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C中的多个提示出现问题++

cout cin if-statement c++

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amwalker · 技术社区 · 3 年前

我如何重写以下代码,使每个输出都有自己的提示(参见所需结果)-g++(Ubuntu 9.4.0-1ubuntu1~20.04.1)9.4.0

#include <iostream>
#include <string>

using std::cin;
using std::cout;
using std::string;

int main() {

    cout << "Enter a numeric value followed by a unit abbreviation (km,mi): ";
    double initial_value;
    string initial_unit;
    cin >> initial_value >> initial_unit; 

    double conversion_factor = {initial_value *1.609};
    double conversion_factor2 = {initial_value /1.609};
    std::string miles = "mi";
    std::string kilometers = "km";

    if (initial_unit == miles) {
       cout << initial_value << " " << initial_unit << " "<< "is" << " " << conversion_factor << "\n" << "km";
    }
    if (initial_unit == kilometers) {
       cout << initial_value << " " << initial_unit << " "<< "is" << " " << conversion_factor2 << "\n" << "mi";
    }
    else (initial_unit != kilometers, miles);
    {
       cout << "Unknown unit, please try again";
    }

    return 0;
}

我得到的结果:

Enter a numeric value followed by a unit abbreviation (km,mi): 10 mi
10 mi is 16.09
kmUnknown unit, please try again

期望结果示例:

Enter a numeric value followed by a unit abbreviation (km,mi): 
10 mi
10 mi is 16.09 km  
16.09 km
16.09 km is 10 mi  
20000 leagues
Unknown unit, please try again  
0.5 mi
0.5 mi is 0.8045 km

2 回复 | 直到 3 年前

Remy Lebeau 3 年前

声明 else (initial_unit != kilometers, miles); 是完全错误的。

首先 ; 不应该在那里。这导致您的代码输出 Unknown unit 消息,不管是什么 initial_unit 实际上设置为。

但更重要的是,您不能像尝试的那样将单个值与逗号分隔的值列表进行比较 comma operator 和 operator precedence ,表达式 initial_unit != kilometers, miles 将被解释为 (initial_unit != kilometers), miles 因此表达式的结果将是 miles 不管怎样 initial_unit 实际上设置为。

您必须单独比较每个值,如下所示:

else if ((initial_unit != kilometers) && (initial_unit != miles))

但是,你已经比较过了 initial_unit 反对 kilometers 和 英里 在那句话之前,所以没有必要再对它们进行比较。声明应该只是 else 本身,例如:

if (initial_unit == miles) {
   ...
}
else if (initial_unit == kilometers) { // <-- you forgot the 'else' here
   ...
}
else { // <-- everything else!
   ...
}

现在,话虽如此,要做到你的要求,你需要运行环 ,其中每次迭代都会提示用户输入新的输入,例如:

#include <iostream>
#include <string>

using std::cin;
using std::cout;
using std::string;

int main() {

    const std::string miles = "mi";
    const std::string kilometers = "km";

    cout << "Enter a numeric value followed by a unit abbreviation (km,mi):\n";

    double value;
    string unit;

    while (cin >> value >> unit) {

        if (unit == miles) {
            cout << value << " " << unit << " is " << (value * 1.609) << " " << kilometers << '\n';
        }
        else if (unit == kilometers) {
            cout << value << " " << unit << " is " << (value / 1.609) << miles << '\n';
        }
        else {
            cout << "Unknown unit, please try again\n";
        }
    }

    return 0;
}

paddy 3 年前

你搞得一团糟。以下代码存在多个问题。

if (initial_unit == miles) {
   cout << initial_value << " " << initial_unit << " "<< "is" << " " << conversion_factor << "\n" << "km";
}
if (initial_unit == kilometers) {
   cout << initial_value << " " << initial_unit << " "<< "is" << " " << conversion_factor2 << "\n" << "mi";
}
else (initial_unit != kilometers, miles);
{
   cout << "Unknown unit, please try again";
}

让我们将其分解,看看实际发生了什么:

// Output miles->km conversion
if (initial_unit == miles) {
   cout << initial_value << " " << initial_unit << " "<< "is" << " " << conversion_factor << "\n" << "km";
}

// This condition is unrelated to the first
if (initial_unit == kilometers) {
   cout << initial_value << " " << initial_unit << " "<< "is" << " " << conversion_factor2 << "\n" << "mi";
} else {
    initial_unit != kilometers;  // unused inequality test
    miles;                       // unused result of comma operator
}

// This is always executed
cout << "Unknown unit, please try again";

正确的条件句链是:

if (initial_unit == miles) {
   cout << initial_value << " " << initial_unit << " is " << conversion_factor << "km\n";
}
else if (initial_unit == kilometers) {
   cout << initial_value << " " << initial_unit << " is " << conversion_factor2 << "mi\n";
}
else {
   cout << "Unknown unit, please try again\n";
}