eval simple numeric expressions.

c/mul/main.c

#include "mpc.h"
long eval_op(long x, char* op, long y) {
	if (strcmp(op, "+") == 0) { return x + y; }
	if (strcmp(op, "-") == 0) { return x - y; }
	if (strcmp(op, "*") == 0) { return x * y; }
	if (strcmp(op, "/") == 0) { return x / y; }
	return 0;
}
long eval(mpc_ast_t* t) {
	if (strstr(t->tag, "number")) {
		return atoi(t->contents);
	}
	char* op = t->children[1]->contents;
	long x = eval(t->children[2]);
	int i = 3;
	while (strstr(t->children[i]->tag, "expr")) {
		x = eval_op(x, op, eval(t->children[i]));
		i++;
	}
	return x;
}
int main(int argc, char** argv) {
	mpc_parser_t* Number = mpc_new("number");
	mpc_parser_t* Operator = mpc_new("operator");
		mpc_result_t r;
		if (mpc_parse("<stdin>", input, Lang, &r)) {
			mpc_ast_print(r.output);
			mpc_ast_t* t = r.output;
			mpc_ast_print(t->children[1]);
			long result = eval(t->children[1]);
			printf("%li\n", result);
			mpc_ast_delete(r.output);
		} else {
			mpc_err_print(r.error);