C++ - Mathematical Calculations

/**************************
program title: Assaignment 3
Author: Kenneth Gardiner, 18013601
description. program runs functions to find the best resistor for a circuit
*****************************/
#include <iostream>
using namespace std;
double input();
double Calculate(double reqGain, double stdR[], double& R1, double& R2, double& R3, double& R4);
void Output(double reqGain, double ActGain, double& R1, double& R2, double& R3, double& R4);


int main()
{
	cout << "Voltage Divider" << endl;

	double R1, R2, R3, R4;
	double ActGain;
	double stdR[] = { 1.0, 1.2, 1.5, 1.8, 2.2, 2.7, 3.3, 3.9, 4.7, 5.6,
		6.8, 8.2, 10, 12, 15, 18, 22, 27, 33, 39, 47, 56, 68, 82, 100, 1000000000 };
	double reqGain = input();
	if (reqGain != 0)
	{
		ActGain = Calculate(reqGain, stdR, R1, R2, R3, R4);
		Output(reqGain, ActGain, R1, R2, R3, R4);
	}
	cout << "End Of Program" << endl;
	system("pause");
}





double input()
{
	double gain;
	bool error;
	do
	{
		cout << "Enter gain between 0 and 1 " << endl;
		cin >> gain;
		if (cin.fail())
		{
			cin.clear();
			cin.ignore(1000, '\n');
			error = true;
			cout << endl << "Input Error, enter a number" << endl;
		}
		else if (gain >= 0 && gain <= 1)
		{
			error = false;
		}
	} while (error == true);
	return gain;
}

double Calculate(double reqGain, double stdR[], double& R1, double& R2, double& R3, double& R4)
{
	int NUM_RES = 26;
	double error;
	double Total;
	double calcGain;
	double P1;
	double P2;

	double bestR1 = stdR[0];
	double bestR2 = stdR[0];
	double bestR3 = stdR[0];
	double bestR4 = stdR[0];
	P1 = (R1 * R2) / (R1 + R2);
	P2 = (R3 * R4) / (R3 + R4);
	double BestGain = P2 / (P1 + P2);
	double bestError = fabs(reqGain - BestGain);
	double BestTotal = P2 + P1;

	for (int i1 = 0; i1 < NUM_RES - 1; i1++)
	{
		R1 = stdR[i1];
		for (int i2 = i1; i2 < NUM_RES; i2++)
		{
			R2 = stdR[i2];
			for (int i3 = 0; i3 < NUM_RES - 1; i3++)
			{
				R3 = stdR[i3];
				for (int i4 = i3; i4 < NUM_RES; i4++)
				{
					R4 = stdR[i4];
					P1 = (R1 * R2) / (R1 + R2);
					P2 = (R3 * R4) / (R3 + R4);
					calcGain = P2 / (P1 + P2);
					Total = P1 + P2;
					error = fabs(reqGain - calcGain);

					if (error < bestError)
					{
						bestR1 = stdR[i1];
						bestR2 = stdR[i2];
						bestR3 = stdR[i3];
						bestR4 = stdR[i4];

						bestError = error;
						BestTotal = Total;
						BestGain = calcGain;
					}
					else if (error == bestError && Total > BestTotal)
					{
						bestR1 = stdR[i1];
						bestR2 = stdR[i2];
						bestR3 = stdR[i3];
						bestR4 = stdR[i4];

						bestError = error;
						BestTotal = Total;
						BestGain = calcGain;
					}
				}
			}
		}
	}
	R1 = bestR1;
	R2 = bestR2;
	R3 = bestR3;
	R4 = bestR4;
	return BestGain;
}

void Output(double reqGain, double ActGain, double& R1, double& R2, double& R3, double& R4)
{
	cout << "Required Gain = " << reqGain << endl;
	cout << "Actual Gain = " << ActGain << endl;
	cout << "Resistor 1 = " << R1 << " Kohms" << endl;
	cout << "Resistor 2 = " << R2 << " Kohms" << endl;
	cout << "Resistor 3 = " << R3 << " Kohms" << endl;
	cout << "Resistor 4 = " << R4 << " Kohms" << endl;



}