recursive_postfix_eval.c

/* Solves the puzzle: Puzzle: recursive postfix evaluation.
   Ref: http://rgrig.blogspot.com/2009/03/postfix-expressions.html
*/
#include <stdio.h>

#define F 0x40
char eval(int * pr) {
	char ch;
	int val;
	switch((ch = getchar())) {
	case '+': case '-': case '*': case '|': return ch;
	default:
		if(ch >= '0' && ch <= '9') {
			*pr = ch - '0';
			while(1) {
				switch((ch = eval(&val))) {
				case '+': case '-': case '*': return ch^F;
				case '|': return '|';
				case '+'^F: *pr += val; break;
				case '*'^F: *pr *= val; break;
				case '-'^F: *pr -= val; break;
				}
			}
		}
	}
	return ' ';
}

int main() {
	int r;
	eval(&r);
	printf("%d\n", r);
}