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后缀表达式出现意外的Python错误。有人能解释我为什么会出现这个错误吗?

stack python

-1

Saketh · 技术社区 · 1 年前

from collections import deque
def postfixExpression(expression: str) -> int:
    if not expression:
        return -1
    expression_list = expression.split(" ")
    stack = deque()
    for ex in expression_list:
        print(ex, stack)
        try:
            stack.append(int(ex))
        except ValueError:
            b = stack.pop()
            a = stack.pop()
            switcher = {
                "+" : a + b,
                "-" : a - b,
                "*" : a * b,
                "/" : int(a / b),
                "^" : a ** b,
            }
            if ex not in switcher.keys():
                raise ValueError(f"Wrong expression-> {ex}")
            else:
                stack.append(switcher.get(ex))
    return stack.pop()

expression = "2 3 7 - 7 + 9 7 2 + - * *"
print(f"\nResult {postfixExpression(expression)}")

以上是python中的Postfix评估程序。它适用于大多数情况。

但当我使用输入时 "2 3 7 - 7 + 9 7 2 + - * *" ,它在给予 ZeroDivisionError ,甚至认为我没有使用除法( / )输入中。

错误消息:

$ python temp/temp.py 
4 deque([])
6 deque([4])
* deque([4, 6])
2 deque([24])
1 deque([24, 2])
- deque([24, 2, 1])
+ deque([24, 1])
7 deque([25])
7 deque([25, 7])
+ deque([25, 7, 7])
7 deque([25, 14])
6 deque([25, 14, 7])
+ deque([25, 14, 7, 6])
* deque([25, 14, 13])
* deque([25, 182])
3 deque([4550])
7 deque([4550, 3])
- deque([4550, 3, 7])
7 deque([4550, -4])
+ deque([4550, -4, 7])
9 deque([4550, 3])
7 deque([4550, 3, 9])
2 deque([4550, 3, 9, 7])
+ deque([4550, 3, 9, 7, 2])
- deque([4550, 3, 9, 9])
* deque([4550, 3, 0])
Traceback (most recent call last):
  File "C:\Users\...\temp.py", line 11, in postfixExpression
    stack.append(int(ex))
ValueError: invalid literal for int() with base 10: '*'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "C:\Users\...\temp.py", line 52, in main
    print(f"\nresult {postfixExpression(expression)}")
  File "C:\Users\...\temp.py", line 19, in postfixExpression
    "/" : int(a / b),
ZeroDivisionError: division by zero

当我发表评论时 "/" : int(a / b), 切换器dict中的行给出了预期的答案,即0。

有人能解释我为什么会出现这个错误(main)以及如何解决它吗?

2 回复 | 直到 1 年前

lxop 1 年前

问题是你正在评估 全部的 的操作 switcher 词典因此,无论代码遇到哪种运算符,它都会计算堆栈顶部两个值的和、差、积和商。然后,它查看操作符,以决定实际使用哪一个。在这种情况下,您的 b 值为零,所以它确实试图除以零。

有多种方法可以解决此问题,包括(但不限于):

使用 lambda 中的函数 切换器 字典,并且只调用所需的

            switcher = {
                "+" : lambda: a + b,
                "-" : lambda: a - b,
                "*" : lambda: a * b,
                "/" : lambda: int(a / b),
                "^" : lambda: a ** b,
            }
            if ex not in switcher:
                raise ValueError(f"Wrong expression-> {ex}")
            else:
                stack.append(switcher[ex]())  # <- note the added () here

只需使用 match 语句直接在代码中计算适当的值,而不是使用字典(这需要Python 3.10+)

    match ex:
        case "+":
            val = a + b
        case "-":
            val = a - b
        case "*":
            val = a * b
        case "/":
            val = int(a / b)
        case "^":
            val = a ** b
        case _:
            raise ValueError(f"Wrong expression-> {ex}")
    stack.append(val)

使用 if - elif 链直接在代码中计算适当的值,而不是使用字典

    if ex == "+":
        val = a + b
    elif ex == "-":
        val = a - b
    elif ex == "*":
        val = a * b
    elif ex == "/":
        val = int(a / b)
    elif ex == "^":
        val = a ** b
    else:
        raise ValueError(f"Wrong expression-> {ex}")
    stack.append(val)

在任何情况下,我都建议您将计算操作拉到它自己的函数中,而不是将其嵌入到循环的中间。有点像

def calculate_value(op, a, b):
    ...  # Something based on what I've shown above


def postfixExpression(expression: str) -> int:
    if not expression:
        return -1
    expression_list = expression.split(" ")
    stack = deque()
    for ex in expression_list:
        print(ex, stack)
        try:
            stack.append(int(ex))
        except ValueError:
            b = stack.pop()
            a = stack.pop()
            stack.append(calculate_value(ex, a, b))
    return stack.pop()

