RPN (Reverse Polnish Notation) Calculator

main.c

/*
 * Simple reverse polnish notation calculator.
 *
 * Copyright (c) 2016 Thomas Richner
 * MIT License, https://opensource.org/licenses/MIT
 */
#include <stdio.h>
#include <stdint.h>
#include <stdlib.h>

void panic(char* msg){
	fprintf(stderr, "%s\n", msg);
	exit(-1);
}

int64_t eval(char** str){
	printf("%s\n", *str);

	// ready next character
	char op = **str;
	(*str)++;

	// end of string
	if(op == 0){
		panic("error: invalid input.");
	}

	// a number literal
	if(op >= '0' && op <= '9'){
		return op - '0';
	}

	// recursively evaluate both arguments
	int64_t exp1 = eval(str);
	int64_t exp2 = eval(str);

	// operate on the arguments
	int64_t ret;
	switch(op){
		case '+':
			ret = exp1 + exp2;
			break;
		case '-':
			ret = exp1 - exp2;
			break;
		case '*':
			ret = exp1 * exp2;
			break;
		case '/':
			ret = exp1 / exp2;
			break;
		default:
			panic("error: invalid operation.");
			break;
	}
	// * + X - X /
	return ret;
}

// Usage: ./a.out '+5++89*92'
int main(int argc, char** argv){
	if(argc < 2){
		panic("Not enough arguments.");
	}
	char** strptr = &(argv[1]);
	int64_t result = eval(strptr);
	if(**strptr != '\0'){
		panic("error: invalid input.");
	}
	printf("Result: %li\n", result);
}