C 语言编程，Stephen Kochan，第 11 章，练习 2

Question

这是我在堆栈上的第一个问题。

Stephen Kochan 所著的 Programming in C (Third Edition) 一书中第 11 章 - 指针中的练习 2 有问题。

问题说： 编写一个名为 insertEntry 的函数，将新条目插入到链表中。让该过程将指向要插入的列表条目的指针（本章定义的结构条目类型）作为参数，以及指向列表中要插入新条目的元素的指针。

我的代码是这样的：

#include <stdio.h>


struct entry
{
    int  value;
    struct entry *next;
};


void insertEntry ( struct entry newEntry, struct entry EntryNo ){

    newEntry.next = EntryNo.next;
    EntryNo.next = &newEntry;
}


int main (){
    struct entry n1, n2, n3, n4, newEntry;

struct entry *listPointer = &n1; // with this pointer we mark the start of the list
n1.value = 100;
n1.next = &n2;
n2.value = 200;
n2.next = &n3;
n3.value = 300;
n3.next = &n4;
n4.value = 400;
n4.next = (struct entry *) 0; // we pinpoint that n4 will be the last entry of our list

newEntry.value = 340;

printf("The list before adding a new entry:\n");
while ( listPointer != (struct entry*) 0 ){
    printf("%i\n", listPointer->value );
    listPointer = listPointer->next;
}
listPointer = &n1;

insertEntry ( newEntry, n3 );

printf("The list after adding a new entry:\n");
while ( listPointer != (struct entry*) 0 ){
    printf("%i\n", listPointer->value );
    listPointer = listPointer->next;
}

return 0;

}

输出是：

The list before adding a new entry:
100
200
300
400
The list after adding a new entry:
100
200
300
400

然而，当我编写如下所示的代码时，我得到了所需的输出（这是值 300 和 400 中的新条目。

#include <stdio.h>


struct entry
{
    int  value;
    struct entry *next;
};


int main (){
struct entry n1, n2, n3, n4, newEntry;

struct entry *listPointer = &n1; // with this pointer we mark the start of the list
n1.value = 100;
n1.next = &n2;
n2.value = 200;
n2.next = &n3;
n3.value = 300;
n3.next = &n4;
n4.value = 400;
n4.next = (struct entry *) 0; // we pinpoint that n4 will be the last entry of our list

newEntry.value = 340;

printf("The list before adding a new entry:\n");
while ( listPointer != (struct entry*) 0 ){
    printf("%i\n", listPointer->value );
    listPointer = listPointer->next;
}
listPointer = &n1;

newEntry.next = n3.next;
n3.next = &newEntry;

printf("The list after adding a new entry:\n");
while ( listPointer != (struct entry*) 0 ){
    printf("%i\n", listPointer->value );
    listPointer = listPointer->next;
}

return 0;

}

输出是：

The list before adding a new entry:
100
200
300
400
The list after adding a new entry:
100
200
300
340
400

所以问题出在第一个代码上的函数上，比如添加的条目的值没有注册（或者没有从函数正确返回）。我不知道，因为我在这里很困惑。如果有人可以帮助我，我将不胜感激。提前谢谢你。

Answer 1

您需要将对象的地址传递给您的函数。函数原型将是：

void insertEntry ( struct entry *newEntry, struct entry *EntryNo )

调用函数时，传递地址为：

insertEntry ( &newEntry, &n3 );

此外，由于我们已经修改了函数以将指针作为参数，因此我们还需要在函数内进行一些修改：

void insertEntry ( struct entry *newEntry, struct entry *EntryNo ){

(*newEntry).next = (*EntryNo).next;
(*EntryNo).next = newEntry;

}

工作代码：http://ideone.com/qSdsy0

Answer 2

由于您将对象直接传递给您的函数，因此它适用于对象的副本，而不是对象本身。为了直接处理对象，您需要传递对象的地址。