使用堆栈python评估postfix

Question

My task is to convert a fully parenthesized infix expression. Example

(((54+56)+(4+73))+(9+7))

to postfix. Then evaluate the postfix. The expression is input from the user. I have to use a class called Stack already written for me. It must not be modified:

class Stack:
    def __init__(self):
        self.theStack=[]

    def top(self):
        if self.isEmpty():
            return "Empty Stack"
        else:
            return self.theStack[-1]

    def isEmpty(self):
        return len(self.theStack)==0

    def push(self,item):
        self.theStack.append(item)

    def pop(self):
        if not self.isEmpty():
            temp=self.theStack[-1]
            del(self.theStack[-1])
            return temp
        else:
            return "Empty Stack"

The first problem I have is that when the user inputs for example 54, while using stack, 5 and 4 are two different elements. How can I turn it into one?

Here is the code I have so far so evaluating the postfix:

OPERATOR=["+","-","*", "/"]

def evaluatePostfix(Postfix):
    eStack=Stack()
    for n in Postfix:
        if n not in OPERATOR and n!="(" and n!=")":
            eStack.push(n)
        if n in OPERATOR:
            math=eStack.pop()+n+eStack.pop()
            eval(math)

I know the problem is the second to last line but I'm not sure how to fix it

Answer 1

There are a few questions you're asking (and your answer isn't complete, as far as leading you to your ultimate goal of evaluating a postfix expression), but let me address the one you ask:

"The first problem I have is that when the user inputs for example 54, while using stack, 5 and 4 are two different elements. How can I turn it into one?"

If you are committed to scanning the input one character at a time, then the simplest approach is to use a temporary variable. Here's a quick and dirty way to do it in Python.

infix = "54 + 490"
postfix = ""
a_number = None
OPERATOR = ["+", "-", "*", "/"]

for n in infix:
    if n not in OPERATOR and n != "(" and n != ")":
        if a_number is None:
            a_number = n
        else:
            a_number = a_number + n

    if n is ")" or n in OPERATOR:
         if a_number is not None:
             postfix += a_number
             a_number = None


if a_number is not None:
    postfix += a_number
    a_number = None

print postfix

Answer 2

when you write:

for n in Postfix

you're iterating over the characters in Postfix one at a time. You might do better to convert Postfix to a list of strings with a helper function along these lines to pad the operators if they're not padded

def foo(str):
 newstr = ""
 for x in str:
  if x not in "0123456789. ":
   newstr += " " + x + " "
  else:
   newstr += x
 return newstr

so now you can change

for n in Postfix

to

for n in foo(Postfix).split()

and it should solve the problem of not handling numbers properly.

split() splits a string into a list of nonwhitespace strings, eg "hello world" would become ["hello", "world"]