Puzzle on the second stack in postfix calculator - c

Question

I'm reading "TCPL" recently.There exist a classic case "postfix calculator" in section 4.3. I have a question on the case:
See the case code(copy from the book):
#include<stdio.h>
#include<stdlib.h>
#include<ctype.h>
#define MAXOP 100
#define NUMBER '0'
#define MAXVAL 100 /* maxmium depth of val stack */
#define BUFFSIZE 100
int sp = 0;
double val[MAXVAL];
int bufp = 0; /* next free size in buf*/
char buf[BUFFSIZE]; /* buffer in ungetch */
int getch(void);
void ungetch(int);
int getop(char []);
void push(double);
double pop(void);
int main()
{
int type;
double op2;
char s[MAXOP];
while ((type = getop(s)) != EOF) {
switch (type) {
case NUMBER:
push(atof(s));
break;
case '+':
push(pop() + pop());
break;
case '*':
push(pop() * pop());
break;
case '-':
op2 = pop();
push(pop() - op2);
break;
case '/':
op2 = pop();
if (op2 != 0.0) {
push(pop() / op2);
}
else
{
printf("error: zero divisor. \n");
}
break;
default:
printf("\t%.8g\n", pop());
break;
}
}
return 0;
}
/* push: push value f onto value stack */
void push(double f)
{
if (sp < MAXVAL) {
val[sp++] = f;
}
else {
printf("error: stack full, can't push.\n");
}
}
/* pop: pop and return value on the top of the stack */
double pop(void)
{
if (sp > 0) {
return val[--sp];
}
else {
printf("error: stack empty.\n");
return 0.0;
}
}
/* 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 != '.' ) { /* operator */
return c;
}
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;
}
/* getch: get a (possibly pushed-back) character */
int getch(void)
{
return (bufp > 0)? buf[--bufp] : getchar();
}
/* ungetch: push character back on input */
void ungetch(int c)
{
if (bufp >= BUFFSIZE) {
printf("ungetch: too many characters in buufer.\n");
}
else {
buf[bufp++] = c;
}
}
There is a global char array buf, It is like a stack and the last two function are to 'pop' and 'push' on buf.
My question is in which case does the stack buf contains more than one element?
The only chance can the ungetch() be called is in function getop(), however in getop() the function getch() is called at least once.

Answer 1

My question is in which case does the stack buf contains more than one element?
never, buf can be just a char, it is only used to memorize the char placed just after a number to not lost it
The code can be modified to not use it :
/* getch: get a (possibly pushed-back) character */
int getch(void)
{
return getchar();
}
/* ungetch: push character back on input */
void ungetch(int c)
{
ungetc(c, stdin);
}
Compilation and execution :
pi#raspberrypi:/tmp $ gcc pf.c
pi#raspberrypi:/tmp $ ./a.out
1 2 3 4 + + +
10
Or using the fact only one char can be saved :
int SavedChar = EOF;
/* getch: get a (possibly pushed-back) character */
int getch(void)
{
if (SavedChar == EOF)
return getchar();
int r = SavedChar;
SavedChar = EOF;
return r;
}
/* ungetch: push character back on input */
void ungetch(int c)
{
SavedChar = c;
}
Compilation and execution :
pi#raspberrypi:/tmp $ gcc pf.c
pi#raspberrypi:/tmp $ ./a.out
1 2 3 4 + + +
10