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 arr1 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.

Nick · Accepted Answer · 2017-03-18 17:39:59Z

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.).

answered Mar 18, 2017 at 17:39

Nick

6,8761 gold badge25 silver badges34 bronze badges

Sign up to request clarification or add additional context in comments.

3 Comments

Phaneeth Over a year ago

I see. Thanks for the answer. Appreciate it.

Phaneeth Over a year ago

Consider this example.

struct Node{      int data;      Node* next; };  int main(){    Node* test;    if(test == NULL){      cout << "test is null";    } }

So why in this case it is NULL

Nick Over a year ago

In that case, if it is NULL, it is by random chance (that is, it would not be necessarily be NULL as specified in the language). It may be more likely to be NULL because of stack usage semantics, but that's an aside. Really, this is why there's a large difference between someone who thinks something is correct because it's working, and someone who understands why something is or is not working. I suggest reading up on C++ more if you want to be in the latter category.

g_d · Accepted Answer · 2017-03-18 17:39:37Z

0

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. e.g. 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)

edited Mar 18, 2017 at 17:39

answered Mar 18, 2017 at 17:37

g_d

7078 silver badges20 bronze badges

1 Comment

Phaneeth Over a year ago

Have a problem with Node** arr not with the vector.

Collectives™ on Stack Overflow

Array of Nodes: Initialization

2 Answers 2

3 Comments

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

3 Comments

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related