Clarification in declaring an array

Question

int getch(void);
void ungetch(int);
/* getop: get next character or numeric operand */
int getop(char s[])
{
  int i, c;
  while ((s[0] = c = getch()) == ' ' || c == '\t')
    ;
  s[1] = '\0';
  if (!isdigit(c) && c != '.')
    return c; /* not a number */
  i = 0;
  if (isdigit(c)) /* collect integer part */
  while (isdigit(s[++i] = c = getch()))
    ;
  if (c == '.') /* collect fraction part */
    while (isdigit(s[++i] = c = getch()))
      ;
  s[i] = '\0';
  if (c != EOF)
    ungetch(c);
  return NUMBER;
}

#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;
}

The above code is from K&R.

In the above code, I see the array buf[] is used to a maximum of index 1, but it is defined to have a size of 100. Is the definition ok or there is a huge wastage in memory?

I pretty much think this is a bad style of programming.

I'm asking for getop() func only,not for general getch() and ungetch()

I'm a beginner and I'm sorry if my question is invalid :P

"*I see the array buf[ ] is used to a maximum of index 1*" -- Where? — Spikatrix, May 25 '15 at 06:44
What if you call `ungetch` 2 or more times? So no, it is not waste of memory? This allows ungetting all the character back into buffer. — Mohit Jain, May 25 '15 at 06:45
Mohit Jain and Cool Guy, forgive me for not posting full code.Now it's updated with full code — ThunderPunch, May 25 '15 at 06:52
If only `getop()` is used, then you could argue 1 byte will be enough ... I wouldn't go as far as saying 100 bytes is a *huge* waste of memory, though. — Ja͢ck, May 25 '15 at 07:07
I think you are supposed to assume that the `getch` and `ungetch` functions might be used by other code in the program too, so that there are already chars ungetch'd by the time this function is called — M.M, May 25 '15 at 08:24

score 1 · Answer 1 · answered May 25 '15 at 06:57

Disregarding the "style of programming", these are functions without anything to run them (ie. a main function). As such, you can't tell how these will be used but you can understand how they can be used. Now, suppose ungetch is called in a loop, without getch being called at all. In each iteration, bufp will grow by 1, and buf will slowly fill up to the point where bufp is equal to BUFSIZE and the "too many characters" will be printed. Next, if getch is called in a loop AFTER buf is full, bufp will shrink by 1 for each iteration untill buf is empty and getchar will be used for the next character.

Yeah, so the whole buffer is in fact used, or it might be used depending on how `getch` and `ungetch` and called. The OP's claim that only two bytes are used is wrong. — David Grayson, May 25 '15 at 07:20

Clarification in declaring an array

1 Answers1