How do you dynamically allocate a matrix

Question

How do you dynamically allocate a 2D matrix in C    I have tried based on what I already know    include  lt iostream gt   int main        int rows      int cols      int   arr      arr   new int rows  cols        It works for one parameter  but now for two  What should I do

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The other answer describing arrays of arrays are correct.
BUT if you are planning of doing a anything mathematical with the arrays - or need something special like sparse matrices you should look at one of the many maths libs like TNT before re-inventing too many wheels

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const int nRows   20  const int nCols   10  int   name  nCols    new int nRows  nCols   std  memset name  0  sizeof int    nRows   nCols     row major contiguous memory name 0  0    1    first element name nRows-1  nCols-1    1    last element delete   name

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or you can just allocate a 1D array but reference elements in a 2D fashion   to address row 2  column 3  top left corner is row 0  column 0    arr 2   MATRIX WIDTH   3   where MATRIX WIDTH is the number of elements in a row

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I have this grid class that can be used as a simple matrix if you don t need any mathematical operators          Represents a grid of values     Indices are zero-based      template lt class T gt  class GenericGrid       public          GenericGrid size t numRows  size t numColumns            GenericGrid size t numRows  size t numColumns  const T  amp  inInitialValue            const T  amp  get size t row  size t col  const           T  amp  get size t row  size t col            void set size t row  size t col  const T  amp  inT            size t numRows   const           size t numColumns   const       private          size t mNumRows          size t mNumColumns          std  vector lt T gt  mData       template lt class T gt  GenericGrid lt T gt   GenericGrid size t numRows  size t numColumns       mNumRows numRows       mNumColumns numColumns        mData resize numRows numColumns       template lt class T gt  GenericGrid lt T gt   GenericGrid size t numRows  size t numColumns  const T  amp  inInitialValue       mNumRows numRows       mNumColumns numColumns        mData resize numRows numColumns  inInitialValue       template lt class T gt  const T  amp  GenericGrid lt T gt   get size t rowIdx  size t colIdx  const       return mData rowIdx mNumColumns   colIdx       template lt class T gt  T  amp  GenericGrid lt T gt   get size t rowIdx  size t colIdx        return mData rowIdx mNumColumns   colIdx       template lt class T gt  void GenericGrid lt T gt   set size t rowIdx  size t colIdx  const T  amp  inT        mData rowIdx mNumColumns   colIdx    inT      template lt class T gt  size t GenericGrid lt T gt   numRows   const       return mNumRows      template lt class T gt  size t GenericGrid lt T gt   numColumns   const       return mNumColumns

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arr   new int cols rows     If you either don t mind syntax    arr row   cols   col    Aij    or use operator   overaloading somewhere  This may be more cache-friendly than array of arrays  or may be not  more probably you shouldn t care about it  I just want to point out that a  array of arrays is not only solution  b  some operations are more easier to implement if matrix located in one block of memory  E g   for int i 0 i  lt  rows cols   i     matrix i  someOtherMatrix i     one line shorter than  for int r 0 i  lt  rows   r    for int c 0 i  lt  cols   s       matrix r  c  someOtherMatrix r  c     though adding rows to such matrix is more painful

