimensions and etc  Usage its usage is similar to c   arrays  Matrix lt int gt  A 1  2   Matrix lt int gt  B 2  3   Matrix lt int gt  result 1  3    A 0  0    7  A 0  1    10   B 0  0    1  B 0  1    4  B 0  2    2  B 1  0    1  B 1  1    2  B 1  2    100   result   A   B   result dump matrix     Result  Matrix view  -            -   17 48 1014   -            -  Here is the documentation and Github page

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For a matrix class  you want to stay away from overloading the    operator  See C   FAQ 13 10  Also  search the web for some freeware Matrix classes    Worst case  they can give you guidance   Best case  less software that you have to write and debug

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There is no  quot canonical quot  way to do the matrix in C    STL does not provide classes like  quot matrix quot   However there are some 3rd party libraries that do  You are encouraged to use them or write your own implementation  You can try my implementation derived from some public implementation found on the internet

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You could use a template like     include  lt iostream gt  using std  cerr  using std  endl     qt4type typedef unsigned int quint32   template  lt typename T gt  void deletep T  amp      template  lt typename T gt  void deletep T   amp  ptr        delete ptr      ptr   0    template lt typename T gt  class Matrix       public          typedef T value type          Matrix      cols 0    rows 0    data new T 0    auto delete true              Matrix quint32 rows  quint32 cols  bool auto del   true            bool exists quint32 row  quint32 col  const          T  amp  operator   quint32 row  quint32 col           T operator   quint32 row  quint32 col  const          virtual  Matrix             int size   const   return  rows    cols            int rows   const   return  rows            int cols   const   return  cols        private          Matrix const Matrix  amp            quint32  rows   cols          mutable T    data          const bool auto delete     template lt typename T gt  Matrix lt T gt   Matrix quint32 rows  quint32 cols  bool auto del     rows rows    cols cols   auto delete auto del         data   new T rows   cols     template lt typename T gt  inline T  amp  Matrix lt T gt   operator   quint32 row  quint32 col        return  data  cols   row   col     template lt typename T gt  inline T Matrix lt T gt   operator   quint32 row  quint32 col  const       return  data  cols   row   col      template lt typename T gt  bool Matrix lt T gt   exists quint32 row  quint32 col  const       return  row  lt   rows  amp  amp  col  lt   cols      template lt typename T gt  Matrix lt T gt    Matrix         if auto delete           for int i   0  c   size    i  lt  c    i                 will do nothing if T isn t a pointer             deletep  data i                        delete     data     int main         Matrix lt  int  gt  m 10 10       quint32 i   0      for int x   0  x  lt  10    x            for int y   0  y  lt  10    y    i                m x  y    i                      for int x   0  x  lt  10    x            for int y   0  y  lt  10    y                cerr  lt  lt        lt  lt  x  lt  lt        lt  lt  y  lt  lt          lt  lt  m x y   lt  lt  endl                       edit  fixed a typo

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In C   you can use like this   matrix  p   new matrix    After that   delete p

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nota bene   This answer has 20 upvotes now  but it is not intended as an endorsement of std  valarray   In my experience  time is better spent installing and learning to use a full-fledged math library such as Eigen  Valarray has fewer features than the competition  but it isn t more efficient or particularly easier to use   If you only need a little bit of linear algebra  and you are dead-set against adding anything to your toolchain  then maybe valarray would fit  But  being stuck unable to express the mathematically correct solution to your problem is a very bad position to be in  Math is relentless and unforgiving  Use the right tool for the job     The standard library provides std  valarray lt double gt   std  vector lt  gt   suggested by a few others here  is intended as a general-purpose container for objects  valarray  lesser known because it is more specialized  not using  specialized  as the C   term   has several advantages    It does not allocate extra space  A vector rounds up to the nearest power of two when allocating  so you can resize it without reallocating every time   You can still resize a valarray  it s just still as expensive as realloc     You may slice it to access rows and columns easily  Arithmetic operators work as you would expect    Of course  the advantage over using C is that you don t need to manage memory  The dimensions can reside on the stack  or in a slice object   std  valarray lt double gt  matrix  row   col       no more  no less  than a matrix matrix  std  slice  2  col  row       pi     set third column to pi matrix  std  slice  3 row  row  1       e     set fourth row to e

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you could do it with a template  if the matrix size is known at compile-time    template  lt int width  int height gt  class Matrix      double data height  width            member functions

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C   is mostly a superset of C   You can continue doing what you were doing   That said  in C    what you ought to do is to define a proper Matrix class that manages its own memory   It could  for example be backed by an internal std  vector  and you could override operator   or operator   to index into the vector appropriately  for example  see  How do I create a subscript operator for a Matrix class  from the C   FAQ    To get you started   class Matrix   public      Matrix size t rows  size t cols       double amp  operator   size t i  size t j       double operator   size t i  size t j  const   private      size t mRows      size t mCols      std  vector lt double gt  mData      Matrix  Matrix size t rows  size t cols    mRows rows     mCols cols     mData rows   cols       double amp  Matrix  operator   size t i  size t j        return mData i   mCols   j      double Matrix  operator   size t i  size t j  const       return mData i   mCols   j        Note that the above doesn t do any bounds-checking  and I leave it as an exercise to template it so that it works for things other than double

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There s lots of subtleties in setting up an efficient and high quality matrix class  Thankfully there s several good implementations floating about   Think hard about whether you want a fixed size matrix class or a variable sized one  i e  can you do this      These tend to be fast and allocated on the stack  matrix lt 3 3 gt  M     or do you need to be able to do this     These are slower but more flexible and partially allocated on the heap  matrix M 3 3      There s good libraries that support either style  and some that support both  They have different allocation patterns and different performances    If you want to code it yourself  then the template version requires some knowledge of templates  duh   And the dynamic one needs some hacks to get around lots of small allocations if used inside tight loops

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