#include <stdio.h>
#define PI 3.1415926 //结尾不需要分号
#define area(r) (PI*(r)*(r)) //整体带括号，内部变量带括号 注意：macro中的变量是没有类型的
int main()
{
    double radius;
    printf("input a radius: ");
    scanf("%lf", &radius);
    if (radius < 0) {
        //printf("radius can't be negative.\n");
        puts("radius can't be negative.");
    }
    else {
        printf("The area of a circle with a %f radius is %f\n", radius, area(radius));
    }
}