C++中小数点输出格式

在《算法竞赛入门经典》一书中

习题1-5　打折　（discount）
一件衣服95元，若消费满300元，可打八五折。输入购买衣服件数，输出需要支付的金额（单位：元），保留两位小数。

我编写的代码为

#include<iostream>
#include<iomanip>
using namespace std;
int main(void){
    double s;
    cin>>s;

    if(s*95>=300){
        cout<<setiosflags(ios::fixed)<<setprecision(2)<<s*95*0.85<<endl;
    } 

    else{
        cout<<setiosflags(ios::fixed)<<setprecision(2)<<s*95<<endl;
    }

    return 0;
}

于是将C++中如何显示不同格式的小数查了一下：

Floating point output can be changed with

<< setiosflags( ios::fixed ) : never use scientific notation //绝对不使用科学记数法。

<< setiosflags( ios::scientific ) : always use scientific notation //使用科学记数法。

<< resetiosflags( ios::floatfield ) : restores default (use fixed or scientific notation based on the precision)//重置为缺省。

<< (re)setiosflags( ios::showpoint ) : controls if the trailing zeros and decimal point will be output (default is no) //控制小数点后面的零是否显示。

<< setprecision( digits ) : if fixed or scientific format is selected, controls the number of digits after the decimal place, otherwise controls the total number of significant digits// 如果fixed或者scientific已经确定了，则控制小数点后面的位数；否则，控制所有位数。

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

int main(void){
    double S = 0.000000123;
    double A = 456.7;
    double B = 8910000000000.0;

    cout << "default precision is "
         << cout.precision() << endl;    // 6

    cout << "    S = " << S << endl;    // 1.23e-07
    cout << "    A = " << A << endl;    // 456.7
    cout << "    B = " << B << endl;    // 8.91e+12

    cout << setiosflags(ios::showpoint)
         << "ios::showpoint ON" << endl;

    cout << "    S = " << S << endl;    // 1.23000e-07
    cout << "    A = " << A << endl;    // 456.700
    cout << "    B = " << B << endl;    // 8.91000e+12

    cout << resetiosflags(ios::showpoint)
         << "ios::showpoint OFF" << endl;
    cout << setiosflags(ios::fixed)
         << "ios::fixed ON" << endl;

    cout << "    S = " << S << endl;    // 0.000000
    cout << "    A = " << A << endl;    // 456.700000
    cout << "    B = " << B << endl;    // 8910000000000.000000

    cout << resetiosflags(ios::fixed)
         << setiosflags(ios::scientific)
         << "ios::scientific ON" << endl;

    cout << "    S = " << S << endl;    // 1.230000e-07
    cout << "    A = " << A << endl;    // 4.567000e+02
    cout << "    B = " << B << endl;    // 8.910000e+12
}

输出为：

default precision is 6
    S = 1.23e-007
    A = 456.7
    B = 8.91e+012
ios::showpoint ON
    S = 1.23000e-007
    A = 456.700
    B = 8.91000e+012
ios::showpoint OFF
ios::fixed ON
    S = 0.000000
    A = 456.700000
    B = 8910000000000.000000
ios::scientific ON
    S = 1.230000e-007
    A = 4.567000e+002
    B = 8.910000e+012

--------------------------------
Process exited after 0.02482 seconds with return value 0
请按任意键继续. . .

参考

https://www.cs.duke.edu/courses/cps149s/fall99/resources/n2.html