dynamic memory allocation in C++
 

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

Re: dynamic memory allocation in C++
 

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.

Re: dynamic memory allocation in C++
 

Correcting myself slightly...

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

Re: dynamic memory allocation 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.

Re: dynamic memory allocation in C++
 

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

Re: dynamic memory allocation in C++
 

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

Re: dynamic memory allocation in C++
 

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.

Re: dynamic memory allocation in C++
 

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

Re: dynamic memory allocation in C++
 

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);

...........

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

Re: dynamic memory allocation in C++
 

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);

Re: dynamic memory allocation in C++
 

sorry

there are still some problems.

Re: dynamic memory allocation in C++
 

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.

Re: dynamic memory allocation in C++
 

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

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

Re: dynamic memory allocation in C++
 

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]; }