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PREPROCESSING UNIT 8 PART 2


ifelse
Introduction
#if allows you to define more generalized conditions. Multiple conditions, which are connected by relational operators such as AND(&&), OR(||), are allowed.The else directive lets us specify the action if the #if condition is not true.
Program
Suppose we have three files:

file1.h
#define USD 1
file2.h
#define UKP 1

file3
#include <stdio.h>
#include <file1.h>              //A

#if (defined (USD))            // B
         #define currency_rate 46
#else
        #define currency_rate 100             //C
#endif                          //D

main()
{
    int rs;
    rs = 10 * currency_rate;    //H
    printf ("%d\n", rs);
}
Explanation
  1. Statement B indicates the ifelse directive.
  2. If the identifier USD is defined, the currency rate is taken as 46; otherwise, the currency rate is taken as 100.
  3. Since USD is defined in file1.h, the currency rate is taken as 46.
Point to Remember
The ifelse directive allows us to take action if the condition is not satisfied.
ifelif
Introduction
ifelif allows us to take one action if there are multiple decision points. For example, if you want to take the currency rate of 1 if USD and UKP are not defined, you can write the following program.
Program
Suppose we have three files:
file1.h
#define USD 1

file2.h
#define UKP 1

file3
#include <stdio.h>
#include <file1.h>             //A

#if (defined (USD))                  // B
     #define currency_rate 46
#elif (defined (UKP))
     #define currency_rate 100       //C
#else
     # define currency_rate 1        //D
#endif

main()
{
    int rs;
    rs = 10 * currency_rate;         //H
    printf ("%d\n", rs);
}
Explanation
  1. Statement B includes the ifelif directive. It is similar to the else directive.
  2. #elif appears only after #if, #ifdef, #ifndef, and #elif.
  3. #elif is similar to #else but it is followed by a condition.
  4. You can have as many #elif directives as you want.
  5. If USD is defined, then the currency rate is 46; otherwise, if UKP is defined, then the currency rate is 100; otherwise, the currency rate is 1.
  6. In this case, if you remove the statement include file1.h at position A, then USD and UKP are not defined and currency rate is taken as 1.
Points to Remember
  1. #elif is similar to #else but it is followed by a condition.
  2. #elif allows taking action in the case of multiple decision points.
ERROR DIRECTIVE
Introduction
The error directive is used to specify an error message for a specific situation. In the following program, the error message is displayed if USD and UKP are not defined.
Program
Suppose we have three files:
file1.h
#define USD 1

file2.h
#define UKP 1

file3
#include <stdio.h>
#include <file1.h>                 //A

#if !defined (USD) || !defined (UKP)     // B
#error "ERROR: NO_CURRENCY rate is specified." //C
#endif

main()
{
    int rs;
    rs = 10 * currency_rate;        //D
    printf ("%d\n", rs);
}
Explanation
  1. Statement B checks whether UKP or USD is defined.
  2. If both are not defined then the preprocessor displays an error.
Points to Remember
  1. The #error directive allows us to specify an error message.
  2. The error message is generated by the preprocessor.



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