I'm writing a reverse Polish calculator. If the input is 1 2 - 4 5 + *, the output should be 9, but I'm getting -9.
Does anyone see where the problem lies?
Thanks!
#include <stdio.h>
#include <stdlib.h> // for atof()
#include <ctype.h>
/* denifitions for main() */
#define MAXOP 100 // max size of operand or operator
#define NUMBER '0' // signal that a number was found
/* definition for push() */
#define MAXVAL 100 // maximum depth of val stack
/* definition for pop() */
#define BUFSIZE 100
/* declarations for main() */
int getop(char []);
void push(double);
double pop(void);
/* declarations for push() */
int sp = 0; // next free stack position
double val[MAXVAL]; // value stack
/* declarations for getop() */
int getch(void);
void ungetch(int);
/* external variables for getch() and ungetch() */
char buf[BUFSIZE]; // buffer for ungetch
int bufp = 0; // next free position in buf
/* reverse Polish calculator */
int main() {
int type;
double op2;
char s[MAXOP];
printf("reverse polish calculator: ");
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("print: \t%.8g\n", pop());
printf("\nPress any key to continue.");
getchar(); getchar();
exit(0);
//break;
default:
printf("error: unknown command %s\n", s);
break;
}
}
return 0;
}
/* push: push f onto value stack */
void push(double f) {
if (sp < MAXVAL)
val[sp++] = f;
else
printf("error: stack full, can't push %g\n", f);
}
/* pop: pop and return tp value from stack */
double pop(void) {
if (sp > 0)
return val[--sp];
else {
printf("error: stack empty\n");
return 0.0;
}
}
/* getop: get next operator 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;
}
/* get a (possibly pushed back) character */
int getch(void) {
return (bufp > 0) ? buf[--bufp] : getchar();
}
/* push character back on input */
void ungetch(int c) {
if (bufp >= BUFSIZE)
printf("ungetch: too many characters\n");
else
buf[bufp++] = c;
}