Reverse polish notation calculator

rpn.awk

# calc.awk -- Reverse polish calculator

# void -> string
function usage() {	
	return instructions "\n" example
}

# number -> void
function push(value) {
	STACK[SP] = value
	SP++
}

# void -> number
function pop() {
	SP--
        return STACK[SP]
}

# string -> void
function op(symbol) {
	# There's only one element on the stack
	if (SP == 1) return
	
	if (SP > 1) {
	        second = pop()
		first = pop()

		if (symbol == "+") result = first + second
		if (symbol == "-") result = first - second
		if (symbol == "*") result = first * second
		if (symbol == "/") result = first / second

		push(result)
	}
}

# string -> bool
function isop(field) {
	if (field ~ operator) return TRUE

	return FALSE
}

BEGIN {
	# Constants to simulate booleans
	TRUE = 1
	FALSE = 0

	program = "calc.awk"
	instructions = program " -- Accepted inputs: one or more NUMBERS separated with WHITESPACE followed by one or more OPERATORS (+, -, *, /)"
	example = "EXAMPLE: 5 1 2 + * (which is equal to the number 15)"

	# Regex
	numbers = "[0-9]"
	space = "\\s"
	operator = "[+|\\-|*|/]"
	
	expression = "((" number space ")+" operator ")+"
}

$0 ~  expression {
	# Holds the current length of STACK, as awk's arrays are actually hash tables
	SP = 0

	# True when input is malformed and the last field is a number and not an operator
	if (!isop($NF)) {
		print "Invalid input (ERROR: line ends with a number, it must end with an operator)"
		print ""
		print usage()
		next
	}
	
	for (i = 1; i <= NF; i++) {
		if (isop($i)) op($i)
		else push($i)
	}

	print pop()
}

$0 !~ expression {
	print ""
	print usage()
}