Using fgets to read lines from a file into Binary Search Tree - c

Question

Currently i'm trying to store each separate line in my file into a string, and then store it in a binary search tree, but a problem occurs. For some reason when I print my BST only the last line is outputted and not the first 3. Below is my code.
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
struct node
{
int count;
char* key;
struct node* left;
struct node* right;
};
struct node *newNode(char* item)
{
struct node* temp = (struct node*)malloc(sizeof(struct node));
temp->key = item;
temp->left = NULL;
temp->right = NULL;
temp->count = 1;
return temp;
};
void printInorder(struct node* root)
{
if(root != NULL)
{
printInorder(root->left);
printf("%s \n", root->key);
printInorder(root->right);
}
}
struct node* insert(struct node* node, char* key)
{
if(node == NULL)//When tree is empty
return newNode(key);
if(strcmp(key, node->key) < 0)
node->left = insert(node->left, key);
if(strcmp(key, node->key) > 0)
node->right = insert(node->right, key);
return node;
};
int main()
{
struct node *root = NULL;
int i = 0;
char str[100];
FILE* fp;
fp = fopen("textFile.txt", "r");
if ((fp = fopen("textFile.txt","r")) == NULL)
{
printf("Could not open textFile.txt\n");
exit(1);
}
while(fgets(str, 100, fp) != NULL)
{
++i;
root = insert(root, str);
printf("%3d: %s", i, str);
}
printf("bst printed\n");
printInorder(root);
return 0;
}
textFile.txt contains
bob is working.
david is a new hire.
alice is bob's boss.
charles doesn't like bob.
And when the bst is printed the only line that is outputted is the last one
Charles doesn't like bob.
Any help would really be appreciated.

Answer 1

Notice that when you create a node with newNode, you store a copy of the pointer passed into it, rather than a copy of the string being pointed at. This means that every time you insert a value into the tree, it stores a pointer to the str buffer in main. In other words, after you do your first insertion, things look like this:
+------------+
| BST Node | str
+------------+ +---+---+---+---+---+...+---+
| key | ---------> | b | o | b | | i | | 0 |
+------------+ +---+---+---+---+---+...+---+
When you then read the next line of the file, you're overwriting str with the contents of that line, so the picture looks like this:
+------------+
| BST Node | str
+------------+ +---+---+---+---+---+...+---+
| key | ---------> | d | a | v | i | d | | 0 |
+------------+ +---+---+---+---+---+...+---+
Notice that your BST now acts as though it contains "david is a new hire" even though you never inserted that value. As a result, when you try inserting "david is a new hire" into the BST, nothing happens.
The same thing happens for the next few reads, until eventually you read the final line of the file, when things look like this:
+------------+
| BST Node | str
+------------+ +---+---+---+---+---+...+---+
| key | ---------> | c | h | a | r | l | | 0 |
+------------+ +---+---+---+---+---+...+---+
This is why you're only seeing the line about Charlie at the end - the BST is directing you to the single shared copy of the buffer.
To fix this, make your BST store copies of the strings passed into it, or copy the strings before storing them in the tree. For example, you might have the newNode function call strdup to get its own copy of the string to store:
struct node *newNode(char* item)
{
struct node* temp = (struct node*)malloc(sizeof(struct node));
temp->key = strdup(item); // <--- here!
/* TODO: Error-handling! */
temp->left = NULL;
temp->right = NULL;
temp->count = 1;
return temp;
};
That should fix your issue. Just make sure to deallocate everything when you're done!