knr/2.10.c

#include <stdio.h>
int lower(int c);
int main()
{
  printf("%c",lower('X'));
  return 0;
}

/* lower:  convert c to lower case; ASCII only */
int lower(int c)
{
  return (c >= 'A' && c <= 'Z') ? c + 'a' - 'A' : c;
}