How to use #if #elif #else #endif in Macro

Description

Logical directive tests whether an identifier exists as a result of having been created in a previous #define directive.

Syntax

C Conditional Compilation takes the following form:

#if defined identifier 
...
#endif

If identifier is defined, statements between #if and #endif are included in the program code.

If the identifier isn't defined, the statements between the #if and the #endif will be skipped.

Absence of an identifier

Test for the absence of an identifier. In fact, this tends to be used more frequently than the form you've just seen.

The general form of this directive is:

Here, the statements following the #if down to the #endif will be included if the identifier hasn't previously been defined.

Multiple conditions

To define more generalized conditions.

Multiple conditions are connected by relational operators such as AND(&&), OR(||).

#define USA 1// w w w  .  j  a v  a 2 s  .  c o m

#include <stdio.h>

#if ((1>0) && defined USA)
       #define currency_rate 46
#endif

#if (defined (EUP))
   #define currency_rate 100
#endif

main()
{
    int rs;
    rs = 10 * currency_rate;
    printf ("%d\n", rs);
}

The code above generates the following result.

#elif and #else

#ifelif allows us to take one action for multiple decision entries.

#define USA 1/*  w  w w. j av a  2s.  co  m*/
#define EUP 1
#include <stdio.h>


#if (defined (USA))
     #define currency_rate 46
#elif (defined (EUP))
     #define currency_rate 100
#else
     #define currency_rate 1
#endif

main()
{
    int rs;
    rs = 10 * currency_rate;
    printf ("%d\n", rs);
}

The code above generates the following result.

If undefined

#ifndef is used to check whether a particular symbol is defined.

#define USA 1/*from w  ww . j  a  v  a  2  s. co m*/
//#define EUP 1

#include <stdio.h>

#ifndef USA            
   #define currency_rate 100    
#endif               

#ifndef EUP                 
   #define currency_rate 46
#endif               
main()
{
    int rs;
    rs = 10 * currency_rate; 
    printf ("%d\n", rs);
}

The code above generates the following result.

#ifdef

The #ifdef directive makes substitutions based on whether a particular identifier is defined.

#define USA 1//from ww w .j  a va  2  s.  c o m
// #define EUP 1

#include <stdio.h>
#ifdef USA
#define currency_rate 46
#endif

#ifdef EUP
#define currency_rate 100
#endif
main()
{
    int rs;
    rs = 10 * currency_rate;
    printf ("%d\n", rs);
}

The code above generates the following result.