c - 删除条目和在链接列表中插入条目功能对错误条目进行操作

Question

下午好。我正在从 Stephen G. Kochan 所著的《C 语言编程第三版》一书中学习 C 语言。我编写了一些代码，应该从列表中插入和删除某些条目，它确实做到了，问题是，它没有删除正确的条目，并且它没有完全将条目插入到正确的位置。代码如下。任何帮助将不胜感激!

//Insert and Remove entry functions using doubly linked lists
#include <stdio.h>

struct Entry
{
    int Value;
    struct Entry *Previous;
    struct Entry *Next;
};


int main()
{
    void InsertEntry (struct Entry *InsertPosition, struct Entry EntryToInsert);
    void RemoveEntry (struct Entry *EntryToRemove);
    struct Entry N1, N2, N3, N4, N5, Insert, *Start = &N1;

//set initial values
    N1.Value = 10;
    N2.Value = 20;
    N3.Value = 20;
    N4.Value = 30;
    N5.Value = 40;
    Insert.Value = 35;

//link the list

    N1.Next = &N2;
    N2.Next = &N3;
    N3.Next = &N4;
    N4.Next = &N5;
    N5.Next = (struct Entry *) 0;
//Link again

    N1.Previous = &N1;
    N2.Previous = &N1;
    N3.Previous = &N2;
    N4.Previous = &N3;
    N5.Previous = &N4;

    InsertEntry(&N4, Insert);
    RemoveEntry(&N2);

//Display the Lists
    while (Start->Next != (struct Entry *) 0)
    {
        printf("Previous: %i, Current: %i, Next: %i\n",         Start->Previous->Value, Start->Value, Start->Next->Value);
        Start = Start->Next;
    }

    return 0;
}

void InsertEntry (struct Entry *InsertPosition, struct Entry EntryToInsert)
{

    EntryToInsert.Previous = InsertPosition->Previous;
    EntryToInsert.Next = InsertPosition;
    InsertPosition->Previous->Next = &EntryToInsert;

}

void RemoveEntry (struct Entry *EntryToRemove)
{
    EntryToRemove->Previous->Next = EntryToRemove->Next;
}

Answer 1

代码中存在几个问题。

将 InsertEntry 和 RemoveEntry 的声明移至 main() 之前。

初始化Previous字段时，列表头需要Previous = NULL。

打印列表时，需要避免分别打印列表头和尾部的 Previous->Value 和 Next->Value。

在 InsertEntry() 函数中，代码按值传递 EntryToInsert 结构，将副本插入到列表中。我猜您打算传递指针，将原始结构插入列表中。

在InsertEntry()函数中，还需要设置InsertPosition->Next->Previous。

在InsertEntry()函数中设置InsertPosition->Previous->Next和InsertPosition->Next->Previous之前，需要检查是否分别位于列表的头部和尾部。

在RemoveEntry()函数中，还需要设置EntryToRemove->Next->Previous。

在RemoveEntry()函数中，在设置EntryToRemove->Previous->Next和EntryToRemove->Next->Previous之前，需要检查是否分别位于列表的头部和尾部。

在RemoveEntry()函数中，还需要设置EntryToRemove->Previous = NULL和EntryToRemove->Next = NULL。

我建议您尝试自行解决上述每个问题。如果您在修复给定问题时遇到问题，我已包含完整的代码以及我建议的修复方案供您查看。

struct Entry
{
    int Value;
    struct Entry *Previous;
    struct Entry *Next;
};

void InsertEntry(struct Entry *InsertPosition, struct Entry *EntryToInsert);
void RemoveEntry(struct Entry *EntryToRemove);

int main()
{
    struct Entry N1, N2, N3, N4, N5, Insert, *Start = &N1;

    //set initial values
    N1.Value = 10;
    N2.Value = 20;
    N3.Value = 20;
    N4.Value = 30;
    N5.Value = 40;
    Insert.Value = 35;

    //link the list

    N1.Next = &N2;
    N2.Next = &N3;
    N3.Next = &N4;
    N4.Next = &N5;
    N5.Next = NULL;
    //Link again

    N1.Previous = NULL;
    N2.Previous = &N1;
    N3.Previous = &N2;
    N4.Previous = &N3;
    N5.Previous = &N4;

    InsertEntry(&N4, &Insert);
    RemoveEntry(&N2);

    //Display the Lists
    while (Start != (struct Entry *) 0)
    {
        printf("Previous: ");
        if (Start->Previous != NULL)
        {
            printf("%i", Start->Previous->Value);
        }
        else
        {
            printf("NULL");
        }
        printf(", Current: %i, Next: ", Start->Value);
        if (Start->Next != NULL)
        {
            printf("%i", Start->Next->Value);
        }
        else
        {
            printf("NULL");
        }
        printf("\n");
        Start = Start->Next;
    }

    return 0;
}

void InsertEntry(struct Entry *InsertPosition, struct Entry *EntryToInsert)
{
    EntryToInsert->Previous = InsertPosition->Previous;
    EntryToInsert->Next = InsertPosition;
    if (InsertPosition->Previous != NULL)
    {
        InsertPosition->Previous->Next = EntryToInsert;
    }
    InsertPosition->Previous = EntryToInsert;

}

void RemoveEntry(struct Entry *EntryToRemove)
{
    if (EntryToRemove->Previous != NULL)
    {
        EntryToRemove->Previous->Next = EntryToRemove->Next;
    }
    if (EntryToRemove->Next != NULL)
    {
        EntryToRemove->Next->Previous = EntryToRemove->Previous;
    }
    EntryToRemove->Previous = NULL;
    EntryToRemove->Next = NULL;
}