for(i = 0; i < n; i++){ a[i] += b[i];}
for(i = 0; i < n; i++){ a[i] += b[i]; }
for(i = 0; i < n; i+=4){ // suppose that n % 4 == 0 a[i] += b[i]; a[i+1] += b[i+1]; a[i+2] += b[i+2]; a[i+3] += b[i+3];}
for(i = 0; i < n; i+=4){ // suppose that n % 4 == 0 a[i] += b[i]; a[i+1] += b[i+1]; a[i+2] += b[i+2]; a[i+3] += b[i+3]; }