如何从R环境中的ls（）获取所有对象的dim（）[重复](How to get the dim() of all of the objects obtaining from ls() in R environment [duplicate])

 Python支持一个内置的机制来处理列表，你可以很容易地做这样的事情： 
 
list = []
list.append(10)
list.insert(0, 20)
list.append(30)
list.insert(1, 50)
 
 有关更多示例，请参阅官方文档https://docs.python.org/3/tutorial/datastructures.html 。 
 
 无论如何，如果你想通过你的自我编程一个双链表，我已经对你的代码做了一些修改： 
 
class createnode:
    def __init__(self, data):
        self.data = data
        self.next = None
        self.prev = None


class Unordered_list:
    def __init__(self):
        self.head = None

    def buildlst(self, data):
        node = createnode(data)
        if self.head is None:
            self.head = node
        else:
            node.next = self.head
            self.head.prev = node  # <---
            #  node.prev = None  <---- this is not necessary
            self.head = node

    def buildlstend(self, data):
        node = createnode(data)
        ptr = self.head
        while ptr.next:
            ptr = ptr.next
        ptr.next = node
        node.prev = ptr  # <---

    def insertbeforenode(self, data, srch_data):
        if srch_data == self.head.data: # <-- if you try to insert before the head
            self.buildlst(data)
        else:
            node = createnode(data)
            ptr = self.head
            while ptr:
                if ptr.data == srch_data:
                    node.prev = ptr.prev
                    node.next = ptr
                    ptr.prev = node
                    node.prev.next = node  # <--
                    break  # <--- when you find your node you have to quit from the loop
                else:
                    ptr = ptr.next

    def printlist(self):
        temp = self.head
        while (temp):
            print(temp.data)
            temp = temp.next
 
 我已经把箭头（＃< - ）放在了我进行更改的地方。 在处理指针和其他错误时存在一些错误。 请试试这个，让我知道！ 

Python supports a built-in mechanism to work with lists, you could easily do something like this:
 
list = []
list.append(10)
list.insert(0, 20)
list.append(30)
list.insert(1, 50)
 
See the official documentation for more examples https://docs.python.org/3/tutorial/datastructures.html.
 
Anyway if you want to program a double linked list by your self I have made some changes to your code:
 
class createnode:
    def __init__(self, data):
        self.data = data
        self.next = None
        self.prev = None


class Unordered_list:
    def __init__(self):
        self.head = None

    def buildlst(self, data):
        node = createnode(data)
        if self.head is None:
            self.head = node
        else:
            node.next = self.head
            self.head.prev = node  # <---
            #  node.prev = None  <---- this is not necessary
            self.head = node

    def buildlstend(self, data):
        node = createnode(data)
        ptr = self.head
        while ptr.next:
            ptr = ptr.next
        ptr.next = node
        node.prev = ptr  # <---

    def insertbeforenode(self, data, srch_data):
        if srch_data == self.head.data: # <-- if you try to insert before the head
            self.buildlst(data)
        else:
            node = createnode(data)
            ptr = self.head
            while ptr:
                if ptr.data == srch_data:
                    node.prev = ptr.prev
                    node.next = ptr
                    ptr.prev = node
                    node.prev.next = node  # <--
                    break  # <--- when you find your node you have to quit from the loop
                else:
                    ptr = ptr.next

    def printlist(self):
        temp = self.head
        while (temp):
            print(temp.data)
            temp = temp.next
 
I have put an arrow (# <--) where I made the changes. There were a couples of mistakes on working with pointers and others inaccuracies. Please try this and let me know!