error C2106: '=' : left operand must be l-value выдаёт ошибку на сортировку - C++

sqrt(double( x[i] * x[i] +  y[i] * y[i])) = sqrt(double( x[i+1] * x[i+1] +  y[i+1] * y[i+1])) ;
sqrt(double( x[i+1] * x[i+1] +  y[i+1] * y[i+1]))=Buf;