评估中缀表达式python [英] Evaluating an infix expression python

问题描述

我的任务是使用堆栈计算完全括号中的中缀表达式.已经为我编写了一个 Stack 类，我不能更改或修改 Stack 类.

My task is to evaluate a fully parenthesized infix expression using a stack. A Stack class has been written for me and I must not change or modify the Stack class.

以下是如何计算中缀表达式的分步说明:

Here are the step by step directions for how to evaluate the infix expression:

只需从左到右扫描表达式.如果它不是 a )，则将其压入堆栈.当您遇到 a ) 时，从堆栈中弹出 4 次，进行数学运算并将值压入堆栈.最后，堆栈中将只有一个值，这就是答案.

Just scan the expression from left to right. If it is anything other than a ), push it onto the stack. When you encounter a ), pop from the stack 4 times, do the math and push the value onto the stack. At the end you will have just one value in the stack and that will be the answer.

代码如下:

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"

这是我目前的代码:

def evaluateInfix(Input):

    xStack=Stack()

    for n in Input:
        if n!=")":
            print "Pushing %s into the stack" %n
            xStack.push(n)

        if n==")":
            math=xStack.pop()+xStack.pop()+xStack.pop()

            last=xStack.pop()

            for j in math:

                print "    Popping %s from stack" %j

            print "    Popping %s from stack" %last

            evaluation=eval(math)

            xStack.push(evaluation)

            print "Pushing %d into stack" %evaluation

这是我的代码运行示例:

Here is an example of my code running:

Enter a fully parenthesized expression that has non-negative integer operands and using            only + - * and ( )
Please enter the expression: ((9+9)+(9+9))
Pushing ( into the stack
Pushing ( into the stack
Pushing 9 into the stack
Pushing + into the stack
Pushing 9 into the stack
    Popping 9 from stack
    Popping + from stack
    Popping 9 from stack
    Popping ( from stack
    Pushing 18 into stack
Pushing + into the stack
Pushing ( into the stack
Pushing 9 into the stack
Pushing + into the stack
Pushing 9 into the stack
    Popping 9 from stack
    Popping + from stack
    Popping 9 from stack
    Popping ( from stack
Pushing 18 into stack
Traceback (most recent call last):
  File "project2.py", line 252, in <module>
    main()
  File "project2.py", line 246, in main
    Infix=evaluateInfix(Input)
  File "project2.py", line 164, in evaluateInfix
    math=xStack.pop()+xStack.pop()+xStack.pop()
TypeError: unsupported operand type(s) for +: 'int' and 'str'

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