530: Mainline

The getchar function obtains integer-cast characters from keyboard input or input redirected from a file; the putchar function displays characters. If getchar encounters an error or the end of a file, it returns whatever value the macro EOF expands to, which is -1 in most implementations. The following program, an iterative filter, converts lower-case letters to upper-case letters, because each upper-case letter, viewed as an integer, is equally offset from the corresponding lower-case letter:

#include  
int transform_letter (int arg) { 
  int offset = 'A' - 'a'; 
  if ((int) 'a' <= arg && arg <= (int) 'z') 
    return arg + offset; 
  else return arg; 
} 
main ( ) { 
  int input; 
  while (EOF != (input = getchar ( ))) 
    putchar (transform_letter (input)); 
} 

Note that getchar casts input characters into integers of type int, thereby requiring a cast at the point where those characters are compared with character constants.