Matrices

Hi,

I would like to find a determinant of a 2*2 matrix.
But there seems to be an error while it is running.

Code:

s=det2(&det,&m1,2,3);
printf("Value of determinant was..%d\n",det);

int det2(int *det,matrix *m1,int row,int col){
  int A,B,C,D;
  row=2;
  col=3;
  A=(*m1).arr[row][col];
  B=(*m1).arr[row+1][col];
  C=(*m1).arr[row][col+1];
  D=(*m1).arr[row+1][col+1];
  *det=(A*D)-(B*C);
  return(0);
}

You might want to post the smallest and simplest compilable program that demonstrates the problem.

Where's main()?
Post the complete source code.

here is the code.
getMatrix is another function used to obtain matrix from keyboard.

Code:

int det2(int *val,matrix *m,int row,int col);
main(){
  matrix m1, m2;
  int s,det;
s=getMatrix(&m1);
s=det2(&det,&m1,2,3);
 printf("Value of determinant was..%d\n",det);


int det2(int *det,matrix *m1,int row,int col){
  int A,B,C,D;
  row=2;
  col=3;
  A=(*m1).arr[row][col];
  B=(*m1).arr[row+1][col];
  C=(*m1).arr[row][col+1];
  D=(*m1).arr[row+1][col+1];
  *det=(A*D)-(B*C);
  return(0);
}

I assume the matrix is square since that's the algorithm for equating the determinant of a 2x2 square matrix.
However, if it's square,

Code:

...

  row=2;
  col=3;
  A=(*m1).arr[row][col];
  B=(*m1).arr[row+1][col];

...

column 3 won't exist, a square matrix is 2 x 2.
Also you're probably indexing the array columns & rows from 1 on paper, but in C arrays are indexed from 0.

Also,

Code:

A = (*m1).arr[row][col];

Can be written as m1->arr[row][col];

Consider the following:

Code:

A =| a  b |
   | c  d |

The det(A) = A[0][0].A[1][1] - A[0][1].A[1][0]

Where, A[row][column] (0-based).

If it's a 2X2 matrix, row 2 or col 2 won't exist either. The indices will be only 0 and 1.

Perhaps you have a larger matrix and are only picking a 2X2 square out of it?

Todd

Firstly thanks.

And yes, I have a larger matrix and I'm extracting the 2 by 2.

Thats why i picked row[2]col[3], which is the first number then the next number of the matrix should be [2+1][3] right?

"next number" for what?

[2+1][3] will be the number below (as we humans view a matrix) [2][3].

BTW, are you processing this matrix via calculating the cofactors, or is this some other application?

For example,

1 2
3 4

I meant if 1 is [2][3] when 2 will be [3][3] and 3 will be [2][4].

No, im am merely taking the matrices and applying the formula.

No, it's the other way around - in row-major order.

1 2
3 4

1 = [2][3]
2 = [2][4]
3 = [3][3]
4 = [3][4]

Todd

Thanks all. I try doing it again tmrw. For some reason my laptop doesnt allow the compiler to work. I'll hv to do it on the school's PC.

my laptop doesnt allow the compiler to work

Some specially designed laptop with compiler disabled environment?..