为什么即使分配给另一个相同类型的结构，该结构仍保持为 NULL？

Question

我正在尝试生成字符串的二叉树。列表中包含的结构是使用以下代码创建的：

typedef struct WordNode {
    char *word;
    unsigned count;
    struct WordNode *left;
    struct WordNode *right;
} wordnode;

我在 main 函数中首先将根节点初始化为 NULL，如下所示：

// The initial struct begins empty.
wordnode *head = NULL;

这是因为初始结构将是给定输入文件中包含的第一个单词。一旦它被初始化为 NULL，我就开始处理文件内容：

// While the end of file marker has not been reached...
while (EOF != fscanf(inHandle, BUFFMT, wordBuffer)) {
    // Clean the word currently in the wordBuffer.
    cleanWord(wordBuffer);
    
    // If the buffer contains an actual word...
    if (strlen(wordBuffer) > 0) {
        
        // Add one to the total word count.
        totalWords++;

        placeWordInTree(wordBuffer, head);
        
    }
}

placeWordInTree 函数处理输入文件的每个单词（占第一个空根节点）：

void placeWordInTree(char *word, wordnode *currNode) {
    // If the current node is empty, initialize it to a newly generated node.
    if (currNode == NULL) {
        currNode = generateNode(word);
        
    // If the base case has not been satisfied...   
    } else {
        // Store the result of comparing the given word and the current structs word.
        int strComp = strcmp(word, currNode->word);
    
        // If the words are equal, add one to the current structs counter.
        if (strComp == 0) {
            currNode->count++;
        
        // If the given word is less than the struct word, recursively call this function
        // with the given node and the current structs left node.   
        } else if (strComp < 0) {
            placeWordInTree(word, currNode->left);
        
        // If the given word is greater than the struct word, recursively call this
        // function with the given word and the current structs right node.
        } else if (strComp > 0) {
            placeWordInTree(word, currNode->right);
        }
    }
}

这个函数检查当前节点是否为NULL。如果是，则将 generateNode 函数使用给定单词生成的新结构分配给当前结构。如果当前结构不为空，它将给定字与当前结构字进行比较，然后将计数器加一或尝试递归地将节点放置在当前节点左结构中（如果它小于当前结构字）或正确的结构（如果更多）。

generateNode 函数生成一个像这样的节点：

wordnode *generateNode(char *word) {
    // Allocates enough memory to hold a new word node.
    wordnode *newNode = malloc(sizeof(wordnode));
    
    // Allocates enough memory to store the new nodes given word.
    newNode->word = malloc(strlen(word)+1);
    
    // Copies the given word into the new nodes word field.
    strncpy(newNode->word, word, strlen(word));
    
    // Sets the new nodes count to 1, and its branches to NULL.
    newNode->count = 1;
    newNode->left = NULL;
    newNode->right = NULL;
    
    // Return the new node.
    return newNode;
}

我遇到的问题是程序似乎认为在 placeWordInTree 函数中检查 head 结构时始终为 NULL。当放置 printf 语句进行调试时，它告诉我 placeWordInTree 函数的每个可能结果何时执行，它总是执行代码，就好像 head 结构总是 NULL。更令人困惑的是，当打印树的内容时，head 结构包含的单词始终是输入文件的第一个单词。经过长时间的调试没有成功，树甚至不再打印了。

我检查了文件输入，它工作正常。我检查了显示功能，它工作正常（但我将它放在下面以防万一。）我检查了主要功能，一切看起来都很好。问题似乎只是 placeWordInTree 函数的 else 语句初始 if 语句永远不会执行。

unsigned displayTree(wordnode *currNode) {
    unsigned treeLength = 0;
    
    // If the current node is NULL, return 0;
    if (currNode == NULL) {
        return treeLength;
        
    // If the current node is not null, recursively call displayList on its left and
    // right pointers and add their return value to the total tree node count.  
    } else {
        printf("\n%6u\t%s", currNode->count, currNode->word);
        treeLength += displayTree(currNode->left);
        treeLength += displayTree(currNode->right);
    }
    
    return treeLength++;
}

Answer 1

你需要在 placeWordInTree 函数中通过引用传递节点。在 C 中，这是通过将指针传递给要更改的所需变量来实现的。

void placeWordInTree(char *word, wordnode **currNode)

并且在调用函数时添加操作符的地址。

placeWordInTree(wordBuffer, &head);