Change string to upper case without strupr

#include<stdio.h>
 
void upper_string(char*);
 
main()
{
   char string[100];
 
   printf("Enter a string to convert it into upper case\n");
   gets(string);
 
   upper_string(string);
 
   printf("Entered string in upper case is \"%s\"\n", string);
 
   return 0;
}
 
void upper_string(char *string)
{
   while(*string)
   {
       if ( *string >= 'a' && *string <= 'z' )
       {
          *string = *string - 32;
       }
       string++;
   }
}