Reverse Polish Notation

expr.py

# avaliador de expressao em notacao polonesa reversa
funcoes = {
    "+": lambda a, b: a + b,
    "-": lambda a, b: a - b,
    "*": lambda a, b: a * b,
    "/": lambda a, b: a / b
}


def avalia(expressao):
    pilha = []
    for token in expressao.split(' '):
	if token.isdigit():
		pilha.append(int(token))
	else:
		operacao = funcoes[token]
		op2, op1 = pilha.pop(), pilha.pop()
		pilha.append(operacao(op1, op2))
    return pilha.pop()


assert(avalia("3 3 +") == 6)
assert(avalia("2 5 *") == 10)
assert(avalia("5 20 15 - -") == 0)