数据甚至不超过20
直接暴力打表

l=[2.5,1.5,0.5,-0.5,-1.5,-4,-11.5,-22,-35.5,-52,3.5,4,4.5,5,5.5,6,6.5,7,7.5,8]
print("%.3f"%l[int(input())])

#include <stdio.h>
#include <iostream>
using namespace std;
double x;
int main()
{
    cin>>x;
    if(x<5) printf("%.3f",-x+2.5);
    else if(x<10) printf("%.3f",2-1.5*(x-3)*(x-3));
    else printf("%.3f",x/2-1.5);
    return 0;
}

x=float(input())
if      0<=x<5:    print('%.3f'%(-x+2.5))
elif    5<=x<10:   print('%.3f'%(2-1.5*(x-3)*(x-3)))
elif    10<=x<20:  print('%.3f'%(x/2-1.5))

代码必须要整齐