应要求:测试代码使用 λ 功能:

from collections import deque
def postfixExpression(expression: str) -> int:
    if not expression:
        return -1
    expression_list = expression.split(" ")
    stack = deque()
    for ex in expression_list:
        print(ex, stack)
        try:
            stack.append(int(ex))
        except ValueError:
            b = stack.pop()
            a = stack.pop()
            switcher = {
                "+" : lambda: a + b,
                "-" : lambda: a - b,
                "*" : lambda: a * b,
                "/" : lambda: int(a / b),
                "^" : lambda: a ** b,
            }
            if ex not in switcher.keys():
                raise ValueError(f"Wrong expression-> {ex}")
            else:
                stack.append(switcher.get(ex)())
    return stack.pop()

expression = "2 3 7 - 7 + 9 7 2 + - * *"
print(f"\nResult {postfixExpression(expression)}")

带输出

2 deque([])
3 deque([2])
7 deque([2, 3])
- deque([2, 3, 7])
7 deque([2, -4])
+ deque([2, -4, 7])
9 deque([2, 3])
7 deque([2, 3, 9])
2 deque([2, 3, 9, 7])
+ deque([2, 3, 9, 7, 2])
- deque([2, 3, 9, 9])
* deque([2, 3, 0])
* deque([2, 0])

Result 0

OM222O 1 年前

我相信这就是你真正想要实现的目标,但很难说,因为你没有提供一个你手工解决的例子或预期的输出:

from collections import deque

switcher = {
    "+" : lambda a,b:a + b,
    "-" : lambda a,b:a - b,
    "*" : lambda a,b:a * b,
    "/" : lambda a,b:a // b,
    "^" : lambda a,b:a ** b,
}

def postfixExpression(expression: str) -> int:
    if not expression:
        return -1
    expression_list = expression.split(" ")
    stack = deque()
    for ex in expression_list:
        print(ex, stack)
        if ex.isnumeric():
            stack.append(int(ex))
        elif ex in switcher:
            stack.append(switcher[ex](stack.pop(),stack.pop()))
        else:
            raise ValueError(f"Wrong expression-> {ex}")
    return stack.pop()

expression = "2 3 7 - 7 + 9 7 2 + - * *"
print(f"\nResult {postfixExpression(expression)}")

编辑:只是提供的解决方案之间的快速比较,以下是代码:

from collections import deque

def postfixExpression_OP_LAMBDA(expression: str) -> int:
    if not expression:
        return -1
    expression_list = expression.split(" ")
    stack = deque()
    for ex in expression_list:
        try:
            stack.append(int(ex))
        except ValueError:
            b = stack.pop()
            a = stack.pop()
            switcher = {
                "+" : lambda a,b: a + b,
                "-" : lambda a,b: a - b,
                "*" : lambda a,b: a * b,
                "/" : lambda a,b: int(a / b),
                "^" : lambda a,b: a ** b,
            }
            if ex not in switcher.keys():
                raise ValueError(f"Wrong expression-> {ex}")
            else:
                stack.append(switcher.get(ex)(a,b))
    return stack.pop()

def postfixExpression_MATCH(expression: str) -> int:
    if not expression:
        return -1
    expression_list = expression.split(" ")
    stack = deque()
    for ex in expression_list:
        try:
            stack.append(int(ex))
        except ValueError:
            b = stack.pop()
            a = stack.pop()
            match ex:
                case "+":
                    val = a + b
                case "-":
                    val = a - b
                case "*":
                    val = a * b
                case "/":
                    val = int(a / b)
                case "^":
                    val = a ** b
                case _:
                    raise ValueError(f"Wrong expression-> {ex}")
            stack.append(val)
    return stack.pop()

switcher_ = {
    "+" : lambda a,b:a + b,
    "-" : lambda a,b:a - b,
    "*" : lambda a,b:a * b,
    "/" : lambda a,b:a // b,
    "^" : lambda a,b:a ** b,
}

def postfixExpression_OM(expression: str) -> int:
    if not expression:
        return -1
    expression_list = expression.split(" ")
    stack = deque()
    for ex in expression_list:
        if ex.isnumeric():
            stack.append(int(ex))
        elif ex in switcher_:
            stack.append(switcher_[ex](stack.pop(),stack.pop()))
        else:
            raise ValueError(f"Wrong expression-> {ex}")
    return stack.pop()

expression = "2 3 7 - 7 + 9 7 2 + - * *"

以及用于比较的快速基准: 我的解决方案是快3到4倍,这取决于你所做的和正确使用Lambda。如果重命名a或b,@lxop提供的解决方案将失败,因为他依赖于极差的变量作用域(此处有更多详细信息: https://www.youtube.com/watch?v=fZE6ZWde-Os )。