#include<bits/stdc++.h>
using namespace std;
double x,y;
int main(){
	cin>>x;
	if(x>=0&&x<5) y=2.5-x;
	if(x>=5&&x<10) y=2-1.5*(x-3)*(x-3);
	if(x>=10&&x<20) y=x/2-1.5;
	cout<<fixed<<setprecision(3)<<y<<endl;
}

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

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

#include<cstdio>
using namespace std;
int main()
{
    double x,y;
    scanf("%lf",&x);
    if (0<=x&&x<5) y=-x+2.5;
      else if (5<=x&&x<10) y=2-1.5*(x-3)*(x-3);
      else if (10<=x&&x<20) y=x/2.0-1.5;
    printf("%.3lf",y);
    return 0;
}

#include<bits/stdc++.h> using namespace std; int main(){ double x; cin>>x; if(0<=x&&x<5){ cout<<fixed<<setprecision(3)<<-x+2.5;

}

else  if(5<=x&&x<10){
	cout<<fixed<<setprecision(3)<<2-1.5*(x-3)*(x-3);}


else if	
(10<=x&&x<20){
	cout<<fixed<<setprecision(3)<<x/2-1.5;}

return 0;

}

😄 看看我的最基础题解

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

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

#include 
using namespace std;
void f(float x);
int main(int argc, char **argv){
float x;
cin >> x;
f(x);
return 0;
}
void f(float x){
double y;
if (x < 5){
y = -x + 2.5;
printf("%.3f",y);
}else if(x < 10){
y = 2 - 1.5 * (x - 3) * (x - 3);
printf("%.3f",y);
}else if(x < 20){
y = (x / 2) - 1.5;
printf("%.3f",y);
}
}

#include<iostream>
#include<iomanip>
using namespace std;
long double x,y;
int main(){
	cin>>x>>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;
	return 0;
}