Display() resets the entire array C++ - c++

Question

I tried creating a hash map storing its inputs as linked list nodes using separated chaining. The first display function gives a desirable output, but the next one resets the entire array to empty nullptr. I used the same class object so shouldn't it give the same result each time? Was it because of the destructor somehow? I thought it may be because I inserted a new item so I deleted it and the same thing still persists. My only suspect is the display() function but please point out if the problem comes from somewhere else.
Sorry if the post is so long, I want to make sure everyone can see the entire code to spot the problem.
#include <iostream>
#include <string>
#include "C:\Users\admin\source\repos\hash-library-master\sha3.cpp"
using namespace std;
const int TABLE_SIZE = 11;
struct HashNode
{
string key;
string value;
HashNode* next;
};
class HashMap {
private:
HashNode **table;
public:
//each element of table will be a root pointer to their respective chain
HashMap() {
table = new HashNode*[TABLE_SIZE];
for (int i = 0; i < TABLE_SIZE; i++)
{
table[i] = nullptr;
}
}
//hashing algorithm using SHA3 (courtesy of Stephan Brumme)
string hashFunc(string input)
{
string key;
SHA3 sha3;
key = sha3(input);
return key;
}
//insert new node
void insert(string key, string value)
{
//using hashing function to calculate hash index from string variable key
int hash = 0;
for (int a = 0; a < key.length(); ++a)
hash += key[a];
hash = hash % TABLE_SIZE;
//create new node to store data
HashNode* newNode = new HashNode;
newNode->value = value;
newNode->key = key;
newNode->next = nullptr;
//check and insert new node to front of line
if (table[hash] == nullptr)
table[hash] = newNode;
else
{
newNode->next = table[hash]->next;
table[hash]->next = newNode;
}
}
void display()
{
for (int i = 0; i < TABLE_SIZE; ++i)
{
if (table[i] == nullptr)
cout << i << " NULL" << endl;
else
{
while (table[i] != nullptr)
{
cout << i << " " << table[i]->value << "; ";
table[i] = table[i]->next;
if (table[i] == nullptr)
cout << "(end of chain)" << endl;
}
}
}
}
~HashMap() {
for (int i = 0; i < TABLE_SIZE; i++)
if (table[i] != NULL)
delete table[i];
delete[] table;
}
};
Driver
#include <iostream>
#include <string>
#include "hashMap.h"
using namespace std;
int main()
{
HashMap obj;
//test insert
obj.insert("5", "3100 Main St, Houston TX ");
obj.insert("5", "2200 Hayes Rd, Austin TX");
obj.insert("226", "1775 West Airport St, San Antonio TX");
obj.insert("273", "3322 Walnut Bend, Houston TX");
obj.insert("491", "5778 Alabama, Waco TX");
obj.insert("94", "3333 New St, Paris TX");
obj.display(); //resolved
cout << endl << endl;
//testing new hashing algorithm
string input, key;
cout << "Please enter any new address you want to store: ";
cin >> input;
key = obj.hashFunc(input); //create hash key
obj.insert(key, input);
obj.display(); //resets the array somehow
return 0;
}
Output
0 1775 West Airport St, San Antonio TX; (end of chain)
1 NULL
2 3322 Walnut Bend, Houston TX; (end of chain)
3 NULL
4 5778 Alabama, Waco TX; (end of chain)
5 NULL
6 NULL
7 NULL
8 NULL
9 3100 Main St, Houston TX ; 9 2200 Hayes Rd, Austin TX; (end of chain)
10 3333 New St, Paris TX; (end of chain)
//where display() resets
Please enter any new address you want to store: ewrewrw
0 NULL
1 ewrewrw; (end of chain)
2 NULL
3 NULL
4 NULL
5 NULL
6 NULL
7 NULL
8 NULL
9 NULL
10 NULL

Answer 1

Make all member functions that are not supposed to change the object itself const. This ensures that the function can be used when the object is used in a const context and will also enable the compiler to help you if you make a mistake. It will give you compilation errors if you try modifying the object in the function and will therefore complain about the line table[i] = table[i]->next; where you make changes to the object in your current code.
So start by making the function const and fix the errors. With the fixes in place it could look something like this:
void display() const // const added
{
for (int i = 0; i < TABLE_SIZE; ++i)
{
if (table[i] == nullptr)
cout << i << " NULL" << endl;
else
{
// using a temporary pointer, ptr, to go through the list
for(HashNode* ptr = table[i]; ptr != nullptr; ptr = ptr->next)
{
cout << i << " " << ptr->value << "; ";
}
cout << "(end of chain)" << endl;
}
}
}

Answer 2

Your display function is setting all elements of table to nullptr by looping until they become nullptr.
You should use another pointer variable for iterating to avoid this destruction.
void display()
{
for (int i = 0; i < TABLE_SIZE; ++i)
{
if (table[i] == nullptr)
cout << i << " NULL" << endl;
else
{
HashNode *p = table[i]; // another pointer variable for iterating
while (p != nullptr)
{
cout << i << " " << p->value << "; ";
p = p->next;
if (p == nullptr)
cout << "(end of chain)" << endl;
}
}
}
}