#include <stdio.h>                                          
   #include <ctype.h>                                          
                                                               
   main()                                                      
   {                                                           
       char alphabet;                                          
       printf("Enter an alphabet");                            
       putchar('\n');   /* move to next line */
                                       
       alphabet = getchar();                                   
       if (islower(alphabet))                                  
          putchar(toupper(alphabet));                          
       else                                                    
          putchar(tolower(alphabet));                          
   }

                                                   
  
 
Output                                                      
   Enter an alphabet                                           
   a                                                           
   A                                                           
   Enter an alphabet                                           
   Q                                                           
   q                                                           
   Enter an alphabet                                           
   z                                                           
   Z