Sorry, this is not a cfd question, but I know there are lots of people now using C++ in cfd programming.

Is there any way of dynamic memory allocation for a 2 or 3 d matrix in C++? I'm not looking for allocating the memory by defining a matrix as an array (1 d matrix), but something keeping the matrix as it is.

Thanks. - mm

There is no direct support for 2d or 3d matrices in either bare C++ or it's STL (standard template library).

This is one of the reasons you'll get loads of hits if you do a web search for "C++ matrix class". Trouble is, most of them aren't quite what you want (too much or too little functionality) and people generally want cash for them.

Correcting myself slightly...

There is no direct support for dynamically allocatable and resizeable 2d and 3d matrices in C++.

Any matrix, whether 2-D, 3-D, 4-D, or higher, is ultimately stored in RAM as a 1-D structure. Computer memory is 1-D. Only through the sophisticated use of multiple pointers can you create a higher-dimensional data structure.

In C++ you can try the following:

void main() {

double **mat2,***mat3;

int i,j,k,n1,n2,n3;

// Allocation of a 2 and 3 dimensional matrix

n1 = n2 = n3 = 10;

mat2 = new double*[n1];

mat3 = new double**[n1];

for (i=0; i<<n1>n1; i++)

{

mat2[i] = new double[n2];

mat3[i] = new double*[n2];

for (j=0; j<<n2>n2; j++)

mat3[i][j] = new double[n3];

}

// Access to matrix elements

for (i=0; i<<n1>n1; i++)

for (j=0; j<<n2>n2; j++)

{

mat2[i][j] = 1;

for (k=0; k<<n3>n3; k++)

mat3[i][j][k] = 2;

}

// Deallocation

for (i=0; i<<n1>n1; i++)

{

for (j=0; j<<n2>n2; j++)

delete [] mat3[i][j];

delete [] mat3[i];

delete [] mat2[i];

}

delete [] mat2;

delete [] mat3; }

Best regards

Markus

Thank you very much, Markus. It works perfectly. -- Malik

I met the same problem. But mine is more difficult. In your code, the n1,n2,n3 is known by the compiler. What should I do when they are not const? For example, I read grid data from a file. But before that I don't know the number of the total vetex. Code read the size from the grid data file, maybe ni*nj*nk. Then I want to create a 3D array with size of ni*nj*nk. I tried in Visual C++ 6.0. But the operator "new" dosen't surport non-constant size.

I can't believe that the new operator does accept only constant integers as parameters. If I have to know the array sizes at compile time, I need no dynamic memory allocation at all and thus no new operator.

I do not know the Visual C++ compiler, but I use this kind of dynamic memory allocation for years with a lot of different C++ compilers on many different machines.

By the way, in my program n1,n2, and n3 are not constant. They could be read in from a file. A constant n1 is defined in C++, e.g., by

const int n1=10;

Best regards

Markus

I've crack te problem. Just like the following: ----------------------

FILE * f;

int i,j,k;

float x,y,z;

//point *points;

f=fopen("grid.dat","r");

fscanf(f,"%d %d %d",&i,&j,&k);

const int ni=i;

const int nj=j;

const int nk=k;

printf("%d,%d,%d\n",ni,nj,nk);

//read the size of grid

point ***points = (point ***)new point[ni*nj*nk]; //create 3d array. point is a class

for(k=0;k<nk;k++)

for(j=0;j<nj;j++)

for(i=0;i<ni;i++) {

fscanf(f," %f %f %f",&x,&y,&z);

...........

----------------------

these code is not needed. ---------------- fscanf(f,"%d %d %d",&i,&j,&k);

const int ni=i;

const int nj=j;

const int nk=k;

----------------

new operater does surport non-constant size

just use -------------------- int ni,nj,nk;

fscanf(f,"%d %d %d",&ni,&nj,&nk);

sorry

there are still some problems.

This is just simple C pointer/array equivalence stuff. You'd get much more help by posting to comp.lang.c. Just don't mention you're using a C++ compiler.

If you want to make use of C++ features, make yourself a Matrix class and use STL vectors in it. Then you can do nice overloads to make your code look like your equations.

BTW, the relevent comp.lang.c FAQ page can be found online at:

http://www.eskimo.com/~scs/C-faq/s6.html

Thanks. Now I've known how to do it.

point *** points = new point**[ni]; for (i=0;i<ni;i++) {

points[i] = new point*[nj];

for (j=0;j<nj;j++)

points[i][j] = new point[nk]; }