C LANGUAGE: PREPROCESSORS

Conditional compilation

Macro	Meaning
(i) #if (expression) statements (A) #else statements (B) #endif	If the given condition is true then the statements (A) will be compiled and executed otherwise statements (B) will be compiled and executed
(ii) #ifdef macroname statements (A) #endif	If the specified macro is defined then the set of statements (A) are compiled otherwise not.
(iii) #ifdef macroname statements (A) #else statements  (B) #endif	If the specified macro is defined then the set of statements (A) are compiled otherwise set of statements (B) are compiled
(iv) #ifndef macroname statements (A) #endif	If the specified macro is not  defined then the set of statements (A) are compiled otherwise not.
(v) #ifndef macroname statements (A) #else statements (B) #endif	If the specified macro is not defined then the set of statements (A) are compiled otherwise set of statements (B) are compiled
(vi) #undef macroname	On execution of this statement the specified macro is undefined.

Examples:1

#if (expression)
statements (A)
#else
statements (B)
#endif

#include<stdio.h>
#include<conio.h>
#define a 10
 void main()
 {
     //clrscr();
     #if(a>1)
        printf("Hello");
     #else
        printf("Hi");
     #endif
 getch();
 }

/* Output */
Hello

#include<stdio.h>
#include<conio.h>
#define a 10
 void main()
 {
     //clrscr();
     #if(a>100)
        printf("Hello");
     #else
        printf("Hi");
     #endif
 getch();
 }

/* Output */
Hi

Example:2

#ifdef macroname
statements
#endif

#include<stdio.h>
 //#include<conio.h>
 #define a 10
 void main()
 {
     int i;
     //clrscr();
     printf("hello\n");
     #ifdef a
     for(i=1;i<=10;i=i+2)
    {
        printf("%d\n",i);
    }
    #endif
    printf("hi\n");
    return(0);
 }

/* Output */
hello
1
3
5
7
9
hi

Example:3

#ifdef macroname
statements
#else
statements
#endif

#include<stdio.h>
//#include<conio.h>
#define a 10
 int main()
 {
     int i;
     //clrscr();
     printf("hello\n");
     #ifdef a
     for(i=1;i<=10;i=i+2)
    {
        printf("%d\n",i);
    }
    #else
    for(i=2;i<=10;i=i+2)
    {
        printf("%d\n",i);
    }
    #endif
    printf("hi\n");
    return(0);
 }

/* Output */
hello
1
3
5
7
9
hi

Example:4

#ifndef macroname
statements
#endif

#include<stdio.h>
//#include<conio.h>
#define a 10
int main()
 {
     int i;
     //clrscr();
     printf("hello\n");
     #ifndef a
     for(i=1;i<=10;i=i+2)
    {
        printf("%d\n",i);
    }
    #endif
    printf("hi\n");
    return(0);
 }

/* Output */
hello
hi

Example:5

#ifndef macroname
statements
#else
statements
#endif

#include<stdio.h>
#include<conio.h>
#define a 10
int main()
 {
     int i;
     //clrscr();
     printf("hello\n");
     #ifndef a
     for(i=1;i<=10;i=i+2)
    {
        printf("%d\n",i);
    }
    #else
    for(i=2;i<=10;i=i+2)
    {
        printf("%d\n",i);
    }
    #endif
    printf("hi\n");
    return(0);
 }

/* Output */
hello
2
4
6
8
10
hi

Example:6

#undef macroname

#include<stdio.h>
//#include<conio.h>
#define a 10
int main()
 {
     int i;
     //clrscr();
     printf("hello\n");
     #ifdef a
     for(i=1;i<=10;i=i+2)
    {
        printf("%d\n",i);
    }
    #else
    for(i=2;i<=10;i=i+2)
    {
        printf("%d\n",i);
    }
    #endif
    printf("hi\n");
    #undef a
    printf("hello\n");
    #ifdef a
    for(i=1;i<=10;i=i+2)
    {
        printf("%d\n",i);
    }
    #else
    for(i=2;i<=10;i=i+2)
    {
        printf("%d\n",i);
    }
    #endif
    printf("hi\n");
    return(0);
 }

/* Output */
hello
1
3
5
7
9
hi
hello
2
4
6
8
10
hi

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