C++ 如何用C++流输出小数点后3位数字？

How to output with 3 digits after the decimal point with C++ stream?

提问by Alex Lapchenko

Given a variable of float type, how to output it with 3 digits after the decimal point, using iostream in C++?

给定一个float类型的变量，如何在C++中使用iostream输出小数点后3位？

回答by dasblinkenlight

Use setfand precision.

使用setf和precision。

#include <iostream>

using namespace std;

int main () {
    double f = 3.14159;
    cout.setf(ios::fixed,ios::floatfield);
    cout.precision(3);
    cout << f << endl;
    return 0;
}

This prints 3.142

这打印 3.142

回答by mask8

This one does show "13.141"

这个确实显示“13.141”

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

int main(){
    double f = 13.14159;
    cout << fixed;
    cout << setprecision(3) << f << endl;
    return 0;
}

回答by paxdiablo

You can get fixed number of fractional digits (and many other things) by using the iomanipheader. For example:

您可以通过使用iomanip标题获得固定数量的小数位数（以及许多其他内容）。例如：

#include <iostream>
#include <iomanip>

int main() {
    double pi = 3.141592653589;
    std::cout << std::fixed << std::setprecision(2) << pi << '\n';
    return 0;
}

will output:

将输出：

3.14

Note that both fixedand setprecisionchange the stream permanently so, if you want to localise the effects, you can save the information beforehand and restore it afterwards:

请注意，fixed和setprecision永久更改流，因此，如果您想本地化效果，您可以事先保存信息并在之后恢复它：

#include <iostream>
#include <iomanip>

int main() {
    double pi = 3.141592653589;

    std::cout << pi << '\n';

    // Save flags/precision.
    std::ios_base::fmtflags oldflags = std::cout.flags();
    std::streamsize oldprecision = std::cout.precision();

    std::cout << std::fixed << std::setprecision(2) << pi << '\n';
    std::cout << pi << '\n';

    // Restore flags/precision.
    std::cout.flags (oldflags);
    std::cout.precision (oldprecision);

    std::cout << pi << '\n';

    return 0;
}

The output of that is:

其输出是：

3.14159
3.14
3.14
3.14159

回答by uneet7

If you want to print numbers with precision of 3 digits after decimal, just add the following thing before printing the number cout << std::setprecision(3) << desired_number. Don't forget to add #include <iomanip>in your code.

如果要打印精度为小数点后 3 位的数字，只需在打印数字之前添加以下内容cout << std::setprecision(3) << desired_number。不要忘记添加#include <iomanip>您的代码。