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problem in multiplying arrays

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# 1  
Old 12-05-2011
problem in multiplying arrays
Hi, this is my code.It's simple : there are 2 2D arrays and the multiplied to C.
Code:
#include<stdio.h>
#include<sys/shm.h>
#include<sys/stat.h>
#include<stdlib.h>

main()
{
        int *A;      //A[2][3]
        int *B;      //B[3][4]
        int *C;      //C[2][4]
        int i,j,x,k,d;
        int id;
        id = shmget(IPC_PRIVATE,4096, IPC_CREAT|0666);
        A = (int *)shmat(id,NULL, 0);
        x=1;

        for(i=0;i<2;i++)
        {
                for(j=0;j<3;j++)
                {
                        A[j*3+i]=x;
                        x++;
                }
        }

        B = (int *)shmat(id,NULL, 0);
        x=12;
        for(i=0;i<3;i++)
        {
                for(j=0;j<4;j++)
                {
                        B[j*4+i]=x;
                        x--;
                }
        }


        printf("A:\n");
        for(i=0;i<2;i++)
        {
                for(j=0;j<3;j++)
                {
                        printf("%d\t",A[j*3+i]);
                }
                printf("\n");
        }

        printf("B:\n");
        for(i=0;i<3;i++)
        {
                for(j=0;j<4;j++)
                {
                        printf("%d\t",B[j*4+i]);
                }
                printf("\n");
        }

        //Multiply to C[2][4], k common A,B
        C = (int *)shmat(id,NULL, 0);
        for(i=0;i<2;i++)
        {
                for(j=0;j<4;j++)
                {
                        for(k=0;k<3;k++)
                        {
                                d=d+A[k*3+i]*B[j*4+k];
                        }
                        C[j*4+i]=d;
                        d=0;
                }
        }

        printf("C:\n");
        for(i=0;i<2;i++)
        {
                for(j=0;j<4;j++)
                {
                        printf("%d\t",C[j*4+i]);
                }
                printf("\n");
        }
        shmdt(A);
        shmdt(B);
        shmdt(C);
        shmctl(id,IPC_RMID,NULL);
}

A should be:
1 2 3
4 5 6
and B:
12 11 10 9
8 7 6 5
4 3 2 1
but what i take is that:
Code:
A:
12	2	3	
8	11	6	
B:
12	11	10	9	
8	7	6	5	
4	3	2	1	
C:
172	1915	1738	1561	
16720	32032223	29070862	26109501

C is also wrong...
Any ideas??
Last edited by giampoul; 12-05-2011 at 03:48 PM..
# 2  
Old 12-05-2011
Indenting your program so I can read it.
Code:
#include<stdio.h>
#include<sys/shm.h>
#include<sys/stat.h>
#include<stdlib.h>

main()
{
        int *A;      //A[2][3]
        int *B;      //B[3][4]
        int *C;      //C[2][4]
        int i,j,x,k,d;
        int id;
        id = shmget(IPC_PRIVATE,4096, IPC_CREAT|0666);
        A = (int *)shmat(id,NULL, 0);
        x=1;

        for(i=0;i<2;i++)
        {
                for(j=0;j<3;j++)
                {
                        A[j*3+i]=x;
                        x++;
                }
        }

        B = (int *)shmat(id,NULL, 0);
        x=12;
        for(i=0;i<3;i++)
        {
                for(j=0;j<4;j++)
                {
                        B[j*4+i]=x;
                        x--;
                }
        }


        printf("A:\n");
        for(i=0;i<2;i++)
        {
                for(j=0;j<3;j++)
                {
                        printf("%d\t",A[j*3+i]);
                }
                printf("\n");
        }

        printf("B:\n");
        for(i=0;i<3;i++)
        {
                for(j=0;j<4;j++)
                {
                        printf("%d\t",B[j*4+i]);
                }
                printf("\n");
        }

        //Multiply to C[2][4], k common A,B
        C = (int *)shmat(id,NULL, 0);
        for(i=0;i<2;i++)
        {
                for(j=0;j<4;j++)
                {
                        for(k=0;k<3;k++)
                        {
                                d=d+A[k*3+i]*B[j*4+k];
                        }
                        C[j*4+i]=d;
                        d=0;
                }
        }

        printf("C:\n");
        for(i=0;i<2;i++)
        {
                for(j=0;j<4;j++)
                {
                        printf("%d\t",C[j*4+i]);
                }
                printf("\n");
        }
        shmdt(A);
        shmdt(B);
        shmdt(C);
        shmctl(id,IPC_RMID,NULL);
}

---------- Post updated at 01:42 PM ---------- Previous update was at 01:38 PM ----------

Code:
        A[0]=0xdeadbeef;
        printf("%x %x %x\n", A[0], B[0], C[0]);

Code:
deadbeef deadbeef deadbeef

A, B, and C are all pointers to the same memory. You're getting the exact same segment ID three times in a row, which gives you the exact same segment three times in a row.

