gauss backward

#include<bits/stdc++.h>
using namespace std;
double x[100];
double y[100][100];
double find_p(double x1,int n)
{
    double temp;
    int k=1,s=4;
    if(n==0)
    {
        return 1;
    }
    for(int j=1; j<=n; j++)
    {
        if(j==1)
        {
            temp=x1;
            continue;
        }
        if(j==s)
        {
            s=s+2;
            if(j%2==0)
                k++;
        }
        if(j%2==0)
        {
            temp=temp*(x1+k);

        }
        else
        {
            temp=temp*(x1-k);

        }

    }

    return temp;
}


int fact(int i)
{
    int f=1;
    for(int j=i; j>=1; j--)
    {
        f*=j;

    }
    return f;
}
int main()
{
    int n,p;
    int q;
    cout<<"enter the total element of x and y:";
    cin>>n;
    for(int i=0; i<n; i++)
    {
        cout<<"enter the element of x:";
        cin>>x[i];
        cout<<"enter the element of y[][]:";
        cin>>y[i][0];

    }
    for(int i=1; i<n; i++)
    {
        for(int j=0; j<n-i; j++)
        {
            y[j][i]=y[j+1][i-1]-y[j][i-1];

        }
    }
    cout<<"difference table:"<<endl;
    for(int i=0; i<n; i++)
    {
        cout<<x[i]<<"\t";
        for(int j=0; j<n-i; j++)
        {
            cout<<y[i][j]<<"\t";
        }
        cout<<endl;
    }
    if(n%2==0)
    {
        p=n/2;
        q=x[p];
    }
    else
    {
        p=n/2;
        q=x[p];
    }
    double value,x1;
    cout<<"enter desired value:";
    cin>>value;
    x1=(value-q)/(x[1]-x[0]);
    double sum=0;
    int k=1;
    cout<<q<<endl;

    for(int i=0; i<n; i++)
    {
        if(i==1)
        {
            p--;
        }
        if(i==k+2)
        {
            k=k+2;
            p--;

        }

        sum=sum+(find_p(x1,i)*y[p][i])/fact(i);


    }
    cout<<"desired y="<<sum;


}