[英]Pointer to root node of B-tree after call to a function begins pointing to a child ( should continue pointing to root instead )
下面的B樹代碼是從CLRS 3rd Edition->高級數據結構->第18章獲取的算法的代碼表示。
我已經為btree_insert
, btree_split_child
和btree_insert_nonfull
函數編寫了代碼。
從CLRS轉移 :代碼遵循具有無限制分支的生根樹的LEFT-CHILD,RIGHT-SIBLING表示。
Btree的最小度為3。
/* Representing rooted trees with unbounded branching :-
*
* LEFT-CHILD, RIGHT-SIBLING representation : Instead of having a pointer to each of its children,
* however, each node x has only two pointers:
* x.left-child points to the leftmost child of node x, and
* x.right-sibling points to the sibling of x immediately to its right.
*
* If x has no children, then x.left-child = NIL, and if node x is the rightmost child of its parent,
* then x.right-sibling = NIL.
*
*/
/* assume t = 3 situation
* Therefore, lowerbound on #keys = 2 and upperbound = 2t - 1 keys, i.e 5 keys */
#include <stdio.h>
#include <stdlib.h>
#define DEGREE 3
struct s_btree_node
{
int leaf;
int total_keys;
int keys[2 * DEGREE - 1];
struct s_btree_node * left_child;
struct s_btree_node * right_sibling;
};
/* helper functions for btree structure */
/* pointer to the root node of the BTree structure */
static struct s_btree_node * proot = NULL;
/* allocate new node on the heap */
struct s_btree_node * new_node(void);
void btree_split_child (struct s_btree_node * proot, int i);
void btree_insert_nonfull (struct s_btree_node * proot, int key);
void btree_insert_node(struct s_btree_node * pnode, int key);
int main(int argc, char * argv[])
{
proot = new_node();
btree_insert_node(proot, 8);
btree_insert_node(proot, 1);
btree_insert_node(proot, 11);
btree_insert_node(proot, 5);
btree_insert_node(proot, 13);
btree_insert_node(proot, 7);
btree_insert_node(proot, 28);
btree_insert_node(proot, 2);
}
struct s_btree_node * new_node()
{
struct s_btree_node * pnode = (struct s_btree_node *)malloc(sizeof(struct s_btree_node));
pnode->leaf = 1;
pnode->total_keys = 0;
pnode->left_child = NULL;
return pnode;
}
void btree_split_child (struct s_btree_node * pnode, int i)
{
/* pnew_split_node is z in CORMEN */
struct s_btree_node * pnew_split_node = new_node();
/* CORMEN: line 2 */
/* Traverse tree to get the correct right-sibling. Conceptually y is x's i-th child */
int j = 0;
struct s_btree_node * pstud = pnode->left_child;
/* Get x.c_suffix_i */
while (pstud != NULL && j < i) {
pstud = pstud->right_sibling;
j++;
}
/* poriginal_split_node is y in CORMEN */
struct s_btree_node * poriginal_split_node = pstud;
/* CORMEN: line 3 */
pnew_split_node->leaf = poriginal_split_node->leaf;
/* CORMEN: line 4 */
pnew_split_node->total_keys = DEGREE - 1;
/* CORMEN: line 5 */
j = 0;
while (j <= DEGREE - 2) {
pnew_split_node->keys[j] = poriginal_split_node->keys[j + DEGREE];
j++;
}
/* CORMEN: line 7 */
if (poriginal_split_node->leaf == 0) {
int k = 1;
/* find starting child node */
struct s_btree_node * poriginal_split_node_child = poriginal_split_node->left_child;
while (k < k + DEGREE) {
poriginal_split_node_child = poriginal_split_node_child->right_sibling;
k++;
}
