C/ctype.h/isalpha

//Declaration:  int isalpha(int ch); 
//Return:       returns nonzero if ch is a letter or zero if not. 
  #include <ctype.h>
  #include <stdio.h>
  int main(void)
  {
    char ch;
    for(;;) {
       ch = getchar();
       if(ch == ".") {
          break;
       }
       if(isalpha(ch)) {
          printf("%c is a letter\n", ch);
       }
    }
    return 0;
  }
         
/*
a
a is a letter
a
a is a letter
d
d is a letter
.
*/