Question

So I’m trying to create my own BST for a spellchecker because I want to additional functionality (find nearby nodes for suggestions). Anyways, I can create the root node, but after that it doesn’t work. For example, if I have the following code:

BST myTree;
myTree.add("rootElement");
myTree.add("abcChild");

If I make root (node *root) public and check for myTree.root->left !=NULL || myTree.root->right != NULL, I get a seg fault. I don’t understand. Here’s some of my code:

struct node {
  string data;
  node *left;
  node *right;
};


void BST::add(string newData)
{
  //Find a position                                                                                                                         
  if (root == NULL){
    root = new node;
    root->data = newData;
    root->left = NULL;
    root->right = NULL;

  }
  else{ //remember to include string                                                                                                        

    if(root->data.compare(newData) < 0){

      // Add to left                                                                                                                        

      addRecursive(root->left, newData);

    }
    else{
      // Add to right                                                                                                                       
      addRecursive(root->right, newData);
    }
  }
}


void BST::addRecursive(node *currentNode, string newData)
{
  if (currentNode == NULL){

    currentNode = new node;
    currentNode->data = newData;
    currentNode->left = NULL;
    currentNode->right = NULL;

  }
  else{ //remember to include string                                                                                                  

    if(currentNode->data.compare(newData) < 0){

      // Add to left                                                                                                                  

      addRecursive(currentNode->left, newData);

    }
    else{
      // Add to right                                                                                                                 
      addRecursive(currentNode->right, newData);
    }
  }
}

What’s the deal?

Answer 1

In add, when you do

root = new node;

root is a class variable, so that’s not a problem and is the correct way to do it. However, in addRecursive, when you are doing

currentNode = new node;

currentNode is a pointer that is passed by value to your function, so you are only making the local variable currentNode point to another place in memory. You need to pass the pointers by reference so that when you modify the parameter, it modifies the original instead of just the local variable. Just make the signature of your function addRecursive to void addRecursive(node*& currentNode, const string& newData). This will make the pointers be passed to the function by reference.

Also notice that I changed string newData to const string& newData. That is so you avoid making a copy of the string in memory every time you call the function. You should make that change in all of your functions when you don’t need to modify a copy of the string passed to the function, to improve efficiency.