What I've learnt:

setiosflags(ios::fixed)<<setprecision(1):fixed:将一个浮点数表示为一个定点整数和小数点和小数部分的格式;setprecision(1):小数部分的精度为1位

#include <iostream>
#include<math.h>
#include<iomanip>
using namespace std;

const double E=2.718281828;

void cal_TD(double x,double y)
{
    double e=6.11*pow(E,(5417.7530*((1/273.16)-(1/(y+273.16)))));
    double h=(0.5555)*(e-10.0);
    double humidex=x+h;
    cout<<"T "<<setiosflags(ios::fixed)<<setprecision(1)<<x<<" "<<"D "<<setprecision(1)<<y<<" "<<"H "<<setprecision(1)<<humidex<<endl;
    
}

void cal_TH(double x,double y)
{
    double h=y-x;
    double e=h/(0.5555)+10.0;
    double exponent=(log(e/6.11))/(log(E));
    double dewpoint=1/((1/273.16)-(exponent/5417.7530))-273.16;
    cout<<"T "<<setiosflags(ios::fixed)<<setprecision(1)<<x<<" "<<"D "<<setprecision(1)<<dewpoint<<" "<<"H "<<setprecision(1)<<y<<endl;
                       
}

void cal_HD(double x,double y)
{
    double e=6.11*pow(E,(5417.7530*((1/273.16)-(1/(y+273.16)))));
    double h=(0.5555)*(e-10.0);
    double temperature=x-h;
    cout<<"T "<<setiosflags(ios::fixed)<<setprecision(1)<<temperature<<" "<<"D "<<setprecision(1)<<y<<" "<<"H "<<setprecision(1)<<x<<endl;

}
int main(int argc, const char * argv[]) {
    char a,b;
    double x,y;
    while(cin>>a&&a!='E')
    {
        cin>>x>>b>>y;
        if(a=='T'&&b=='D')
            cal_TD(x,y);
        if(a=='T'&&b=='H')
            cal_TH(x, y);
        if(a=='H'&&b=='D')
            cal_HD(x,y);
        if(a=='D'&&b=='T')
            cal_TD(y,x);
        if(a=='H'&&b=='T')
            cal_TH(y,x);
        if(a=='D'&&b=='H')
            cal_HD(y,x);
    }
    return 0;
}