def calc(expr):
"Processes an expression written in Polish notation from left to right"
# Create Stack objects for the operators and operands
operatorStack = Stack()
operandStack = Stack()
# Split expression into a list (temporary)
expr = expr.split()
for token in expr:
# Check if the token is an operator
if token in ["+", "-", "*", "/"]:
operatorStack.push(token)
pendingOperand = False
# Check if the operand is a valid number
elif is_number(token):
operand = token
# Check if a calculation should be done
if pendingOperand == True:
while operandStack.height() != 0:
operand_1 = operandStack.pop()
operator = operatorStack.pop()
# print("Evaluating " + str(operand_1) + " " + operator + " " + str(operand))
operand = evaluate(operator, operand_1, operand)
# Push the number to the operand stack
operandStack.push(operand)
pendingOperand = True
else: # Default if the token is not a operator or cannot be a float
print(str(token) + " is not a valid token.")
# Return result
return operandStack.pop()
