Python第五天-简易计算器

使用python实现简易计算器-支持加减乘除和括号

priority_dict = {  # 操作符优先级字典
    "+": 1,
    "-": 1,
    "*": 2,
    "/": 2
}

operands_dict = {  # 操作符函数字典
    "+": lambda x, y: x + y,
    "-": lambda x, y: x - y,
    "*": lambda x, y: x * y,
    "/": lambda x, y: x / y,
}


def calculator(expression):
    suffix = _infix2suffix(expression)
    return _calsuffix(suffix)


def _calsuffix(suffix):
    '''
    计算后缀表达式
    :param suffix: 后缀表达式
    :return:
    '''
    stack = []
    for item in list(suffix):
        '''
        是数字，入栈
        是操作符，出栈2个，计算，入栈
        '''
        if item.isdecimal():
            stack.append(item)
        elif item in operands_dict.keys():
            num2 = float(stack.pop())
            num1 = float(stack.pop())
            stack.append(operands_dict[item](num1, num2))
        else:
            raise KeyError("illegal expression")
    if len(stack) != 1:
        raise KeyError("illegal expression")
    return stack.pop()


def _infix2suffix(expression):
    '''
    中缀表达式转后缀表达式
    :param expression: 中缀表达式
    :return: 后缀表达式
    '''
    result = []  # 结果列表
    stack = []  # 栈
    for item in expression:
        '''
        是数字，直接添加到结果列表
        是左括号，入栈
        是右括号，依次出栈，直到左括号
        是操作符，栈为空或栈顶为左括号或当前操作符比栈顶操作符优先级高，直接入栈
                其他情况，依次出栈，直到栈为空或栈顶为左括号或当前操作符比栈顶操作符优先级高，当前操作符入栈
        '''
        if item.isdecimal():
            result.append(item)
        else:
            if item == "(":
                stack.append(item)
            elif item == ")":
                while True:
                    if len(stack) == 0:
                        raise KeyError("illegal expression")
                    pop_item = stack.pop()
                    if pop_item == "(":
                        break
                    result.append(pop_item)
            elif item in priority_dict.keys():
                if len(stack) == 0 or 
                        stack[len(stack) - 1] == "(" or 
                        priority_dict[item] > priority_dict[stack[len(stack) - 1]]:
                    stack.append(item)
                else:
                    while True:
                        if len(stack) == 0 or 
                                stack[len(stack) - 1] == "(" or 
                                priority_dict[item] > priority_dict[stack[len(stack) - 1]]:
                            break
                        result.append(stack.pop())
                    stack.append(item)
            else:
                raise KeyError("illegal expression")
    while len(stack) > 0:
        result.append(stack.pop())
    return "".join(result)

if __name__ == "__main__":
    print(calculator("1+2/(3*3-5)*3"))  # 1233*5-/3*+