# calc3 - infix calculator

NF > 0 {
	f = 1
	e = expr()
	if (f <= NF) printf("error at %s\n", $f)
	else printf("\t%.8g\n", e)
}

function expr(  e) {		# term | term [+-] term
	e = term()
	while ($f == "+" || $f == "-")
		e = $(f++) == "+" ? e + term() : e - term()
	return e
}

function term(  e) {		# factor | factor [*/] factor
	e = factor()
	while ($f == "*" || $f == "/")
		e = $(f++) == "*" ? e * factor() : e / factor()
	return e
}

function factor(  e) {	  # number | (expr)
	if ($f ~ /^[+-]?([0-9]+[.]?[0-9]*|[.][0-9]+)$/) {
		return $(f++)
	} else if ($f == "(") {
		f++
		e = expr()
		if ($(f++) != ")")
			printf("error: missing ) at %s\n", $f)
		return e
	} else {
		printf("error: expected number or ( at %s\n", $f)
		return 0
	}
}