Question

我正在从事 Codecademy 项目，但无法真正找到我的问题的解决方案。

给定 weight 和 planet-number 作为输入，本练习的目标是使用 switch 语句返回您需要根据 converted-weight 到达的 planet-number你想去。

计算 converted-weight 后，我想根据 planet-number 是否在有效范围内打印一些条件消息。我在比较 planet-number 的有效范围时遇到困难（例如，在这种情况下为 0-6）。这是我尝试过的代码：

#include <iostream>

int main() {

  double weight;
  int planet;
  double planetw;

  std::cout << "What is your Earth weight?\n";
  std::cin >> weight;

  std::cout << "What Planet number would you like to fight on? \n";
  std::cin >> planet;

switch (planet) {
case 1:
  planetw = weight * 0.38;
  break;
case 2:
  planetw = weight * 0.91;
  break;
case 3:
  planetw = weight * 0.38;
  break;
case 4:
  planetw = weight * 2.34;
  break;
case 5:
  planetw = weight * 1.06;
  break;
case 6:
  planetw = weight * 0.92;
  break;
case 7:
  planetw = weight * 1.19;
  break;
default:
  std::cout << "That is not a planet option\n";
  break;

}
 
if (planet) != range (0,6) {
  std::cout << "Pick a valid planet number next time.\n";
} else {
 std::cout << "Your traget weight to fight there is " << weight << " .\n";
}
}

Answer 1

由于您已经在 switch 情况下处理了所需的值范围，因此您可以使用已经存在的 default 情况，例如：

#include <iostream>

int main() {

  double weight;
  int planet;
  double planetw;

  std::cout << "What is your Earth weight?\n";
  std::cin >> weight;

  std::cout << "What Planet number would you like to fight on? \n";
  std::cin >> planet;

  switch (planet) {
  case 1:
    planetw = weight * 0.38;
    break;
  case 2:
    planetw = weight * 0.91;
    break;
  case 3:
    planetw = weight * 0.38;
    break;
  case 4:
    planetw = weight * 2.34;
    break;
  case 5:
    planetw = weight * 1.06;
    break;
  case 6:
    planetw = weight * 0.92;
    break;
  case 7:
    planetw = weight * 1.19;
    break;
  default:
    std::cout << "That is not a planet option\n";
    std::cout << "Pick a valid planet number next time.\n";
    return 0;
  }
 
  std::cout << "Your target weight to fight there is " << planetw << " .\n";

  return 0;
}

但是，要回答您的问题，您可以进行如下范围检查：

#include <iostream>

int main() {

  double weight;
  int planet;
  double planetw;

  std::cout << "What is your Earth weight?\n";
  std::cin >> weight;

  std::cout << "What Planet number would you like to fight on? \n";
  std::cin >> planet;

  switch (planet) {
  case 1:
    planetw = weight * 0.38;
    break;
  case 2:
    planetw = weight * 0.91;
    break;
  case 3:
    planetw = weight * 0.38;
    break;
  case 4:
    planetw = weight * 2.34;
    break;
  case 5:
    planetw = weight * 1.06;
    break;
  case 6:
    planetw = weight * 0.92;
    break;
  case 7:
    planetw = weight * 1.19;
    break;
  default:
    std::cout << "That is not a planet option\n";
    break;
  }
 
  if (planet < 1 || planet > 7) {
    std::cout << "Pick a valid planet number next time.\n";
  } else {
    std::cout << "Your target weight to fight there is " << planetw << " .\n";
  }

  return 0;
}

如果你真的想使用像 if (planet != range(....)) 这样的实际语法，那当然是可行的，但需要更多的设置工作。你可以这样做：

#include <iostream>

struct sRange
{
  int minValue;
  int maxValue;

  bool operator==(int value) const {
    return (value >= minValue && value <= maxValue);
  }

  bool operator!=(int value) const {
    return (value < minValue || value > maxValue);
  }
};

bool operator==(int value, const sRange &rhs) {
    return rhs == value;
}

bool operator!=(int value, const sRange &rhs) {
    return rhs != value;
}

sRange range(int minValue, int maxValue) {
    return sRange{minValue, maxValue};
}

int main() {

  double weight;
  int planet;
  double planetw;

  std::cout << "What is your Earth weight?\n";
  std::cin >> weight;

  std::cout << "What Planet number would you like to fight on? \n";
  std::cin >> planet;

  switch (planet) {
  case 1:
    planetw = weight * 0.38;
    break;
  case 2:
    planetw = weight * 0.91;
    break;
  case 3:
    planetw = weight * 0.38;
    break;
  case 4:
    planetw = weight * 2.34;
    break;
  case 5:
    planetw = weight * 1.06;
    break;
  case 6:
    planetw = weight * 0.92;
    break;
  case 7:
    planetw = weight * 1.19;
    break;
  default:
    std::cout << "That is not a planet option\n";
    break;
  }
 
  if (planet != range(1, 7)) { // or: if (range(1, 7) != planet)
    std::cout << "Pick a valid planet number next time.\n";
  } else {
    std::cout << "Your target weight to fight there is " << planetw << " .\n";
  }

  return 0;
}

Answer 2

因为您在 if 语句中询问范围，所以我不使用 switch 语句回答您的问题。

你可以这样做

if (planet > 7 || planet < 1) {
    std::cout << "That is not a planet option\n";
    std::cout << "Pick a valid planet number next time.\n";
}
else {
    double ratio[7] = {0.38, 0.91, 0.38, 2.34, 1.06, 0.92, 1.19};
    planetw = weight * ratio[planet-1];
    std::cout << "Your target weight to fight there is " << planetw << " .\n";
}

Answer 3

你可以简单地把它写成两个条件来实现你的范围逻辑：

if (planet > 7 || planet < 1) {
}
else {}

注意：你从索引 1 开始，所以你的范围是 1 到 7

这些指令有时会在汇编代码 jg（如果更大则跳转）和 jl（如果更低则跳转）中进行翻译，但是编译器使用的一种适用于无符号整数的技巧是减去低端，然后检查它是否小于或等于到最大界限。

int tmp = planet - 1;
if (tmp > 6) { ... must be a valid planet message... }

您可以在 if 语句中使用数字范围吗？

3 个答案: