上代码：

1.普通

#include<bits/stdc++.h>
using namespace std;

int main(){
    double x;
    cin>>x;
    cout << fixed << setprecision(3);
    if(x<5) cout << (-x)+2.5;
    else if(x<10) cout << 2.0-1.5*(x-3)*(x-3);
    else cout << x/2-1.5;
    return 0;
}

2.函数

#include<bits/stdc++.h>
using namespace std;

double f(double x){
    if(x<5) return (-x)+2.5;
    else if(x<10) return 2.0-1.5*(x-3)*(x-3);
    else return x/2-1.5;
}
int main(){
    double x;
    cin>>x;
    cout << fixed << setprecision(3)<<f(x);
    return 0;
}

3.傻瓜式

#include<bits/stdc++.h>
using namespace std;

#include<bits/stdc++.h>
using namespace std;

int main(){
    double x;
    cin >> x;
    const double eps = 1e-9;
    if (abs(x - 0.0) < eps) cout << fixed << setprecision(3) << 2.500 << endl;
    else if (abs(x - 1.0) < eps) cout << fixed << setprecision(3) << 1.500 << endl;
    else if (abs(x - 2.0) < eps) cout << fixed << setprecision(3) << 0.500 << endl;
    else if (abs(x - 3.0) < eps) cout << fixed << setprecision(3) << -0.500 << endl;
    else if (abs(x - 4.0) < eps) cout << fixed << setprecision(3) << -1.500 << endl;
    else if (abs(x - 5.0) < eps) cout << fixed << setprecision(3) << -4.000 << endl;
    else if (abs(x - 6.0) < eps) cout << fixed << setprecision(3) << -11.500 << endl;
    else if (abs(x - 7.0) < eps) cout << fixed << setprecision(3) << -22.000 << endl;
    else if (abs(x - 8.0) < eps) cout << fixed << setprecision(3) << -35.500 << endl;
    else if (abs(x - 9.0) < eps) cout << fixed << setprecision(3) << -52.000 << endl;
    else if (abs(x - 10.0) < eps) cout << fixed << setprecision(3) << 3.500 << endl;
    else if (abs(x - 11.0) < eps) cout << fixed << setprecision(3) << 4.000 << endl;
    else if (abs(x - 12.0) < eps) cout << fixed << setprecision(3) << 4.500 << endl;
    else if (abs(x - 13.0) < eps) cout << fixed << setprecision(3) << 5.000 << endl;
    else if (abs(x - 14.0) < eps) cout << fixed << setprecision(3) << 5.500 << endl;
    else if (abs(x - 15.0) < eps) cout << fixed << setprecision(3) << 6.000 << endl;
    else if (abs(x - 16.0) < eps) cout << fixed << setprecision(3) << 6.500 << endl;
    else if (abs(x - 17.0) < eps) cout << fixed << setprecision(3) << 7.000 << endl;
    else if (abs(x - 18.0) < eps) cout << fixed << setprecision(3) << 7.500 << endl;
    else if (abs(x - 19.0) < eps) cout << fixed << setprecision(3) << 8.000 << endl;
    else {
        double y;
        if (x >= 0 && x < 5) {
            y = -x + 2.5;
        } else if (x >= 5 && x < 10) {
            y = 2 - 1.5 * (x - 3) * (x - 3);
        } else if (x >= 10 && x < 20) {
            y = x / 2 - 1.5;
}
        cout << fixed << setprecision(3) << y << endl;
    }
    
    return 0;
}