C知識回顧(三)

#include 
/*
 *1.     
 */
#define PI 3.141592654          //     double

/*
 *2.     
 */
#define swap(x,y)\
x=x+y;\
y=x-y;\
x=x-y

/*
 *3.       
 */
#define LOG\
    int a =3;\
    int b =4;\
    printf("a: %d
",a);\
    printf("b: %d
",b)

#ifndef A           //         A (if undefine ,    if     n  un-    )

#define A 1         //       A     1

#endif              //    

#define B 2

//#undef A            //     A

#define C 100

int main(int argc, const char * argv[]) {
    printf("PI: %.9lf
",PI);
    
    int x =10;
    int y =20;
    printf("    : x=%d,x=%d
",x,y);
    swap(x,y);
    printf("    : x=%d,x=%d
",x,y);
    
    LOG;
    printf("a=%d,b=%d
",a,b);
    
    printf("-------------
");

    /*
     *4.                        if()
                                    {

                                    }
                                    else-if()
                                    {

                                    }
                                    else-if()
                                    {

                                    }
     */
#ifdef A
    printf("defineA: %d
",A);
#endif
    
#ifdef B
    printf("defineB: %d
",B);
#elif C
    printf("defineC: %d
",C);
#else
    printf("      !
");
#endif
    
    return 0;
}