修改头指针在链表 [英] Modifying head pointer in a linked list

问题描述

我无法理解这种code。我真正需要的是修改头指针指向第一个元素。那么，为什么不*头工作？改变*头的变化值，其中该指针指向那应该工作，对不对？我已阅读通过引用传递/按值传递，但我觉得很难理解。有人可以帮助澄清这一点？
鸭preciate你的帮助。谢谢你。

在C / C ++更容易使具有指针滥用的错误。考虑这个C / C ++ code在列表的前面插入一个元素：

 布尔insertInFront（IntElement *头，int数据）{
  IntElement * newElem =新IntElement;
  如果（newElem！）返回false;  newElem-＆GT;数据=数据;
  头= newElem; //不正确的！
  返回true;
}
 

在preceding code不正确，因为它仅更新头指针的本地副本。正确的版本在传递一个指向头指针：

 布尔insertInFront（IntElement **头，int数据）{
  IntElement * newElem =新IntElement;
  如果（newElem！）返回false;  newElen-＆GT;数据=数据;
  *头= newElem; //正确更新头
  返回true;
}
 

解决方案

您需要帮助理解的区别吧？

想象的功能在第一种情况下来电：

  IntElement *头;
int数据;
...
insertInFront（头，数据）;
 

现在，在这种情况下，该地址指向的头部被放置在栈上，并通过在作为参数insertInFront。当insertInFront做头=为newElement;只（在堆栈）参数被修改。

在第二种情况下，主叫方将是：

  IntElement *头;
int数据;
...
insertInFront（安培;头，数据）;
 

在这种情况下，头部的地址放置在堆栈上，并通过在作为参数传递给insertInFront。当你做*头=为newElement，这在转交地址被取消引用来获得原始表头的地址，而被修改。

I am having trouble understanding this code. All I really need is to modify the head pointer to point to the first element. So why won't *head work ? Changing the value of *head changes where this pointer points to and that should work, right ? I have read the pass by reference/pass by value, but am finding it hard to understand. Can someone help clarify this ? Appreciate your help. Thanks.

In C/C++ it’s easier to make mistakes with pointer misuse. Consider this C/C++ code for inserting an element at the front of a list:

bool insertInFront( IntElement *head, int data ){
  IntElement *newElem = new IntElement;
  if( !newElem ) return false;

  newElem->data = data;
  head = newElem; // Incorrect!
  return true;
}

The preceding code is incorrect because it only updates the local copy of the head pointer. The correct version passes in a pointer to the head pointer:

bool insertInFront( IntElement **head, int data ){
  IntElement *newElem = new IntElement;
  if( !newElem ) return false;

  newElen->data = data;
  *head = newElem; // Correctly updates head
  return true;
}

解决方案

You need help understanding the difference right?

Imagine the caller of the function in the first case:

IntElement *head;
int data;
...
insertInFront (head, data);

Now, in this case, the address pointed to by head is placed on the stack and passed in as an argument to insertInFront. When insertInFront does head = newElement; only the argument (on the stack) is modified.

In the second case, the caller would be:

IntElement *head;
int data;
...
insertInFront (&head, data);

In this case, the address of head is placed on the stack and passed in as an argument to insertInFront. When you do *head = newElement, this passed in address is de-referenced to get the address of the original list head, and that is modified.

这篇关于修改头指针在链表的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！