Thread: Language C ---matrix----

Make a program that read the contents of two matrices and return a matrix with the result of the multiplication.



Multiply two matrices.
Show result in another matrix

Originally Posted by Apprentice123

Make a program that read the contents of two matrices and return a matrix with the result of the multiplication.



Multiply two matrices.
Show result in another matrix

How are the matrices being entered? Through a file? Command-line parameters? Through the console?

Anyway, you can represent the matrices with arrays (you can use two-dimensional arrays, but one-dimensional is easier to work with):

Try something like this (note that this was quickly written, untested, untried, and comes with no guarantees; I leave it to you to make it look "prettier" and add proper error checking):

Code:

/* m_mult - Multiply two matrices
 * result: a pointer to an array of ints of size rows1 * cols2 (allocate beforehand)
 * m1: pointer to the first matrix (array with entries ordered by row and then by column--e.g., ordered such that row m column n is given by m1[m*cols1 + n])
 * rows1: number of rows in first matrix
 * cols1: number of columns in first matrix
 * m2: pointer to second matrix
 * rows2: number of rows in second matrix
 * cols2: number of columns in second matrix
 */
void m_mult(int* result, int* m1, int rows1, int cols1, int* m2, int rows2, int cols2)
{
  if( (rows1 != cols2) || (rows2 != cols1) ) {
    //Can't multiply these matrices
    return;
  }

  //Iterate through each row of the resultant matrix
  for(int i = 0; i < rows1; ++i) {
    //Iterate through each column of the resultant matrix
    for(int j = 0; j < cols2; ++j) {
      //Element i, j of the resultant is given by the dot product of row i of m1 with col j of m2
      int product = 0;
      for(int entry = 0; entry < rows2; ++entry) {
        product += m1[i*cols1 + entry] * m2[entry*cols2 + j];
      }

      //Store the product in row i column j
      result[i*cols2 + j] = product;
    }
  }
}

I could make it much nicer if I could use C++ and its standard library, but this should at least work.