如果用户输入与特定值匹配怎么办？

Question

我正在编写一个简单的 c++ 测量程序，它要求用户输入以选择他们想从哪个单位测量单位和单位？但我不知道结构是否正确（我的 C++ 真的很烂）。这是我的代码：

#include <iostream>
using namespace std;

int main()
{
    int storeFROM, storeTO;
    char mm, cm, m, kk;

    cout << "Enter the initial unit (mm, cm, m, or km): ";
    cin >> storeFROM;

    cout << endl;

    if ((storeFROM != 'mm') || storeFROM != 'cm' || storeFROM != 'm' || storeFROM != 'km') {
        cout << "--> Sorry unit to convert FROM is invalid" << endl;
    }
    else if ((storeFROM == 'mm') || storeFROM == 'cm' || storeFROM == 'm' || storeFROM == 'km') 
    {
        cout << "Enter the initial unit(mm, cm, m, or kk) :";
        cin >> storeTO;
    }

    // Calculate the selected units
    system("pause");
}

我希望有人可以帮助我询问用户输入以询问他们想要哪个单元，这正是它应该显示的方式：

Answer 1

这是正确的代码：

bool isValidUnit(const std::string &unit);
int convertValue(double value, const std::string &unitFrom, const std::string &unitTo);

int main() {

    ...

    std::string storeFROM, storeTO;

    cout << "Enter the initial unit (mm, cm, m, or km): ";
    cin >> storeFROM;

    if (!isValidUnit(storeFROM)) {
        cout << "--> Sorry unit to convert FROM is invalid" << endl;
        ...
    }

    cout << "Enter the initial unit (mm, cm, m, or km): ";
    cin >> storeTO;

    if (!isValidUnit(storeTO)) {
        cout << "--> Sorry unit to convert TO is invalid" << endl;
        ...
    }

    double value;

    cout << "Enter the value in (" << storeFROM << "): ";
    cin >> value;

    double valueConverted = convertValue(value, storeFROM, storeTO);

    cout << "Value in (" << storeTO << "): " << valueConverted << endl;

    return 0;
}

bool isValidUnit(const std::string &unit) {
    return unit == "mm" || unit == "cm" || unit == "m" || unit == "km";
}

double unitMultiplier(const std::string &unit) {
    if (unit == "mm") return 0.001;
    if (unit == "cm") return 0.01;
    if (unit == "m") return 1;
    if (unit == "km") return 1000;
    return 0.;
}

double convertValue(double value, const std::string &unitFrom, const std::string &unitTo) {
    if (unitFrom == unitTo) return value;
    if (value <= 0.) return 0.; // or value :)

    // Get it in meters
    int valueInMetes = value * unitMultiplier(unitFrom);
    return valuesInMeter / unitMultiplier(unitTo);
}

Answer 2

在你的程序中，storeFROM 和storeTO 都是strings，所以你应该将它们声明为strings：

string storeFROM, storeTO;

当将它们与 "cm"、"mm" 等值进行比较时，您应该在 "" 中引用它们，而不是在 '' 中引用它们，因为您绝对不希望它们是 multicharacter literals。它应该是这样的：

if ((storeFROM != "mm") || storeFROM != "cm" 
                        || storeFROM != "m" || storeFROM != "km") 
{
    cout << "--> Sorry unit to convert FROM is invalid" << endl;
}
else if ((storeFROM == "mm") || storeFROM == "cm" 
                             || storeFROM == "m" || storeFROM == "km") 
{
    cout << "Enter the initial unit(mm, cm, m, or kk) :";
    cin >> storeTO;
}

Answer 3

对于 int 类型的对象，您不得输入不可接受的符号，例如字母“m”。

您可以将此代码用作程序的模板。

#include <iostream>
#include <string>
#include <initializer_list>
#include <algorithm>
#include <cstdlib>

std::ostream & display_meazures( const std::initializer_list<const char *> &l,
                                 std::ostream &os = std::cout )
{
    auto it = l.begin();
    for ( size_t i = 0; i < l.size(); i++ )
    { 
        if ( i == l.size() - 1 ) os << "or ";
        os << *it;
        if ( i != l.size() - 1 ) os << ", ";
    }

    return os;
}

int main()
{
   std::string storeFROM, storeTO;

   std::initializer_list<const char *> meazure = { "mm", "cm", "m", "km" };

   while ( true )
   {
      std::cout << "Enter the initial unit (" << display_meazures( meazure ) 
                << "): ";
      std::cin >> storeFROM;

      if ( std::find( meazure.begin(), meazure.end(), storeFROM ) != 
           meazure.end() ) 
      {
        break;
      }

      std::cout << "--> Sorry unit to convert FROM is invalid" << std::endl;
      std::cout << std::endl;
   }

// Calculate the selected units

   std::system( "pause" );
}

Answer 4

string storeFROM, storeTO; // note string thingie
float value1, value2;

cin>>storeFROM;

if ((storeFROM != "mm") || storeFROM != "cm" || storeFROM != "m" || storeFROM != "km") )
{
    cout << "--> Sorry unit to convert FROM is invalid" << endl;
}
else  
{
    cin>>value1;
    cout << "Enter the initial unit(mm, cm, m, or kk) :";
    cin >> storeTO;
    if((storeTO == "mm") || storeTO == "cm" || storeTO == "m" || storeTO == "km"))
    {
       cin>>value2;
          //convert here...
          //hint: use switch-case
    }
}
//end programme

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