#include<stdio.h>
int main(void)
{
   double x,y;//定義兩個(gè)雙精度實(shí)型變量
   printf("Enterx(x>=0):\n");//輸入提示
   scanf("%lf",&x);  //調(diào)用scanf函數(shù)輸入數(shù)據(jù)
   if(x<=15){
          y=4*x/3;
   }
   else{
       y=2.5*x-10.5;
   }
   printf("f(%f)=%.2f\n",x,y); //輸出結(jié)果
   return 0;
 }

f（x）= -1 （? X<0 ）

??????????? 0???? （X=0）

?????????? 2x???? （X>0）

int main()
{
?int i;
?double x,y;
??? scanf("%lf",&x);
?i=1*(x<0)+2*(x==0)+3*(x>0);
?switch(i)
?{
?case 1:y=-1;printf("%0.2f\n",y);break;
?case 2:y=0;printf("%0.2f\n",y);break;
?case 3:y=2*x;printf("%0.2f\n",y);break;
?}

?return 0;
}

#include <stdio.h>
#include <stdlib.h>

int main()
{
??? double x,f;
??? scanf("%lf",&x);
??? if (x<0){
??????? f=-1;
??? }
??? else if(x==0){
??????? f=0;
??? }
??? else if(x>0){
??????? f=2*x;
??? }
??? printf("%0.2f",f);

}