c++ Struct pointer unable to read memory when initialised as NULL

Question

I'm creating a program that incorporates Binary search tree algorithm but I've run into a problem that I'm not sure how to solve. Heres the relevant bits of my code.

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

Thats my struct for the node.

void Insert_Rec(string word, node* ptr) {

if (ptr->data == "") {

    ptr->data = word;
    ptr->left = NULL;
    ptr->right = NULL;
    cout << "overwitten!" << endl;
}
else if (word < ptr->data) {
    if (ptr->left != NULL) {
        cout << "Recursing left!";
        Insert_Rec(word, ptr->left);
    }
    else {
        ptr->data = word;
        ptr->left = NULL;
        ptr->right = NULL;
        cout << "Inserted!";
    }
}

And the problem lies there, the program never enters if(ptr->left != NULL) statement. Looking at my visual studio debugger, the ptr->left shows "" instead of NULL. How can i solve this!? I've tried some of the other solutions here but they are either not relevant or just dont work!!

Answer 1

program never enters if(ptr->left != NULL) statement

well ptr->left starts out NULL, and you never assign anything else to it, so it will stay NULL forever.

if (ptr->left) {
    cout << "Recursing left!";
    Insert_Rec(word, ptr->left);
}
else {
    /* this just overwrites the existing node in-place
       but you should be creating a new node for the left child
    ptr->data = word;
    ptr->left = NULL;
    ptr->right = NULL;
    */
    ptr->left = new node{word, nullptr, nullptr};
    cout << "Inserted!";
}

There are a number of other issues with your code (Vlad-from-Moscow's answer shows a better design for this function, but you really need to fix it at the container class level), but this is the immediate blocker.

Answer 2

Your function implementation does not make sense in whole.

The function can be defined the following way

void Insert_Rec( node **head, const std::string &word ) 
{
    if ( *head == nullptr )
    {
        *head = new node { word, nullptr, nullptr };
    }
    else if ( word < ( *head )->data )
    {
        Insert_Rec( &( *head )->left, word );
    }
    else
    {
        Insert_Rec( &( *head )->right, word );
    }
}  

It is the function that should allocate a new node if it is required.

If you want that the BST would not contain duplicates then change the last else statement to else if statement like

else if ( ( *head )->data < word )

The function can be called the following way

node *head = nullptr;
//...
Insert_Rec( &head, "Hello" );

Also instead of the "double" pointer you can use a referenced type as the type of the first function parameter.

For example

void Insert_Rec( node * &head, const std::string &word ) 
{
    if ( head == nullptr )
    {
        head = new node { word, nullptr, nullptr };
    }
    else if ( word < head->data )
    {
        Insert_Rec( head->left, word );
    }
    else
    {
        Insert_Rec( head->right, word );
    }
} 

Answer 3

You've set node *left=NULL and node *right=NULL in struct. Remove that NULL. Leave only two pointers. Otherwise, it's just null. Anyway, keep in mind that very rarely you want to initialise the pointers in a struct.

struct node {
    // simple is better (-:
    string data;
    struct node *left;
    struct node *right;
};