CODEWITHSUNDEEP
ckernighan-and-ritchie
ThunderPunch
ThunderPunch

Reputation: 483

Clarification in declaring an array

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

Upvotes: 2

Views: 109

Answers (1)

Amit
Amit

Reputation: 46323

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.

Upvotes: 1

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