c++ - do\while 语句将无法正确编译(初学者)

Question

请放轻松，因为我是 C++ 的新手，目前正在上编程入门课:)

我目前在正确编译 do\while 语句时遇到问题。我的目标是输入以下内容，直到输入标记值 -1，这将结束该过程并提供我的值的累计总数。

以下是我需要为每个实体输入的值:

 gallons used 
 starting mileage
 ending mileage
 miles driven
 miles per gallon 

一旦输入标记值，它就应该为所有合并的实体计算以下值:

total gallons used
total miles driven
overall average miles per gallon

以下是我到目前为止的内容，但输入 -1 时并未结束。当输入 -1 时，我还没有添加尚未使用的最终值，因为我不确定如何继续并试图让第 1 部分首先正常工作。下面是我的代码(请原谅评论):

int main(void) 
{

    StartUp();   //indicates beginning of code

    double startmileage{ 0 };              //indicates starting mileage, initialized at the value of 0
    double endmileage{ 0 };                //indicates ending mileage, initialized at the value of 0
    double milesdriven{ 0 };               //indicates total miles driven, initialized at the value of 0
    double gallonsused{ 0 };               //indicates total gallons used, initialized at the value of 0
    long double milespergallon{ 0 };       //indicates total miles per gallon, initialized at the value of 0

    do
    {
        cout << "enter the gallons used:  ";                                //prompts user to enter total number of gallons used for trip
        cin >> gallonsused;                                                 //user enters data

        cout << "enter starting mileage:  ";                                //prompts user to enter start mileage for trip
        cin >> startmileage;                                                //user enters data

        cout << "enter ending mileage:  ";                                  //prompts user to enter end mileage for trip
        cin >> endmileage;                                                  //user enters data

        milesdriven = startmileage + endmileage;                            //calculates total miles driven

        cout << "Miles Driven:  " << milesdriven << endl;                   //outputs total miles driven to screen

        milespergallon = milesdriven / gallonsused;                         //calculates total miles per gallon

        cout << "Miles Per Gallon:  " << milespergallon << endl;            //outputs miles per gallon to screen
    } 

    while (gallonsused != -1);
WrapUp();  //indicates end of code

    return Success;

}

这是正在发生的事情:

enter the gallons used:  12.8
enter starting mileage:  0
enter ending mileage:  277
Miles Driven:  277
Miles Per Gallon:  21.6406
enter the gallons used:  -1
enter starting mileage:

如您所见，它会循环，但不会在输入标记值时停止

如有任何帮助，我们将不胜感激!

谢谢!

Answer 1

在循环结束之前，您不会测试 gallonsused != -1，在用户响应所有其他提示并计算里程之后。

不要使用 do-while()，而是在用户输入后立即测试 gallonsused，然后跳出循环。

您还需要总行驶英里数和使用的总加仑数的变量。每次输入后，您将该行程的值添加到总计中，并在最后将它们用于总计。

double totalmiles = 0;
double totalgallons = 0;

while (true)
    {
        cout << "enter the gallons used:  ";                                //prompts user to enter total number of gallons used for trip
        cin >> gallonsused;                                                 //user enters data
        if (gallonsused == -1) { // Check for sentinel
            break;
        }

        totalgallons += gallonsused;

        cout << "enter starting mileage:  ";                                //prompts user to enter start mileage for trip
        cin >> startmileage;                                                //user enters data

        cout << "enter ending mileage:  ";                                  //prompts user to enter end mileage for trip
        cin >> endmileage;                                                  //user enters data

        milesdriven = startmileage + endmileage;                            //calculates total miles driven

        totalmiles += milesdriven;

        cout << "Miles Driven:  " << milesdriven << endl;                   //outputs total miles driven to screen

        milespergallon = milesdriven / gallonsused;                         //calculates total miles per gallon

        cout << "Miles Per Gallon:  " << milespergallon << endl;            //outputs miles per gallon to screen
    } 

   // Show grand totals
    cout << "Grand totals:" << endl;

    cout << "Total Miles Driven: " << totalmiles << endl;
    cout << "Total Miles Per Gallon: " << totalmiles/totalgallons << endl;

    WrapUp();