# Exercise 7.5 - Postfix calculator using scanf¶

## Question¶

Rewrite the postfix calculator of Chapter 4 to use scanf and/or sscanf to do the input and number conversion.

```/*
* Rewrite the postfix calculator of Chapter 4 to use scanf and/or sscanf to do
* the input and number conversion.
*/
#include <stdio.h>
#include <stdlib.h> /* for atof() */

#define MAXOP 100   /* max size of operand or operator */
#define NUMBER '0'  /* signal that a number was found */

int getop(char[]);

void push(double);

double pop(void);

/* reverse Polish calculator */
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;
case '\n':
printf("\t%.8g\n", pop());
break;
default:
printf("error: unknown command %s\n", s);
break;
}
}
}

#define MAXVAL 100

int sp = 0;
double val[MAXVAL];

void push(double f) {
if (sp < MAXVAL)
val[sp++] = f;
else
printf("error:stack full, cant push %g\n", f);
}

double pop(void) {
if (sp > 0)
return val[--sp];
else {
printf("error: stack empty\n");
return 0.0;
}
}

#include <ctype.h>

int getch(void);

void ungetch(int);

int getop(char s[]) {
int i, c;

while ((s[0] = c = getch()) == ' ' || c == '\t');
s[1] = '\0';

i = 0;
if (!isdigit(c) && c != '.' && c != '-')
return c;

if (c == '-')
if (isdigit(c = getch()) || c == '.')
s[++i] = c;
else {
if (c != EOF)
ungetch(c);
return '-';
}

if (isdigit(c))
while (isdigit(s[++i] = c = getch()));

if (c == '.')
while (isdigit(s[++i] = c = getch()));

s[i] = '\0';
if (c != EOF)
ungetch(c);
return NUMBER;
}

#define BUFSIZE 100

char buf[BUFSIZE];
int bufp = 0;

int getch(void) {
char c;
if (bufp > 0) {
return buf[--bufp];
} else {
scanf("%c", &c);
return c;
}
}

void ungetch(int c) {
if (bufp >= BUFSIZE)
printf("ungetch: too many characters\n");
else
buf[bufp++] = c;
}
```