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Using the double-pointer is by far the best compromise between execution speed optimisation and legibility   Using a single array to store matrix  contents is actually what a double-pointer does   I have successfully used the following templated creator function  yes  I know I use old C-style pointer referencing  but it does make code more clear on the calling side with regards to changing parameters - something I like about pointers which is not possible with references  You will see what I mean            Matrix Allocator Utility      param pppArray Pointer to the double-pointer where the matrix should be allocated       param iRows Number of rows       param iColumns Number of columns       return Successful allocation returns true  else false  template  lt typename T gt  bool NewMatrix T    pppArray                  size t iRows                  size t iColumns       bool l bResult   false     if  pppArray    0     Test if pointer holds a valid address                           I prefer using the shorter 0 in stead of NULL        if      pppArray     0      Test if the first element is currently unassigned                                    The  double-not  evaluates a little quicker in general              Allocate and assign pointer array             pppArray    new T   iRows             if    pppArray     0     Test if pointer-array allocation was successful                            Allocate and assign common data storage array                pppArray  0    new T  iRows   iColumns                if    pppArray  0     0     Test if data array allocation was successful                                  Using pointer arithmetic requires the least overhead  There is no                    expensive repeated multiplication involved and very little additional                    memory is used for temporary variables                 T   l ppRow     pppArray                  T  l pRowFirstElement   l ppRow 0                  for  size t l iRow   1  l iRow  lt  iRows  l iRow                                       l ppRow                      l pRowFirstElement    iColumns                    l ppRow 0    l pRowFirstElement                                  l bResult   true                                            To de-allocate the memory created using the abovementioned utility  one simply has to  de-allocate in reverse           Matrix De-Allocator Utility      param pppArray Pointer to the double-pointer where the matrix should be de-allocated       return Successful de-allocation returns true  else false  template  lt typename T gt  bool DeleteMatrix T    pppArray       bool l bResult   false     if  pppArray    0     Test if pointer holds a valid address             if    pppArray     0     Test if pointer array was assigned                   if    pppArray  0     0     Test if data array was assigned                               De-allocate common storage array                 delete      pppArray  0                                        De-allocate pointer array           delete      pppArray              pppArray    0           l bResult   true                   To use these abovementioned template functions is then very easy  e g                       double l ppMatrix   0     NewMatrix  amp l ppMatrix  3  3      Create a 3 x 3 Matrix and store it in l ppMatrix                    DeleteMatrix  amp l ppMatrix

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You can also use std  vectors for achieving this   using  std  vector lt  std  vector lt int gt   gt   Example   std  vector lt  std  vector lt int gt   gt  a       m   n is the size of the matrix      int m   2  n   4        Grow rows by m     a resize m       for int i   0   i  lt  m     i                  Grow Columns by n         a i  resize n               Now you have matrix m n with default values        you can use the Matrix  now     a 1  0  1      a 1  1  2      a 1  2  3      a 1  3  4     OR for i   0   i  lt  m     i        for int j   0   j  lt  n     j               modify matrix         int x   a i  j

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Here is the most clear  amp  intuitive way i know to allocate a dynamic 2d array in C    Templated in this example covers all cases   template lt typename T gt  T   matrixAllocate int rows  int cols  T   M        M   new T  rows       for  int i   0  i  lt  rows  i             M i    new T cols             return M           int main                 int   M1   matrixAllocate lt int gt  rows  cols  M1       double   M2   matrixAllocate rows  cols  M2

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A matrix is actually an array of arrays   int rows        cols        int   matrix   new int  rows   for  int i   0  i  lt  rows    i      matrix i    new int cols     Of course  to delete the matrix  you should do the following   for  int i   0  i  lt  rows    i      delete    matrix i   delete    matrix      I have just figured out another possibility   int rows        cols        int   matrix   new int  rows   if  rows        matrix 0    new int rows   cols       for  int i   1  i  lt  rows    i          matrix i    matrix 0    i   cols      Freeing this array is easier   if  rows  delete    matrix 0   delete    matrix    This solution has the advantage of allocating a single big block of memory for all the elements  instead of several little chunks  The first solution I posted is a better example of the arrays of arrays concept  though

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Try boost  multi array   include  lt boost multi array hpp gt   int main        int rows      int cols      boost  multi array lt int  2 gt  arr boost  extents rows  cols

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include  lt iostream gt       int main            int rows 4          int cols 4          int   arr           arr   new int  rows           for int i 0 i lt rows i                arr i  new int cols                        statements          for int i 0 i lt rows i                delete   arr i                     delete   arr          return 0

[c++] How do you dynamically allocate a matrix?

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