Gauss - Seidel Method

/// 1807062

#include<bits/stdc++.h>
#include<string.h>
using namespace std;

#define see(x)  cout<<endl<<#x<<" : "<<(x)<<endl;

double a[15];

double get_X(double y,double z)
{
    return (a[4]-a[2]*y-a[3]*z)/a[1];
}

double get_Y(double x,double z)
{
    return (a[8]-a[5]*x-a[7]*z)/a[6];
}

double get_Z(double x,double y)
{
    return (a[12]-a[9]*x-a[10]*y)/a[11];
}

int index = 1;

double get_Values(string s)
{
    int i = 0, to = 0;

    while(s[i]!='x')i++;

    double power = 0, value = 0;
    if(s[0]=='-')to = 1;
    if(i==0)value = 1;
    for(int  j = i-1;j>=to;j--)
        value += pow(10.0,power++) * (s[j]-'0');
    if(to) value = -value;

    a[index++] = value;

    i++; /// '+' or '-'
    char cy = s[i]; /// sign of the value
    while(s[i]!='y')i++;

    power = 0,value = 0;
    int j = i-1;
    if(s[j]==cy)value=1;
    while(1)
    {
        if(s[j]=='+' || s[j]=='-')break;
        value += pow(10.0,power++) * (s[j]-'0'),j--;
    }
    if(cy=='-')value = -value;

    a[index++] = value;

    i++; /// '+' or '-'
    char cz = s[i];
    while(s[i]!='z')i++;

    power = 0,value = 0;
    j = i-1;
    if(s[j]==cz)value=1;
    while(1)
    {
        if(s[j]=='+' || s[j]=='-')break;
        value += pow(10.0,power++) * (s[j]-'0'),j--;
    }
    if(cz=='-')value = -value;

    a[index++] = value;

    i++; /// '='
    to = i;
    power = 0,value = 0;
    if(s[i+1]=='-') to++;
    for(int j = s.size()-1;j>to;j--)
        value += pow(10.0,power++) * (s[j]-'0');
    if(to!=i)value = -value;

    a[index++] = value;
}

double Gauss_Seidel()
{
    double X = 0, Y = 0, Z = 0;

    for(int it = 0; it< 30; it++)
    {
        printf("Iteration No. %d : ",it+1);
        X = get_X(Y,Z);
        Y = get_Y(X,Z);
        Z = get_Z(X,Y);
        printf("X = %lf, Y = %lf, Z = %lf\n",X,Y,Z);
    }
    printf("\nAnswer : \nX = %lf, Y = %lf, Z = %lf\n",X,Y,Z);
}

int main()
{
//    ios::sync_with_stdio(0);
//    cin.tie(NULL);

    ifstream in("input.txt");

    string s;

    puts("The three equations are : ");

    while(in>>s)
    {
        get_Values(s);
        cout<<s<<endl;
    }

    puts("The constants are : ");

    cout<<"a1 = "<<a[1]<<endl;
    cout<<"b1 = "<<a[2]<<endl;
    cout<<"c1 = "<<a[3]<<endl;
    cout<<"d1 = "<<a[4]<<endl<<endl;

    cout<<"a2 = "<<a[5]<<endl;
    cout<<"b2 = "<<a[6]<<endl;
    cout<<"c2 = "<<a[7]<<endl;
    cout<<"d2 = "<<a[8]<<endl<<endl;

    cout<<"a3 = "<<a[9]<<endl;
    cout<<"b3 = "<<a[10]<<endl;
    cout<<"c3 = "<<a[11]<<endl;
    cout<<"d3 = "<<a[12]<<endl<<endl;

    Gauss_Seidel();

    in.close();

    return 0;
}

/*
27x+6y-z=85
6x+15y+z=72
x+y+54z=110

-7x+5y-3z=16
-3x-5y+2z=-8
-5x+3y-7z=0
*/