# 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 */
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;
}
```