What is the purpose of ungetc (or ungetch from K&R)?

Question

Can anyone explain to me the purpose of ungetch? This is from K&R chapter 4 where you create a Reverse Polish Calculator.

I've ran the program without the call to ungetch and in my tests it still works the same.

 int getch(void) /* get a (possibly pushed back) character */
    {
        if (bufp > 0)
        {
            return buf[--bufp];
        }
        else
        {
            return getchar();
        }
    }

    void ungetch(int c) /* push character back on input */
    {
        if (bufp >= BUFSIZE)
        {
            printf("ungetch: too many characters\n");
        }
        else
        {
            buf[bufp++] = c;
        }

}

(I've removed the ternary operator in getch to make it clearer.)

Answer 1

I don't know about the specific example you're referring to (It's probaby 23 years since I read K&R, and that was the first edition.), but often when parsing it's convenient to 'peek' at the next character to see if it is part of what you're currently parsing. For instance, if you're reading a number you want to keep reading digits until you come to a non-digit. Ungetc lets the number reader look at the next character without consuming it so that someone else can read it. In Greg Hewgill's example of "2 3+", the number reader would read the 3 digit, then read the plus sign and know the number is finished, then ungetc the plus sign so that it can be read later.