Question

I ve got two matrices W2 and hiddenLayer and i want to proceed the multiplication of those. W2 size’s 12×50 and hiddenLayer size’s 50×1. The proper code for the above calculation:

 for(int h=0; h<50; h++){
      for(int k=0; k<12; k++){
        outputLayer += W2[k][h]*HiddenLayer[h];
      }
}

or i ve got to put at first k-for??

Answer 1

Matrix multiplication is defined as:

C = AB ⇔ Ci,j = Σk=1..n Ai,k Bk,j for i,j = 1...n (in case of square matrices).

Thus outputLayer is a vector. Since HiddenLayer is a vector too, this isn’t really a matrix multiplication but a matrix vector multiplication, which simplifies the formula above:

b = Ax ⇔ bi = Σk=1..m Ai,k xk for i = 1...n (A is an n x m matrix).

So all in all your code should be something like

for(int row = 0; row < 12; row++){
    outputLayer[row] = 0;
    for(int column = 0;  column < 50; column++){    
        outputLayer[row] += W2[row][column]*HiddenLayer[column];
      }
}