Creating a hashtable using vectors of vectors? - c++

Question

I'm currently trying to write a program that creates a hash table, using vectors of vectors for my collision resolution method.
The problem I am facing is that during runtime, a vector of vectors is created, but all of the Entry vectors inside remain of size 0. I know my put functions are faulty but I don't know where/why.
This is my first time creating a hash table and I'd appreciate any assistance in what the problem might be. My goal is to create a vector of Entry vectors, and each Entry has its associated key and value. After finding the hash value for a new Entry key, it should check the Entry vectors' key values to see if the key already exists. If it does, it updates that key's value.
This is a segment of table.cpp:
Table::Table(unsigned int maximumEntries) : maxEntries(100){
this->maxEntries = maximumEntries;
this->Tsize = 2*maxEntries;
}
Table::Table(unsigned int entries, std::istream& input){ //do not input more than the specified number of entries.
this->maxEntries = entries;
this->Tsize = 2*maxEntries;
std::string line = "";
int numEntries = 0;
getline(input, line);
while(numEntries<maxEntries || input.eof()){ // reads to entries or end of file
int key;
std::string strData = "";
convertToValues(key, strData, line);
put(key, strData); // adds each of the values to the tab;e
numEntries++;
getline(input,line);
}
}
void Table::put(unsigned int key, std::string data){
Entry newEntryObj(key,data); //create a new Entry obj
put(newEntryObj);
}
void Table::put(Entry e){ // creating the hash table
assert(currNumEntries < maxEntries);
int hash = (e.get_key() % Tsize);
Entry newEntry = Entry(e.get_key(), e.get_data());
for(int i = 0; i < hashtable[hash].size(); i++){
if (e.get_key() == hashtable[hash][i].get_key()){
hashtable[hash][i].set_data(e.get_data());
}
else{
hashtable[hash].push_back(newEntry); // IF KEY DOESNT EXIST, ADD TO THE VECTOR
}
}
}
This is Table.h
#ifndef table_h
#define table_h
#include "entry.h"
#include <string>
#include <istream>
#include <fstream>
#include <iostream>
#include <vector>
class Table{
public:
Table(unsigned int max_entries = 100); //Builds empty table with maxEntry value
Table(unsigned int entries, std::istream& input); //Builds table designed to hold number of entires
void put(unsigned int key, std::string data); //creates a new Entry to put in
void put(Entry e); //puts COPY of entry into the table
std::string get(unsigned int key) const; //returns string associated w/ param, "" if no entry exists
bool remove(unsigned int key); //removes Entry containing the given key
friend std::ostream& operator<< (std::ostream& out, const Table& t); //overloads << operator to PRINT the table.
int getSize();
std::vector<std::vector<Entry>> getHashtable();
private:
std::vector<std::vector<Entry>> hashtable; //vector of vectors
int Tsize; //size of table equal to twice the max number of entries
int maxEntries;
int currNumEntries;
#endif /* table_h */
};
and Entry.h:
#include <string>
#include <iosfwd>
class Entry {
public:
// constructor
Entry(unsigned int key = 0, std::string data = "");
// access and mutator functions
unsigned int get_key() const;
std::string get_data() const;
static unsigned int access_count();
void set_key(unsigned int k);
void set_data(std::string d);
// operator conversion function simplifies comparisons
operator unsigned int () const;
// input and output friends
friend std::istream& operator>>
(std::istream& inp, Entry &e);
friend std::ostream& operator<<
(std::ostream& out, Entry &e);
private:
unsigned int key;
std::string data;
static unsigned int accesses;
};

Answer 1

There are various problems with your code, but the answer for your question would be this:
In
void Table::put(Entry e){ // creating the hash table
Have a look at the loop.
for(int i = 0; i < hashtable[hash].size(); i++){
Now, hashtable[hash] is a vector. But initially it doesn't have any elements. So hashtable[hash].size() is 0. So you don't enter the loop.
On top of this, trying to access hashtable[hash] in the first place results in undefined behaviour due to hashtable not being properly resized to Tsize. Try this in your constructor(s):
this->maxEntries = maximumEntries;
this->Tsize = 2*maxEntries;
this->hashtable.resize(this->Tsize);
EDIT:
It would be easier for you to understand if you use std::vector::at function instead of std::vector::operator[]. For example:
void Table::put(Entry e){ // creating the hash table
assert(currNumEntries < maxEntries);
int hash = (e.get_key() % Tsize);
Entry newEntry = Entry(e.get_key(), e.get_data());
for(int i = 0; i < hashtabl.at(hash).size(); i++){
if (e.get_key() == hashtable.at(hash).at(i).get_key()){
hashtable.at(hash).at(i).set_data(e.get_data());
}
else{
hashtable.at(hash).push_back(newEntry); // IF KEY DOESNT EXIST, ADD TO THE VECTOR
}
}
}
Without resizing hashtable, this code would throw an out_of_range exception when you try to do hashtable.at(hash) the first time.
P.S. None of this is tested.