为什么此C++程序不会为第一个输出显示浮点数？

Question

我很好奇为什么如果代码块是一系列代码块中的第一个块，C++为什么不将输出显示为float。让我来解释......拿这个例子：

#include <iostream> 
#include<iomanip> 
using namespace std; 

int main() 
    { 
    double miles; 
    double fahrenheit; 
    double gallons; 
    double pounds; 
    double inches; 

    const double milesToKilometers=1.60; 
    const double fahrenheitToCelsius=5.0/9.0; 
    const double gallonsToLiters=3.9; 
    const double poundsToKilograms=0.45; 
    const double inchesToCentimeters=2.54; 



    // Get miles, then convert from miles to kilometers. 
    cout << "Please tell me how many miles you want converted to kilometers: 
    "; 
    cin >> miles; 

    if (miles < 0) 
     cout<<"You cannot enter negative numbers. Please run the program a 
     again and enter valid number." <<endl<<endl; 
    else 
     cout<<miles<<" miles is equal to "<<setprecision(2) 
     <<fixed<<miles*milesToKilometers<<" kilometers.\n\n"; 


    // Get fahrenheit, then convert from fahrenheit to celsius. 
    cout << "Please tell me how many degrees fahrenheit you want converted to 
    celsius: "; 
    cin >> fahrenheit; 

    if(fahrenheit < 0) 
    cout<<"You cannot enter negative numbers. Please run the program again 
    and enter valid number." <<endl<<endl; 
    else if(fahrenheit > 1000) 
    cout<<"You cannot enter numbers above 1000. Please run the program again 
    and enter valid number." <<endl<<endl; 
    else 
    cout<<fahrenheit<<" degree fahrenheit is equal to "<<setprecision(2) 
    <<fixed<< (fahrenheit-32)*fahrenheitToCelsius <<" celsius.\n\n"; 


    // Get gallons, then convert from gallons to liters. 
    cout << "Please tell me how many gallons you want converted to liters: "; 
    cin >> gallons; 

    if (gallons < 0) 
    cout<<"You cannot enter negative numbers. Please run the program again 
    and enter valid number." <<endl<<endl; 
    else 
    cout<<gallons<<" gallons is equal to "<<setprecision(2) 
    <<fixed<<gallons*gallonsToLiters<<" liters.\n\n"; 


// Get pounds, then convert from pounds to kilograms. 
cout << "Please tell me how many pounds you want converted to kilograms: "; 
cin >> pounds; 

if (pounds < 0) 
    cout<<"You cannot enter negative numbers. Please run the program again 
    and enter valid number." <<endl<<endl; 
else 
    cout<<pounds<<" pounds is equal to "<<setprecision(2) 
<<fixed<<pounds*poundsToKilograms<<" kilograms.\n\n"; 


// Get inches, then convert from inches to centimeters. 
cout << "Please tell me how many inches you want converted to centimeters: 
"; 
cin >> inches; 

if (inches < 0) 
    cout<<"You cannot enter negative numbers. Please run the program again 
    and enter valid number." <<endl<<endl; 
else 
    cout<<inches<<" inches is equal to "<<setprecision(2) 
    <<fixed<<inches*inchesToCentimeters<<" centimeters.\n\n"; 


return 0; 
} 

如果你运行它，并输入任何英里数，（例如，10英里），这首码输出将显示：

Please tell me how many miles you want converted to kilometers:10 
//input>>10 
10 miles is equal to 16.00 kilometers. 

的通知输出说10英里不是10.00。现在观察剩余的输出： 如果对于剩余的码块输入10，“华氏摄氏度，”“加仑到升”，“英寸到厘米”的输出将显示10.00，而不是10。另外，如果转代码块的顺序（例如，将'英里转换为公里'代替第一代码块的第二代码块），第一代码块将总是显示一个整数（例如10）而不是浮点数（例如10.00）。

这是怎么回事？

Answer 1

缺省值是代表精确的整数值的浮点值被不带小数点的，没有任何数字的（未显示）小数点的右边显示。这就是为什么第一个是10。 插入setprecision(2)之后，浮点值是带小数点和两个数字在小数点的右边显示的所有。这就是为什么其余的是10.00。如果您希望第一个显示为10.00，请移动setprecision(2)的插入，以便在显示第一个值之前完成。

Answer 2

你在double声明的变量;尝试将它们声明为float这样它将支持小数点，并且您不需要使用setprecision(n)。 但是！如果你的号码中的一个回来跟小数点后的数字十亿，并要限制随后的小数，继续在那里折腾setprecision(n)回数的量。