static double cylinder (String which){
      double r = num();
      double h = num();
      h = num();
      if(which.equals("volume")){
        return (h × Math.PI × r * r);
      }
      else if(which.equals("surface")){
        return 2 × r × Math.PI × h + 2 × Math.PI × r * r;
      }
      else{
        System.out.println("You need to enter 'volume' or 'surface', input not recognized.");
        return -1.0
      }
    }