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  1. 浮点精度更改
  2. 浮点数格式化
  3. 参考文章
cout输出指定位数的浮点数
2020-08-02
C++

cout输出指定位数的浮点数。

//方便快速复制
#include <iomanip>

cout << std::fixed << std::setprecision(2);

cout << std::defaultfloat;

浮点精度更改

指定输出小数点后几位,包含在<iomanip>头文件中

#include <iostream>
#include <iomanip>

using std::cin;
using std::cout;
using std::endl;

int main(int argc, char* argv[]){
double a = 1.6006;
cout << std::setprecision(2) << a << endl;
return 0;
}

输出:

1.6

单独使用并不会进行补0操作

浮点数格式化

对于浮点数输入/输出的默认格式化,有四个操纵符可对其修改:

十六进制浮点格式化忽略流精度规定

包含在头文件<ios>中,而头文件<ios>包含在<iostream>中。

#include <iostream>
#include <sstream>

void print(const char* textNum, double num)
{
std::cout << "The number " << textNum << " in fixed: " << std::fixed << num << '\n'
<< "The number " << textNum << " in scientific: " << std::scientific << num << '\n'
<< "The number " << textNum << " in hexfloat: " << std::hexfloat << num << '\n'
<< "The number " << textNum << " in default: " << std::defaultfloat << num << '\n';
}

int main()
{
print("0.01 ", 0.01);
print("0.00001", 0.00001);
double f;
std::istringstream("0x1P-1022") >> std::hexfloat >> f;
std::cout << "Parsing 0x1P-1022 as hex gives " << f << '\n';
}

输出:

//十六进制输出结果因人而异,无精度可言
The number 0.01 in fixed: 0.010000
The number 0.01 in scientific: 1.000000e-02
The number 0.01 in hexfloat: 0x1.47ae147ae147bp-7
The number 0.01 in default: 0.01
The number 0.00001 in fixed: 0.000010
The number 0.00001 in scientific: 1.000000e-05
The number 0.00001 in hexfloat: 0x1.4f8b588e368f1p-17
The number 0.00001 in default: 1e-05
Parsing 0x1P-1022 as hex gives 2.22507e-308

参考文章

std::fixed, std::scientific, std::hexfloat, std::defaultfloat

std::fixed

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