[英]Why does sending **head to a function work while reversing an SLL and *head doesn't in C?
I am using Apple Clang 11.00 to compile my code to reverse a linked list.我正在使用 Apple Clang 11.00 编译我的代码以反转链接列表。 I was using this code at first but it didn't work:
我起初使用此代码但它不起作用:
void reverseSLL(node *head)
{
node *cur, *next, *prev;
cur = head;
prev = next = NULL;
if (head == NULL) {
printf("SLL does not Exist.\n");
return;
}
while (cur != NULL) {
next = cur->next;
cur->next = prev;
prev = cur;
cur = next;
}
head = prev;
}
Then I switched to this and it worked:然后我切换到这个并且它起作用了:
void reverseSLL(node **head)
{
node *cur, *next, *prev;
cur = *head;
prev = next = NULL;
if (*head == NULL) {
printf("SLL does not Exist.\n");
return;
}
while (cur != NULL) {
next = cur->next;
cur->next = prev;
prev = cur;
cur = next;
}
*head = prev;
}
I don't understand why is this happening.我不明白为什么会这样。 Can anybody help?
有人可以帮忙吗?
Arguments to functions in C are passed by value, which means that changes made to the value of the argument within the body of the function has no effect once the function returns. C 中函数的参数是按值传递的,这意味着一旦函数返回,对函数体内参数值所做的更改就不起作用。 A simple example:
一个简单的例子:
void foo(int a)
{
a += 10;
}
int main()
{
int b = 0;
printf("before: %d\n", b);
foo(b);
printf("after: %d\n", b);
return 0;
}
This will print:这将打印:
before: 0
after: 0
If you want the calling function ( main
in this case) to see the updated value, you have to pass a pointer to int
(you could also return the updated value, but we'll ignore that here):如果您希望调用函数(在本例中为
main
)查看更新后的值,您必须传递一个指向int
的指针(您也可以返回更新后的值,但我们将在此处忽略):
void foo(int *a)
{
*a += 10;
}
int main()
{
int b = 0;
printf("before: %d\n", b);
foo(&b); /* Note that we are taking the address of 'b' here */
printf("after: %d\n", b);
return 0;
}
In this case our output would be:在这种情况下,我们的输出将是:
before: 0
after: 10
Passing a pointer is no different.传递指针也不例外。 Making changes to the pointer in your function is not going to change it in the calling function so you have to pass a pointer to the pointer.
对函数中的指针进行更改不会在调用函数中更改它,因此您必须将指针传递给该指针。 This is why your second code example works.
这就是您的第二个代码示例有效的原因。
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