Array of Nodes: Initialization

Question

#include <iostream>
#include <vector>
#include <string>
#include <math.h>

using namespace std;

struct Node{
    string data;
    Node* next;
    Node(){
        data = "";
        next = NULL;    
    }       
};

int computeHash(string s, int m){
    int p = 1000000007;
    int x = 263;
    unsigned long long sum = 0;
    unsigned long long val = 0;
    for(int i = 0; i < s.length(); i++){
        val = pow(x, i);
        sum = (sum + s[i] * val) % p;   
    }
    sum = sum % m;
    return sum;
}

int main(){
    int buckets;
    cin >> buckets;
    int n;
    cin >> n;
    string tag;
    string s;
    vector< vector<string> > myStore(n);
    for(int i = 0; i < n; i++){
        cin >> s;
        myStore.at(i).push_back(s);
        cin >> tag;
        myStore.at(i).push_back(tag);   
    }
    Node** arr= new Node*[buckets];
    for(int i = 0; i < n; i++){
        if(!myStore[i][0].compare("add")){
            s = myStore[i][1];
            int hash = computeHash(s,buckets);
            cout << hash << endl;
        }

    }

    return 0;   
}

I am trying to write a program to implement hashing with chains. I am trying to create an array of nodes so that I can append if two strings have the same hash value.

But I am having a problem with the initialization of array of Nodes. I thought the nodes in the array would be pointing to NULL. But when I tried to debug in gdb it is showing some other thing.

Can someone explain where I am wrong on comment about this behavior. Why arr 1 and arr[2] are pointing to some memory location instead of null. I also tried to remove the default constructor but still getting the same results. Any help would be appreciated.

Answer 1

You're allocating an array of pointers. Pointers don't have constructors, or default initialization; you're getting random memory (from the allocation).

If you want the array to be NULL-ed out, you need to do so yourself (eg: memcpy, etc.).

Answer 2

You have initialized vector of size n of vectors of size 0 of strings. Then you want to get the '[1]' (the second element of empty vector of strings) You have to inutialize them separately. eg in "for" cycle.

Updated. Use myStore.at(i).at(1) instead of myStore[i][1] to achieve checking of boundary conditions. (Try it, you will understand that the problem with vector indeed)