Double Hashing

Well, I had to make a program which used 2 hashing functions on keys that it gets from a file and stores them on a table.

The first function should let the user define M (uisng function h(k) = k mod M) and then applies a second hashing function which is predefined (in this case, h2(k) = 8 - k mod 8).

The following code that I whipped up seems to work fine, but this is however a kind of "rough draft" that I made. Hashing is a completely new concept to me and I am curious if I am doing it right (the first hash function gives me the same results as examples given in my book, but the book doesnt have any double hashing examples).

Also, I am just looking for areas where I could improve. I am considering changing the arrays to linked lists, or perhaps even binary trees, but only if that alternative would prove more efficient.

Any other ideas are welcomed.

Code:

#include<iostream>
#include<cmath>
#include<cctype>
#include<fstream>
using namespace std;

int first_hash(int a[], int i, int x, int M)
	{
	if (i > -1) 
		return x = (a[i] + 32 * first_hash(a, i-1, x, M)) % M;
	else
		return (0);
	}

int second_hash(int a[], int i, int x)
	{
	if (i > -1)
		{
		x = (a[i] + 32 * second_hash(a, i-1, x)) % 8;
		return 8 - x;
		}
	else
		return (0);
		
	}
int do_thehash(char a[], int c, int M)
	{
	char d;
	int h = 0, tmp[20];
	for (int i = 0; a[i]; i++, h++)  // reads each char and converts to a corresponding int (A = 1, B = 2, etc)
		{
		d = a[i];       
		if(isupper(d))
			tmp[h] = int(d) - 64;
		else
		        tmp[h] = int(d) - 96;
		}
		h = h - 1; 
		c = first_hash(tmp, h, c, M) + second_hash(tmp, h, c);
		return c;			
	}

void table(int a[], int c, int i)
	{
	a[i] = c;
	i++;
	}
void sort_table(int a[], int x)
	{
	int j = 0, tmp = 0;
	for (int i = 1; i < x; i++)
	    {
	    tmp = a[i]; j = i;
	    while(a[j-1] > tmp)
	    	{
	    	a[j] = a[j -1]; j--;
	    	}
	    a[j] = tmp;
	    }
	}

int main(void)
{
	char input[20];
	int c= 0, M, x = 0, hash_table[100];
	
	/**** File stuf(will move later) ****/
	char ofilename[16], ifilename[16];
	ofstream write;
	ifstream read;

	cout << "Enter an input file: ";
	cin >> ifilename;

	read.open(ifilename);
	if(read.fail())
		{
		cout << "That file does not exist!" << endl;
		exit(1);
		}

	cout << "Enter a value for M: ";
	cin >> M;  
	
	while (read >> input)
		{
		c = do_thehash(input, c, M); 
		table(hash_table, c, x);
		cout << c << endl;
		x++;
		}
	
	
 	sort_table(hash_table, x);

	cout << "Enter an output file to write to: ";
	cin >> ofilename;
	write.open(ofilename);
	int j = 0;
	for(int i = 0; j < x; i++)
		{
		if (i == (hash_table[j]) - 1) 
		{
			cout << 'x' << " -- " << x << " -- "<< hash_table[j] << endl;
			write <<'x'<< endl;
			j++;
		}
		else
			write <<  endl;
		} 
	
	write.close();
	cout << "File written" << endl;

	return 0;
}

you forgot the [/code] at the end...added if for you.