Is there a way to fix the segmentation fault ? I can't seem to figure it out

Question

I'm not sure why I'm getting a segmentation fault in my code?

 class Node {
    public:
        int data;
        Node *next;
};

void push_back(Node** head_ref, int new_data) {  
    Node* new_node = new Node(); 
  
    Node *last = *head_ref;
    new_node->data = new_data;  
    new_node->next = NULL;  

    if (*head_ref == NULL) {  
        *head_ref = new_node;  
        return;  
    }  
  
    while (last->next != NULL)  
        last = last->next;  
  
    last->next = new_node;
    
    return;  
}  

int getHash(string initials) {
    string digits;
    
    for (int i = 0; i < 3; i++) {
        switch(initials[i]) {
            case 'A':
            case 'B':
            case 'C':
                digits += "2";
                break;
            case 'D':
            case 'E':
            case 'F':
                digits += "3";
                break;
            case 'G':
            case 'H':
            case 'I':
                digits += "4";
                break;
            case 'J':
            case 'K':
            case 'L':
                digits += "5";
                break;
            case 'M':
            case 'N':
            case 'O':
                digits += "6";
                break;
            case 'P':
            case 'Q':
            case 'R':
            case 'S':
                digits += "7";
                break;
            case 'T':
            case 'U':
            case 'V':
                digits += "8";
                break;
            case 'W':
            case 'X':
            case 'Y':
            case 'Z':
                digits += "9";
                break;
        }
    }
    
    return stoi(digits);
}

int getSize (Node* head) {
    int size = 0;
    bool not_null = true;
    Node *current = head;
    
    while (not_null) {
        if (current->next == NULL) {
            not_null = false;
            return size;
        } else {
            size++;
            current = current->next;
        }
    }
}

int main() {
    int count = 512;
    string initials;
    map<int, int> stats;
    Node* bitvec[count];
    for ( int i = 0; i < count; i++ ) {
        Node *data_node = new Node;
        data_node->data = 1;
        bitvec[i] = data_node;
    }

    for ( int i = 0; i < count; i++ ) {
        for (int j = 0; j < 3; j++) 
            initials.push_back((char)(rand() % 26 + 65));
        
        int hash = getHash(initials);
        
        cout << initials << " - " << hash << " - " << hash % count << endl;
        //cout <<"-"<< count<< endl;
        push_back(&(bitvec[hash % count]), 0);
        
        initials = "";
    }

    cout << "collision array" << endl;

    for ( int i = 0; i < count; i++ ) {
            if ( stats.count( getSize(bitvec[i]) ) <= 0 ) {
                    stats[ getSize(bitvec[i]) ] = 1;
            } else {
                    stats[ getSize(bitvec[i]) ]++;
            }
            cout << setw( 2 ) << getSize(bitvec[i]) << " ";
            if ( ( i + 1 ) % 25 == 0 ) {
                    cout << endl;
            }
    }

    int total = 0;
    cout << endl;
    for ( map<int, int>::iterator it = stats.begin( ); it != stats.end( ); it++ ) {
            if ( it->first != 0 ) {
                    total += it->second;
            }
            cout << it->first << " collisions occurred #" << it->second << endl;
    }

    cout << "Max number of collisions is: " << ( --stats.end( ) )->first << endl;
    cout << "Total collisions: " << total << endl;
    float avg = (float) count / total;
    cout << "average search: " << avg << endl;
  
    return 0;
}

Answer 1

In main

for ( int i = 0; i < count; i++ ) {
    Node *data_node = new Node;
    data_node->data = 1;
    // whoops! Didn't set data_node->next = NULL.
    bitvec[i] = data_node;
}

This mistake means the while (last->next != NULL) and similar tests later in the code fail. This means my comment under the question nailed the location of the bug, but got the cause wrong. Such is life. Should have taken me up on the bet.

This can be simplified to

for ( int i = 0; i < count; i++ ) {
    bitvec[i] = new Node{1, NULL};
}

Which uses Aggregate Initialization to make sure everything is initialized.

This prevents the program from crashing. I have not checked to see if the output is correct.