Thread: File I/O - Hash files

I'm having problems with the basics of binary file i/o. I really don't know too much about file i/o so far, so I'll try to describe my question as best I can.

What I'm trying to do is to create a simple hash file with 7 buckets while each bucket holds 2 entries and utilize them. So far, I been able to create a blank table, but I haven't been able to write to any of the buckets, at least in the way that I want to. I find the correct offset but the blank table gets wiped up until the point where I want to write and after it gets removed. I checked this with a hex editor.

So, what I think i want to ask is - how can I open a existing file to modify in binary? or how do i change one of the entries in the blank hash table/file?

I think what I've put in bold below is what has been causing my problems. I think im lost : /

thanks for any insight/advice.

(most of this is incomplete)

funcs.cpp

Code:

#include "func.h"
#include <iostream>
#include <fstream>
using namespace std;

HashFile::HashFile(int n, int r)
{

	tableSize = n;
	bucketSize = r;
	
	/******************* initialize table assist *********/
	tableAssist = new int[tableSize];
	for (int j = 0; j < tableSize; j++)
	{
		tableAssist[j] = 0;
	}

	/**** create Table size * bucket size blank entries ****/
	DataRec tempDataRec;
	tempDataRec.id = 0;
	ofstream outfile("Bout", ios::out);
	for (int i = 0; i < tableSize * bucketSize; i++)
	{
		outfile.seekp( i * sizeof(DataRec) );
		outfile.write( (char *) &tempDataRec, sizeof(DataRec) );
	}
	outfile.close();
}

int HashFile::store(const DataRec &data_rec)
{

	
	ofstream outfile;
	outfile.open("Bout", ios::out);

	int temp = h(data_rec.id);

	if (tableAssist[temp] == 0)
	{
		outfile.seekp( temp * sizeof(DataRec) );
		outfile.write( (char *) &data_rec, sizeof(DataRec) );
		tableAssist[temp]++;
		return 0;
	}
	if (tableAssist[temp] == 1)
	{
		outfile.seekp( (temp+1) * sizeof(DataRec) );
		outfile.write( (char *) &data_rec, sizeof(DataRec) );
		tableAssist[temp]++;
		return 0;
	}
	if (tableAssist[temp] == 2) //h2 - double hash
	{
		temp = h2(data_rec.id);
	}
	
/*
*/

	return -1;
}


int HashFile::h(int k)
{
	return (k % tableSize);
}

int HashFile::h2(int k)
{
	return (k % (tableSize - 1) + 1);
}

main.cpp

Code:

#include <iostream>
#include "func.h"
using namespace std;

int main()
{
	DataRec temp;
	temp.id = 1;
	HashFile hashobj(7, 2);
	return 0;
}

func.h

Code:

#ifndef FUNC_H
#define FUNC_H
#include <fstream>
//using namespace std;

struct DataRec
{
	int id;
	char name[20];
};

class HashFile
{
public:
	HashFile(int, int);
	int h(int);
	int h2(int);
	int store(const DataRec&);
	int retrieve(DataRec&);
private:
	int tableSize;
	int bucketSize;
	int * tableAssist;
};
#endif