/* Assign rest of the child chain of poriginal_spilt_node to pnew_split_node */
pnew_split_node->left_child = poriginal_split_node_child->right_sibling;
/* Make right_sibling of poriginal_split_node_child point to NULL */
poriginal_split_node_child->right_sibling = NULL;
}
/* CORMEN: line 10 */
/* Remove redundant keys of poriginal_split_node */
poriginal_split_node->total_keys = DEGREE - 1;
int l = 2 * DEGREE - 1;
while (l > poriginal_split_node->total_keys + 1) { /* Keep key 'keys[DEGREE - 1]' till it is promoted to parent */
poriginal_split_node->keys[l - 1] = 0;
l--;
}
/* CORMEN: line 11-13 */
pnew_split_node->right_sibling = poriginal_split_node->right_sibling;
poriginal_split_node->right_sibling = pnew_split_node;
/* CORMEN: line 14-16 */
int k = pnode->total_keys;
while (k > i) {
pnode->keys[k] = pnode->keys[k - 1];
k--;
}
pnode->keys[i] = poriginal_split_node->keys[DEGREE - 1];
/* After key 'key[DEGREE - 1]' from poriginal_split_node has been promoted to parent,
* remove it from poriginal_split_node */
poriginal_split_node->keys[DEGREE - 1] = 0;
/* CORMEN: line 17 */
pnode->total_keys = pnode->total_keys + 1;
}
void btree_insert_nonfull (struct s_btree_node * pnode, int key)
{
int i = pnode->total_keys;
if (pnode->leaf == 1) {
while (i >= 1 && key < pnode->keys[i - 1]) {
pnode->keys[i] = pnode->keys[i - 1];
i = i - 1;
}
pnode->keys[i] = key;
pnode->total_keys = pnode->total_keys + 1;
} else {
while (i >= 1 && key < pnode->keys[i - 1]) {
i = i - 1;
}
i = i + 1;
/* traverse to correct child node */
int j = i;
struct s_btree_node * pchild_node = pnode->left_child;
while (j > 1) {
pchild_node = pchild_node->right_sibling;
j--;
}
if (pchild_node->total_keys == 2 * DEGREE - 1) {
btree_split_child(pnode, i);
if (key > pnode->keys[i - 1]) {
i = i + 1;
}
}
btree_insert_nonfull(pchild_node, key);
}
}
void btree_insert_node (struct s_btree_node * proot, int key)
{
struct s_btree_node * pnode = proot;
if (pnode->total_keys == 2 * DEGREE - 1) {
struct s_btree_node * psplit_node = new_node();
proot = psplit_node;
psplit_node->leaf = 0;
psplit_node->total_keys = 0;
psplit_node->left_child = pnode;
btree_split_child(psplit_node, 0);
btree_insert_nonfull(psplit_node, key);
} else {
btree_insert_nonfull(pnode, key);
}
}
調試會話:
Reading symbols from btree...done.
(gdb) b 74
Breakpoint 1 at 0x4005a5: file BTree.c, line 74.
(gdb) b 247
Breakpoint 2 at 0x400a31: file BTree.c, line 247.
(gdb) b 75
Breakpoint 3 at 0x4005b9: file BTree.c, line 75.
(gdb) run
Breakpoint 1, main (argc=1, argv=0x7fffffffdee8) at BTree.c:74
74 btree_insert_node(proot, 7);
(gdb) p * proot
$1 = {leaf = 1, total_keys = 5, keys = {1, 5, 8, 11, 13}, left_child = 0x0, right_sibling = 0x0}
(gdb) n
Breakpoint 2, btree_insert_node (proot=0x602050, key=7) at BTree.c:247
247 btree_insert_nonfull(psplit_node, key);
(gdb) p * proot
$2 = {leaf = 0, total_keys = 1, keys = {8, 0, 0, 0, 0}, left_child = 0x602010, right_sibling = 0x0}
(gdb) p * proot->left_child
$3 = {leaf = 1, total_keys = 2, keys = {1, 5, 0, 0, 0}, left_child = 0x0, right_sibling = 0x602090}
(gdb) p * proot->left_child->right_sibling
