We have $N\times N$ matrices A, B and C and we wish to evaluate $A=B+C$. $$\mathbf{A}= \mathbf{B}\pm\mathbf{C} \Longrightarrow a_{ij} = b_{ij}\pm c_{ij},$$ In C/C++ this would be coded like

   for(i=0 ; i < N ; i++) {
      for(j=0 ; j < N ; j++) {
         a[i][j] = b[i][j]+c[i][j]