How to efficiently implement the following code in MATLAB

Question

for i1 = 1:M
  for i2 = 1:M
      if i1 == i2
          for g = 1:N
              iS(i1,i2) = iS(i1,i2) + eq(i1,G(g,1))+eq(i2,G(g,2));
          end
      else