$4 = {leaf = 1, total_keys = 2, keys = {11, 13, 0, 0, 0}, left_child = 0x0, right_sibling = 0x0}
(gdb) n
251 }
(gdb) p * proot
$5 = {leaf = 0, total_keys = 1, keys = {8, 0, 0, 0, 0}, left_child = 0x602010, right_sibling = 0x0}
(gdb) n
Breakpoint 3, main (argc=1, argv=0x7fffffffdee8) at BTree.c:75
75 btree_insert_node(proot, 28);
(gdb) p * proot
$6 = {leaf = 1, total_keys = 3, keys = {1, 5, 7, 0, 0}, left_child = 0x0, right_sibling = 0x602090}
(gdb)
當從電話btree_insert_node
返回執行后btree_split_child
到main
功能,只需前行btree_insert_node(proot, 28)
執行,我打印* proot
,給出了一個意想不到的結果。
在調試會話結束時, proot
應該為{leaf = 0, total_keys = 1, keys = {8, 0, 0, 0, 0}, left_child = 0x602010, right_sibling = 0x0}
。 相反,它是{leaf = 1, total_keys = 3, keys = {1, 5, 7, 0, 0}, left_child = 0x0, right_sibling = 0x602090}
。
我是C語言的新手。 任何其他改進我的代碼的建議都將受到贊賞。
在btree_insert_node
, s_btree_node* proot
是指向結構的指針,而不是指向指針的指針。 因此,當您在proot = psplit_node
內修改它時,只會修改本地副本。
您可以改為返回新的根。 這應該工作:
...
struct s_btree_node * btree_insert_node(struct s_btree_node * pnode, int key);
int main(int argc, char * argv[])
{
proot = new_node();
proot = btree_insert_node(proot, 8);
proot = btree_insert_node(proot, 1);
proot = btree_insert_node(proot, 11);
proot = btree_insert_node(proot, 5);
proot = btree_insert_node(proot, 13);
proot = btree_insert_node(proot, 7);
proot = btree_insert_node(proot, 28);
proot = btree_insert_node(proot, 2);
}
...
struct s_btree_node * btree_insert_node(struct s_btree_node * proot, int key)
{
struct s_btree_node * pnode = proot;
if (pnode->total_keys == 2 * DEGREE - 1) {
struct s_btree_node * psplit_node = new_node();
proot = psplit_node;
psplit_node->leaf = 0;
psplit_node->total_keys = 0;
psplit_node->left_child = pnode;
btree_split_child(psplit_node, 0);
btree_insert_nonfull(psplit_node, key);
return proot; //return a new tree-root after insertion
}
else {
btree_insert_nonfull(pnode, key);
return proot; //tree-root was not changed - return it
}
}
在C中,當您將任何內容傳遞給函數時,函數會創建它自己的參數副本,例如int,pointer等。
在函數btree_insert_node
您有一個名為*proot
的參數,您proot
全局變量proot
傳遞給該參數。 假設您的全局proot變量的地址為gpa,其值為gpv。 當您在函數btree_insert_node
傳遞此全局變量時,其局部變量proot(假設其地址為lpa)將獲得gpv的值。 然后當你做proot = psplit_node;
此局部proot變量的值已更改,但全局值未更改。 假設psplit_node
值為psnv,然后在操作之后:
本地proot的地址:lpa,本地proot的值:psnv
全球proot的地址:gpa,全球proot的值:gpv
全球道具的價值保持不變,這不是您的意圖。
解決方案非常簡單, proot
從函數btree_insert_node
刪除參數btree_insert_node
,因為您已經有了全局proot
這應該工作:
int main(int argc, char * argv[])
{
proot = new_node();
struct s_btree_node * a;
btree_insert_node(8);
btree_insert_node(1);
btree_insert_node(11);
btree_insert_node(5);
btree_insert_node(13);
btree_insert_node(7);
btree_insert_node(28);
btree_insert_node(2);
}
...
void btree_insert_node (int key)
{
struct s_btree_node * pnode = proot;
if (pnode->total_keys == 2 * DEGREE - 1) {
struct s_btree_node * psplit_node = new_node();
proot = psplit_node;
psplit_node->leaf = 0;
psplit_node->total_keys = 0;
psplit_node->left_child = pnode;
btree_split_child(psplit_node, 0);
btree_insert_nonfull(psplit_node, key);
} else {
btree_insert_nonfull(pnode, key);
}
}
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