chapter 4 K&R, getch() and ungetch()

[blob84]

Hi, i have problem with these functions: getch() and ungetch(), pag 67 english 2 version K&R.

#define BUFSIZE 100
char buf[BUFSIZE];     /* buffer for ungetch */
int bufp = 0;          /* next free position in buf */
int getch(void) /* get a (possibly pushed-back) character */
{
    return (bufp > 0) ? buf[--bufp] : getchar();
}
void ungetch(int c)    /* push character back on input */
{
    if (bufp >= BUFSIZE)
         printf("ungetch: too many characters\n");
    else
         buf[bufp++] = c;
}

I do not post the entire code of the polish notation calculator, i assume you already know.
What i have understood is:
when getch is called, if the chars counter bufp is > 0, then return the prevoius char buf[--bufp], else call getchar(), this id done to not get back the wrong char, for example a non-digit;
when ungetch is called it stores the characters in the array buf[buf++].
I would be grateful if someone can explain how these functions work, with an example.
Thanks.

[quzah]

The first function reads like this:

if there is a character already in buf
    move the counter back one and then
    return the character that was in the buffer's last used spot
else
    get a character using getchar

The other one just checks to see if the buffer is full, and if it isn't, puts a character in it, updating the position of the end of the buffer.


Quzah.