第9章第1讲编译预处理基本形式

#include"stdio.h"
#define PI 3.14
main()
{
    float l,s,r;
    printf("请输入半径:");
    scanf("%f",&r);
    l=2.0*PI*r;
    s=PI*r*r;
    printf("l=%.2f s=%.2f\n",l,s);
} 

#include"stdio.h"
#define PI 3.14
#define  S(r)  PI*r*r
main( )
{
  float r1=1.5,area;
  area=S(r1); 
  printf("r=%.2f,area=%.2f\n",r1,area);
} 

#include"stdio.h"
#define DEBUG
main()
{
   int x,y;
   x=2;
   y=3;
   x*=x+2;
   y/=y-2;
   #ifdef DEBUG
      printf("x=%d,y=%d\n",x,y);
   #endif
   printf("x+y=%d\n",x+y);
} 

#include"stdio.h"
#define R 1  
main()
{
   char web[50];
   int i=0;
   gets(web);
   while(web[i]!='\0')
   {
        #if(R==1)   
          if(web[i]>='A'&&web[i]<='Z')
          {web[i]=web[i]+32;  i++;}
        #else
           if(web[i]>='a'&&web[i]<='z')
           {web[i]=web[i]-32; i++;}
        #endif
    }  
    puts(web);
}