#include <stdio.h>
#include <stdlib.h>
typedef struct {
  double *data;
  int nrows;
  int ncols;
} Matrix;
void printMatrix(Matrix *M);
void matrixMult(Matrix *A, Matrix *B, Matrix *C);
Matrix *createMatrix(int nrows, int ncols);
void destroyMatrix(Matrix *M);
int main(int argc, char *argv[])
{
  Matrix *A = createMatrix(3, 2);
  A->data[0] = 1.2;
  A->data[1] = 2.3;
  A->data[2] = 3.4;
  A->data[3] = 4.5;
  A->data[4] = 5.6;
  A->data[5] = 6.7;
  printMatrix(A);
  Matrix *B = createMatrix(2, 3);
  B->data[0] = 5.5;
  B->data[1] = 6.6;
  B->data[2] = 7.7;
  B->data[3] = 1.2;
  B->data[4] = 2.1;
  B->data[5] = 3.3;
  printMatrix(B);
  Matrix *C = createMatrix(3, 3);
  matrixMult(A, B, C);
  printMatrix(C);
  destroyMatrix(A);
  destroyMatrix(B);
  destroyMatrix(C);
  return 0;
}
Matrix *createMatrix(int nrows, int ncols)
{
  Matrix *M = malloc(sizeof(Matrix));
  M->data = malloc(nrows * ncols * sizeof(double));
  M->nrows = nrows;
  M->ncols = ncols;
  return M;
}
void destroyMatrix(Matrix *M)
{
  free(M->data);
  free(M);
}
void printMatrix(Matrix *M)
{
  int i, j;
  printf("[\n");
  for (i = 0; i < M->nrows; i++) {
    for (j = 0; j < M->ncols; j++) {
      printf("%6.3f ", M->data[i * M->ncols + j]);
    }
    printf("\n");
  }
  printf("]\n\n");
}
void matrixMult(Matrix *A, Matrix *B, Matrix *C)
{
  if (A->ncols != B->nrows) {
    printf("error: ncols of A does not equal nrows of B\n");
  }
  else {
    int i, j, k;
    double value;
    for (i = 0; i < A->nrows; i++) {
      for (j = 0; j < B->ncols; j++) {
	      value = 0.0;
	      for (k = 0; k < A->ncols; k++) {
	        value += A->data[i * A->ncols + k] * B->data[k * B->ncols + j];
	      }
	      C->data[i * A->nrows + j] = value;
      }
    }
  }
}