#include <stdio.h>
#include <stdlib.h>
#include <math.h>

#define MAXOP 100
#define NUMBER '0'

int getop(char[]);
void push(double);
double pop(void);
double peek();
void ungets();

int
main()
{
	int type, var;
	double op1, op2, r;
	char s[MAXOP];
	char v[26];

	/* 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;
		/* to guarantee the right order, the first value must be stored */
		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 '%':
			op2 = pop();
			op1 = pop();
			if (op1 >= 0.0 && op2 >= 0.0) {
				push((int)op1 % (int)op2);
			} else if (op1 < 0.0 || op2 < 0.0) {
				push(0);
			} else {
				printf("error: zero divisor\n");
			}
			break;
		/* print without popping */
		case 'p':
			printf("%f\n", peek());
			break;
		/* duplicate */
		case 'd':
			op2 = peek();
			push(op2);
			break;
		/* swap top two elements */
		case 's':
			op2 = pop();
			op1 = pop();
			push(op2);
			push(op1);
			break;
		case '=':
			pop();
			if (var > 'a' && var < 'z') {
				v[var - 'a'] = pop();
			}
		case '\n':
			r = pop();
			printf("\t%.8g\n", r);
			break;
		case 'u':
			ungets();
			break;
		default:
			if (type == 'R') {
				push(r);
			} else if (type >= 'a' && type <= 'z') {
				push(v[type - 'a']);
			} else {
				printf("error: unknown command %s\n", s);
			}
			break;
		}
		var = type;
	}
	return 0;
}