c++ - How do I express this? -

i'm trying work out how write following:

total = (value * 0.95 ^ 0) + (value * 0.95 ^ 1) + (value * 0.95 ^ 2) ... 

or:

x = (y * z ^ 0) + (y * z ^ 1) + (y * z ^ 2) + (y * z ^ 3) ... 

this expresses how calculate x 4 iterations, how can express work variable number of iterations? create loop , add values together, i'd find single equation solves this.

i'm using c++ guess isn't language specific problem (sorry literally don't know else ask question!).

any ideas?

thanks, chris.

equation-wise solving, geometric series , can therefore calculated with

double geometric_series(double y, double z, int n) {   return y * (std::pow(z, n) - 1.0) / (z - 1.0); } 

but same result can obtained fun c++ metaprogramming: if know number of iterations in advanced , you're allowed use c++17 features , fold expressions follows

template<std::size_t... n>  double calculate_x(double y, double z, std::index_sequence<n...>) { // [0;n[  auto f = [](double y_p, double z_p, double exp) {    return y_p * std::pow(z_p, exp);  };  return (f(y, z, n) + ...); }  template <std::size_t n> auto calculate_x(double y, double z) {   return calculate_x(y, z, std::make_index_sequence<n>{});  } 

alternatively can done pre-c++17 templates

template <int n> double calculate_x(double y, double z) {   return calculate_x<n-1>(y, z) + (y * std::pow(z, n - 1)); }  template <> double calculate_x<0>(double, double) {   return 0; } 

otherwise simpler solution use loop

double calculate_x_simple(double y, double z, int n) {   double ret = 0.0;   (int = 0 ; < n ; ++i)     ret += y * std::pow(z, i);   return ret; } 

driver code above

int main() {     // x = (y * z ^ 0) + (y * z ^ 1) + (y * z ^ 2) + (y * z ^ 3)    double y = 42.0;    double z = 44.5;    std::cout << (calculate_x<3>(y, z) == calculate_x_simple(y, z, 3)); // 1  }