Since you're getting 4096 bytes of memory anyway, plenty of room for all those small arrays in one call, why not just do this:

Code:
A=(int *)shmat(id,NULL, 0);
B=(A + (3*4));
C=(B + (3*4));

Then you get correct results for A and B, but not C. I'm not quite sure what you're doing with C.
This User Gave Thanks to Corona688 For This Post:
giampoul
# 3  
Old 12-05-2011
Sorry about the lining...i fix it.

Your code is helpful. Now i get the right A and B arrays !
But I get for C that :
Code:
C:
40	34	28	53	
112	97	82	726

but it should be:
Code:
C:
40	34	28	22	
112	97	82	67

Maybe a wrong at the multiply??
I thaught that this:
Code:
for(i=0;i<2;i++)
        {
                for(j=0;j<4;j++)
                {
                        for(k=0;k<3;k++)
                        {
                                d=d+A[k*3+i]*B[j*4+k];
                        }
                        C[j*4+i]=d;
                        d=0;
                }
        }

is the correct multiply code for 2 multidimensional 2Ds arrays.
k=3 is the A columns = B rows.
Is there any other way ??
Last edited by giampoul; 12-05-2011 at 04:04 PM..
# 4  
Old 12-05-2011
Been a while since I did matrix multiplication, going through the math.
This User Gave Thanks to Corona688 For This Post:
giampoul
# 5  
Old 12-05-2011
Quote:
Originally Posted by Corona688
Been a while since I did matrix multiplication, going through the math.
Of course...
i based here http://en.wikipedia.org/wiki/Matrix_multiplication for the multiplication
and here http://sourcecookbook.com/en/recipes...emory-with-ipc for the arrays
# 6  
Old 12-05-2011
You have problems initializing and printing your A array, multiplying by j instead of i so you try to assign 3 rows of 2 instead of 2 rows of 3... Fixing that makes A print rotated 90 degrees to B... Working on it...

---------- Post updated at 02:51 PM ---------- Previous update was at 02:46 PM ----------

You've got your rows and columns mixed up in general I think. A[(row*3) + column] not vice versa.

---------- Post updated at 03:06 PM ---------- Previous update was at 02:51 PM ----------

I found myself making the same mistake as you over and over. When that keeps happening that's a good sign it's time to let the computer start remembering some things instead of the programmer:

Code:
#define OFFSET(COLUMNS, ROW, COL)       ((ROW*COLUMNS)+COL)

...

       A = (int *)shmat(id,NULL, 0);
       B = A + OFFSET(3, 2, 0);
       C = B + OFFSET(3, 4, 0);

...

        x=1;
        for(i=0; i<2; i++)
        for(j=0; j<3; j++)
                A[OFFSET(3, i, j)]=x++;


...

        for(i=0; i<2; i++)
        for(j=0; j<4; j++)
        {
                int d=0, k;
                for(k=0; k<3; k++)
                        d += A[OFFSET(3,i,k)] * B[OFFSET(4,k,j)];

                C[OFFSET(4, j, i)]=d;
        }


...

This User Gave Thanks to Corona688 For This Post:
giampoul
# 7  
Old 12-05-2011
Code:
#define OFFSET(COLUMNS, ROW, COL)       ((ROW*COLUMNS)+COL)

#include<stdio.h>
#include<sys/shm.h>
#include<sys/stat.h>
#include<stdlib.h>
main()
{
  int *A,*B,*C;
  int i,j,x,d,k;
  int id;
  id = shmget(IPC_PRIVATE,4096, IPC_CREAT|0666);
  A = (int *)shmat(id,NULL, 0);
  B = A + OFFSET(3, 2, 0);
  C = B + OFFSET(3, 4, 0);

  x=1;
  for(i=0; i<2; i++)
  {
    for(j=0; j<3; j++)
    {
      A[OFFSET(3, i, j)]=x++;
    }
  }
		
  x=12;
  for(i=0; i<3; i++)
  {
    for(j=0; j<4; j++)
    {
      B[OFFSET(4, i, j)]=x--;
    }
  }

   for(i=0; i<2; i++)
   {
     for(j=0; j<4; j++)
     {
                int d=0, k;
                for(k=0; k<3; k++)
                d += A[OFFSET(3,i,k)] * B[OFFSET(4,k,j)];
                C[OFFSET(4, j, i)]=d;
     }
   }
        
   printf("C:\n");
   for(i=0;i<2;i++)
   {
     for(j=0;j<4;j++)
     {
       printf("%d\t",C[j*4+i]);       
     }
     printf("\n");
   }
}

Yes! That do the job !
If you have the time can you explain me a little these lines:
Code:
B = A + OFFSET(3, 2, 0);
C = B + OFFSET(3, 4, 0)

and what does the OFFSET do ?
Also why its wrong with the 3 shmat() functions? In another program i made where A and B were read by a .txt file the arrays are correct.
I am at beginner level and i am trying a project... Of course this is not all of it. I just split it to pieces so i can do it easier.
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