postfix-calc.c

#include <stdio.h>  //printf()
#include <stdlib.h> //malloc(), free(), exit(), EXIT_FAILURE, atoi()
#include <ctype.h>  //isdigit()
#include <string.h> //(obsolete)? 
#include <assert.h> //assert()
#include "stack.h"  //custom "class" stack

// helper-function
int isnum(char *a) {
	if(*a == '\0') {
		return 1;
	} else {
		return isdigit(*a) && isnum(a+1);
	}
}

// Postfix-Rechner
int calc(char *expr[], int expr_length) {
	int k, n1, n2;
	stack *s = NULL;

	for(k = 0; k < expr_length; k++) {
		if(isnum(expr[k])) {
			stack_push(&s, atoi(expr[k]));
		} else { // Operator erwartet
			n2 = stack_pop(&s);
			n1 = stack_pop(&s);
			switch(expr[k][0]) {
				case '+':
					stack_push(&s, n1+n2);
				break;
				case '-':
					stack_push(&s, n1-n2);
				break;
				case '*':
					stack_push(&s, n1*n2);
				break;
				case '/':
					stack_push(&s, n1/n2);
				break;
				default:
					printf("Syntax error: %s unexpected\n", expr[k]);
				exit(EXIT_FAILURE);
			}
		}
	}
	n1 = stack_pop(&s);
	if(stack_isempty(&s)) return n1;
	else {
		printf("Missing operand\n");
		exit(EXIT_FAILURE);
	}
}

int main(int argc, char *argv[]) {
	printf("%d\n", calc(argv+1, argc-1));
	return 0;
}