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:

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#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;
}

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