Preprocessor Commands

Preprocessor Commands

• The preprocessor Commands are the directives, which give instruction to the compiler(its Preprocessor) to preprocess the information before actual compilation starts.
• All preprocessor directives begin with a # symbol.
• Preprocessor directives do not ends with semicolon.

Preprocessor

• It is a program which process preprocessor commands.
• The preprocessor processes preprocessor command before the compilation of program begins.

Advantages of Preprocessor commands

• Programs easier to develop.
• Easier to read.
• Easier to modify.
• Code becomes more transportable between different machine architectures.

Types of Preprocessor Directives

• Macro Substitution
• It is a process where an identifier in a program is replaced by a predefined string composed of one or more tokens. 
• For eg: #define
• Conditional Compilation
• They are used for conditional compilation of a code block.
• For eg: #ifdef, #undef, #ifndef, #else..etc
• File Inclusive
• It indicates to include a file in our program.
• For eg: #include

#define Preprocessor

• The #define preprocessor directive is used to create symbolic constants.
• The symbolic constant is called a macro and the general form of the directive is:

 #define PI 3.14159
 void  main ()
 {
    printf("Value of PI : %f “ , PI ); 
 }

 //code send to compiler
 void  main ()
 {
    printf("Value of PI :  %f “ , 3.14159 ); 
 }

Function-Like Macros:

• You can use #define to define a macro which will take argument like functions

 #define SQR(a) a*a
 void main()
 {  
    printf("The square of 5 is %d",SQR(5));
    printf("The square of 2+1 is %d",SQR(2+1));
 }

 //code send to compiler
 void main ()
 {  
    printf("The square of 5 is  %f " , 5*5 );
    printf("The square of 2+1 is  %f "" , 2+1 * 2+1 );
 }

#include Directive

• #include Directive is used to include a file into a source file before sending source file for compilation.

Conditional Directives

• Conditional directives are used for conditional compilation of code.
• we can use the #ifdef directive for conditional compilation.
• With #ifdef we can use #else #endif #ifndef ..etc

  #include<stdio.h>
  #define SCIENTIFIC
  #ifdef SCIENTIFIC
  float pi=3.14159;
  #else
  float pi=3.14;
  #endif

  main()
  {
    printf("value of PI is %f",pi);
  }

value of PI is 3.141590