Thread: Matrix vector multiplication using function

Your code, corrected and commented (with a bit of "optimizations"):

Code:

#include <iostream>

// It helps (in terms of optimization to declare immutable
// arguments as 'const'. And, since the first array is incomplete,
// I've declared all of them incomplete... They must have 3 elements.
// Again, 'static' here is for the compiler benefict.
static void matris_muller ( const double a[][3], const double x[], double b[] );

int main()
{
  // Just to be sure I divided the values in groups of 3.
  // Nothing wrong not doing this, but, to me, is clearer.
  //
  // And... just for optimization sake, declaring as 'static const' will make
  // the compiler put those arrays in .rodata section in compile time.
  static const double a_cap[][3] = { { 1, 0, 0 }, { 0, 2, 0 }, { 0, 0, 3 } };
  static const double x[3] = { 1, 1, 1 };

  double b1[3];   // FIXED: We have to allocate all 3 doubles! This time, in the stack.

  matris_muller ( a_cap, x, b1 );

  for ( int i = 0 ; i < 3; ++i )
    std::cout << b1[i] << '\n';   // std::endl is used to flush the stream.
                                  // for std::cout, '\n' will do the same, but cheaper.
}

// FIXED: The name of the function was wrong!
void matris_muller ( const double a[][3], const double x[], double b[] )
{
  for ( int i = 0 ; i < 3 ; ++i )
    for ( int j = 0 ; j < 3; ++j )
      b[j] += a[i][j] * x[j];
}