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#include <ctype.h>
#include <stdio.h>
#include <stdlib.h>
#define MAXOP 100 /* max size of operand or operator */
#define NUMBER '0' /* signal that a number was found */
#define MAXVAL 100 /* max depth of val stack */
#define BUFSIZE 100
// commands: +, -, *, /, %, p (prints the top one)
// c (dups top), s (swaps top two), c (clear stack)
int sp = 0; /* next free stack position */
double val[MAXVAL]; /* value stack */
int bufp = 0; /* next free position in buf */
char buf[BUFSIZE]; /* buffer for ungetch */
/* get next operator or numeric operand */
int getop(char[]);
/* push f onto value stack */
void push(double);
/* pop and return top value from stack */
double pop();
/* returns a value from stack without removing it */
double peek();
/* clears the stack */
void clear();
/* adds commands p (print top val without popping), c (duplicate top val), s
* (swap top two stack positions), d (clear stack) */
int main(int argc, char *argv[]) {
int type;
double op2, op1;
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: division by zero\n");
break;
case '%':
op2 = pop();
if (op2 != 0.0)
push((int)pop() % (int)op2);
else
printf("error: division by zero\n");
break;
case 'p':
peek();
break;
case 'c':
push(peek());
break;
case 's':
op2 = pop();
op1 = pop();
push(op2);
push(op1);
break;
case 'd':
clear();
break;
case '\n':
printf("\t%.8g\n", pop());
break;
default:
printf("error: unknown command %s\n", s);
}
}
return 0;
}
void push(double f) {
if (sp < MAXVAL)
val[sp++] = f;
else
printf("error: stack empty\n");
}
double pop() {
if (sp > 0)
return val[--sp];
else {
printf("error: stack empty\n");
return 0.0;
}
}
double peek() {
if (sp > 0)
return val[sp - 1];
else {
printf("error: stack empty\n");
return 0.0;
}
}
void clear() {
for (; sp > 0; sp--)
;
}
int getch() { return (bufp > 0) ? buf[--bufp] : getchar(); }
void ungetch(int c) {
if (bufp >= BUFSIZE)
printf("ungetch: too many characters\n");
else
buf[bufp++] = c;
}
int getop(char s[]) {
int i, c;
while ((s[0] = c = getch()) == ' ' || c == '\t')
;
s[1] = 0;
if (!isdigit(c) && c != '.' && c != '-')
return c;
i = 0;
if (isdigit(c))
while (isdigit(s[++i] = c = getch()))
;
if (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;
}
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