C++ Hash Chaining Function

Question

I am trying to build a Hash table and with what I have learned using online tutorials I have come up with the following code

#include <iostream>
#include <string>
#include <cstdlib>

using namespace std;
const int SIZE = 100;

int hash(string);

class Node
{
   public:
      Node();
      Node(string);
      int hash(int value);
   private:
      string data;
      Node* next;

   friend class HashTable;
};

Node::Node() {
  data = "";
  next = NULL;
}

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

int Node::hash(int value) {
   int y;
   y = value % SIZE;
}

class HashTable {
public:
    HashTable();
      HashTable(int);
      ~HashTable();
      void insertItem(string);
      bool retrieveItem(string);

private:
      Node* ht;
};

HashTable::HashTable() {
  ht = new Node[SIZE];
}

HashTable::HashTable(int max) {
  ht = new Node[max];
}

HashTable::~HashTable() {
   delete[] ht;
}

void HashTable::insertItem(string name) {
    int val = hash(name);
    for (int i = 0; i < name.length(); i++)
        val += name[i];
}

bool HashTable::retrieveItem(string name) {
    int val = hash(name);
    if (val == 0 ) {
        cout << val << " Not Found " << endl;
    }
    else {
        cout << val << "\t" << ht->data << endl;
    }
}

void print () {
    //Print Hash Table with all Values
}

int main() {

   HashTable ht;
   ht.insertItem("Allen");
   ht.insertItem("Tom");

   ht.retrieveItem("Allen");

   //data.hash(int val);
   //cout << ht;

   system("pause");
   return 0;
}


int hash(string val) {
    int key;
    key = val % SIZE;
}

I am trying to insert string values and validate if the name exists using the retrieveItem function. Also, how do I go about printing the HashTable with values.

Any help will be highly appreciated.

Vish

That my code does not work! What part of the code is wrong? Or if the entire code is wrong, what is the solution? — Vish, May 13 '12 at 17:04
There are a lot of things wrong; how do you know how big the array is if you have two constructors and don't save the length? Why are you trying to use modulus on a string? You're not actually inserting anything in `insertItem` and not returning anything in `retrieveItem`. — Seth Carnegie, May 13 '12 at 17:07
i want to use modulo to get the ascii value of the string and use ascii total % size = index position in table and store the string there. e.g. for string "Allen" total ascii value is 476 % 101 (table size) gives me 76 as the position in the index table to store the data. — Vish, May 13 '12 at 17:13
Steer towards a method signature like `const std::string&` in preference to `string`. [`using namespace std` is something you'll want to avoid](https://stackoverflow.com/questions/1452721/why-is-using-namespace-std-considered-bad-practice), it can cause a lot of confusion and conflict. Passing by `const` reference avoids duplicating the argument and accidental mutations. — tadman, Dec 30 '18 at 21:21

Jerry Coffin · Answer 1 · 2012-05-13T17:32:36.327

There's quite a bit wrong here. Just glancing at a random spot in the middle of the code, I found:

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

This doesn't save the value of the string that was passed in. I'm guessing you really wanted something more like:

Node::Node(string value) : data(value), next(NULL) {}

The next thing I saw was:

int Node::hash(int value) {
   int y;
   y = value % SIZE;
}

This computes a value and puts it into y (a local variable) but doesn't return anything, so the value you've computed is immediately lost. If you attempt to use its return value, you get undefined behavior (actually, I don't remember for sure -- it seems like in C you get UB only if you try to use the return value, but in C++ it's UB to do this at all -- regardless of the technical details, though, it's clearly not what you want and needs to be fixed).

Like I said, I just glanced at one spot in the code and found though -- I doubt they're the only problems. The bottom line is that you probably need to back up a bit, figure out what each function is supposed to do, and go through them one by one and ensure each is doing what it's supposed to.

One other bit of advice: instead of dealing with dynamically allocating an array on your own, I'd used std::vector.

Thanks Jerry! I will look into the functions. I am just starting with C++ so learning as i go... — Vish, May 13 '12 at 17:30

score 0 · Answer 2 · answered May 13 '12 at 18:46

0

just use stl hashmap or for threadsafe hashmap use intel thread building blocks library

answered May 13 '12 at 18:46

Chuck Norrris

284
5
12

Thanks Kay! But I am a beginner so don't know what you mean ;). I am trying to do this using linked list and hash chaining – Vish May 13 '12 at 19:50

C++ Hash Chaining Function

2 Answers2