matrix mult - double ptr in col major

**collymitch** · 03-21-2005

Hey,

I'm creating a matrix-matrix multiplier in C that will use the same format as the GOTO BLAS and ATLAS implementations. I need to allocate memory and store the matrices in COL MAJOR format not in row major as is the standard C representation.

Can you tell me if I am on the right track here:

Code:

//global vars
double **matrixA, **matrixB, **matrixC;

int main(int argc, char *argv[])
{
	int i, c, row, col, j;
	int n_rows = 10; //ASSUME SQUARE FOR NOW!
	int m_cols =  10;
	int mbyn = m_cols * n_rows;
        int i, j;
        double n = 0;

//allocate the number of cols
matrixA = (double**)malloc( sizeof( double* ) * m_cols); 

//allocate the overall size??
matrixA[0] = (double *) malloc( sizeof( double*) * mbyn);
	
for (i=1; i<m_cols; i++)	
{       //for each col allocate the rows
	matrixA[i] = *matrixA + i * n_rows; 
}

I then have a function fill matrix so:

Code:

  for (i=0; i<SIZE; i++) //
    for (j=0; j<SIZE; j++)
      matrix[j][i] = n++;  //or a[i][j];  //fill it col,row - like a graphics point

I think the malloc is correct but i'm not so sure about the filling of the matrix. Is this right??

I want it to work like this:
Image Attachment
(sorry don't know how to display the image in here.)

So that I have one long linear array, that stores one row, then the next row etc. This means that I can skip to the next element one by one the whole way throught the matrix. To get to the next row of a matrix with n cols I would simply move n elements from the start pointer.

thanks for your help,

Colm Mitchell

**swoopy** · 03-21-2005

>So that I have one long linear array, that stores one row, then the next row etc.

The only way to do that is just have a single pointer, and access column and row using pointer arithmetic.

Instead of:
double **matrixA, **matrixB, **matrixC;

You would have:
double *matrixA, *matrixB, *matrixC;

**mrafcho001** · 03-21-2005

i am not sure what you are trying to do but you can use mutli-dimentional arrays to store whole tables.

**Dave_Sinkula** · 03-21-2005

Second example here.

I know this came up here recently and Salem had a really nice solution... this thread: here.

[edit=3]I'm not sure if this is quite what you're after.

Code:

#include <stdio.h>
#include <stdlib.h>

int **foo(int nrows, int ncolumns)
{
   int i, **array2 = malloc(nrows * sizeof *array2);
   if ( array2 )
   {
      array2[0] = malloc(nrows * ncolumns * sizeof *array2[0]);
      if ( array2[0] )
      {
         for ( i = 1; i < nrows; i++ )
         {
            array2[i] = array2[0] + i * ncolumns;
         }
      }
   }
   return array2;
}

void bar(int **array)
{
   free(array[0]);
   free(array);
}

int main(void)
{
   int r, c, i = 0, rows = 3, cols = 5, **myarray = foo(cols, rows);
   for ( c = 0; c < cols; ++c )
   {
      for ( r = 0; r < rows; ++r )
      {
         myarray[c][r] = i++;
         printf("%p : myarray[%d][%d] = %d\n",
                (void*)&myarray[c][r], c, r, myarray[c][r]);
      }
   }
   bar(myarray);
   return 0;
}

/* my output
007A3208 : myarray[0][0] = 0
007A320C : myarray[0][1] = 1
007A3210 : myarray[0][2] = 2
007A3214 : myarray[1][0] = 3
007A3218 : myarray[1][1] = 4
007A321C : myarray[1][2] = 5
007A3220 : myarray[2][0] = 6
007A3224 : myarray[2][1] = 7
007A3228 : myarray[2][2] = 8
007A322C : myarray[3][0] = 9
007A3230 : myarray[3][1] = 10
007A3234 : myarray[3][2] = 11
007A3238 : myarray[4][0] = 12
007A323C : myarray[4][1] = 13
007A3240 : myarray[4][2] = 14
*/

**swoopy** · 03-21-2005

After reading Dave's link, ignore my post.

**collymitch** · 03-24-2005

Ok - i got a bit confused by that so had to ask my supervisor again...

Swoopy you were on the right track - I need to have a single pointer instead of a double so

You would have:
double *matrixA, *matrixB, *matrixC;

would work.

This means that when i go to fill the matrix for example i need to do it not like

PHP Code:


matrix[1][2] = X;

but as

PHP Code:


matrix[0]+ Y = X;

where Y is the Yth element of that row. Meaning that I start a particular column and then move along the array to find the element in the Yth row of that col.

Dave in what way will I need to change the memory allocation because of this?

Thanks for your help guys!

Colly.

**Dave_Sinkula** · 03-24-2005

Originally Posted by collymitch

Dave in what way will I need to change the memory allocation because of this?

Then look at the third example there...

Originally Posted by Dave_Sinkula

Second example here.

To do it column major, you'd do the indexing differently. Maybe something like this.

Code:

#include <stdio.h>
#include <stdlib.h>

int main(void)
{
   size_t rows = 3, cols = 5;
   double *myarray = malloc(rows * cols * sizeof *myarray);
   if ( myarray )
   {
      size_t r, c;
      for ( r = 0; r < rows; ++r )
      {
         for ( c = 0; c < cols; ++c )
         {
            size_t i = c * rows + r;
            myarray[i] = (r + 1) + (c + 1) / 10.0;
            printf("{%d,%d} %p : myarray[%2d] = %g\n", (int)r, (int)c,
                   (void*)&myarray[i], (int)i, myarray[i]);
         }
      }
      free(myarray);
   }
   return 0;
}

Thread: matrix mult - double ptr in col major

Thread Tools

Search Thread

Display

Hybrid View

matrix mult - double ptr in col major

Similar Threads

C - access violation

Compiling c++ intrinsics commands

Copying 2-d arrays

Conversion From C++ To C

Hi, could someone help me with arrays?