#ifdef in C

1. if, #else and #elif: These directives works together and control compilation of portions of the program using some conditions. If the condition with the #if directive evaluates to a non zero value, then the group of line immediately after the #if directive will be executed otherwise if the condition with the #elif directive evaluates to a non zero value, then the group of line immediately after the #elif directive will be executed else the lines after #else directive will be executed.
   Syntax:

   #if macro_condition
      statements
   #elif macro_condition
      statements
   #else
      statements
   #endif
   

   Example:

    

    

    

   #include<iostream>    #define gfg 7     #if gfg > 200    #undef gfg    #define gfg 200 #elif gfg < 50    #undef gfg    #define gfg 50 #else    #undef gfg    #define gfg 100 #endif    int main() {     std::cout << gfg;  // gfg = 50 }

   Output:

   50
   

   Notice how the entire structure of #if, #elif and #else chained directives ends with #endif.

#line number "filename"

number – line number that will be assigned to the next code line. The line numbers of successive lines will be increased one by one from this point on.