rpn.c

#include <errno.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#ifndef STACK_SIZE_MAX
#define STACK_SIZE_MAX 1000
#endif

double stack[STACK_SIZE_MAX];
size_t stack_size;

static double
pop(void) {
	if (stack_size == 0) {
		fputs("stack empty\n", stderr);
		exit(1);
	}

	stack_size--;
	return stack[stack_size];
}

static void
push(double element) {
	if (stack_size == STACK_SIZE_MAX) {
		fputs("stack full\n", stderr);
		exit(1);
	}

	stack[stack_size] = element;
	stack_size++;
}

int
main(int argc, char *argv[]) {
	size_t i;
	char *token, *end;
	double lhs, rhs, value;

	for (i = 1; i < argc; i++) {
		token = argv[i];

		if (strlen(token) == 1 && strspn(token, "+-x/") == 1) {
			rhs = pop();
			lhs = pop();

			switch (*token) {
			case '+':
				value = lhs + rhs;
				break;
			case '-':
				value = lhs - rhs;
				break;
			case 'x':
				value = lhs * rhs;
				break;
			case '/':
				value = lhs / rhs;
				break;
			}
		}
		else {
			errno = 0;
			value = strtod(token, &end);

			if (errno != 0 || end == token || *end != '\0') {
				fprintf(stderr, "invalid token: %s\n", token);
				return 1;
			}
		}

		push(value);
	}

	while (stack_size > 0)
		printf("%g\n", pop());
}