Matrix multiplication

[drey]

I don't know how do this. I know that there are 4 "for" statements...but how????? Can anyone help me...please.. If you don't know what I mean...

http://www.cs.sunysb.edu/~algorith/files/matrix-multiplication.shtml.

PS: Sorry for my English!

 

[Zyrenthian]

ok the down and dirty of what you need to do is

AxB=C
A B C=3x3 matrix
[2 3] [2 3 4] [c11 c12 c13]
[3 4] X [3 4 5] = [c21 c22 c23]
[4 5] [c31 c32 c33]

c11 = 2*2 + 3*3 = 13
c12 = 2*3 + 3*4 = 18
c13 = 2*4 + 3*5 = 23
c21 = 3*2 + 3*3 = 21

etc...

What I suggest is you research matrix multiplication. It is not too difficult once you get the hang of it. Matrix A's Cols must = the Matrix B's rows to do a cross product. You multiply a11 with b11 and add it to a12 x b21 where the first number a11 represents row and the second number a11 represents column.

Good Luck

Matt

 

[Zyrenthian]

Sorry, to do it with C++ you would have arrays

int matrixA[3][2];
int matrixB[2][3];
int matrixC[3][3];

memset(matrixC,0,sizeof(int)*3*3);


int col = 0;
int row = 0;

for(int i = 0; i<3;i++)
{
    for(int j = 0;j<2;j++)
    {
       matrixC[col][row]+= matrix[i][j];
       col++;
       if(col == 3)
       {
             col = 0;
             row++;
       }
    }
}

sorry for the mis-interpretation, I got side tracked and thought you were asking how to do matrix multiplicaiton.

Matt

 

[Zyrenthian]

MAN... 2 for 2... sorry bout not putting in the code tag once again... in need more coffee

I think this should do the trick

Sorry, to do it with C++ you would have arrays

int matrixA[3][2];
int matrixB[2][3];
int matrixC[3][3];

memset(matrixC,0,sizeof(int)*3*3);


int col = 0;
int row = 0;

for(int i = 0; i<3;i++)
{
    for(int j = 0;j<2;j++)
    {
       matrixC[col][row]+= matrix[i][j];
       col++;
       if(col == 3)
       {
             col = 0;
             row++;
       }
    }
}

sorry for the mis-interpretation, I got side tracked and thought you were asking how to do matrix multiplicaiton.

Matt

 

[Zyrenthian]

man.. i have NOT had enough coffee... i am sorry. What I said above is invalid.

Please dis-regard it. As you said you do need 4 for loops

for(int acol = 0;acol<2;acol++)
{
for(int arow = 0;arow<3;arow++)
{
for(int bcol = 0;bcol < 3;bcol++)
{
for(int brow = 0;brow<2;brow++)
{
matrixC[row][col] = matrixA[acol][arow]*matrixB[bcol][brow];

col++;
if(col==3)
{
col = 0;
row++;
}
}
}
}
}

I THINK THIS IS THE ANSWER but with my track record this morning I dont know

PLEASE TEST IT

Matt