Internal Rate of Return (IRR) Calculation

/*===========================================================================
* Title : Internal Rate of Return (IRR) calculation
*
* Author : Vivek Shankar Skandappan (vivekshankars@gmail.com)
*
* Reference : Wikipedia for the IRR formula
*
* Version : 1.0
*
=============================================================================*/

/*  Complile as cc -lm irr.c */

#include <stdio.h>
#include <math.h>

/* Max rate variation considered is from 1% to 50% */
#define LOW_RATE 0.01
#define HIGH_RATE 0.5
#define MAX_ITERATION 1000
#define PRECISION_REQ 0.00000001

double computeIRR(double cf[], int numOfFlows)
{
	int i = 0,j = 0;
	double m = 0.0;
	double old = 0.00;
	double new = 0.00;

	double oldguessRate = LOW_RATE;
	double newguessRate = LOW_RATE;
	double guessRate = LOW_RATE;
	double lowGuessRate = LOW_RATE;
	double highGuessRate = HIGH_RATE;
	double npv = 0.0;
	double denom = 0.0;

	for(i=0; i<MAX_ITERATION; i++)
	{
		npv = 0.00;
		for(j=0; j<numOfFlows; j++)
		{
			denom = pow((1 + guessRate),j);
			npv = npv + (cf[j]/denom);
		}
		//printf("\n NPV for guessRate: %f : %f",guessRate, npv);

		/* Stop checking once the required precision is achieved */
		if((npv > 0) && (npv < PRECISION_REQ))
			break;

		if(old == 0)
			old = npv;
		else
			old = new;

		new = npv;

		if(i > 0)
		{
			if(old < new)
			{
				if(old < 0 && new < 0)
					highGuessRate = newguessRate;
				else
					lowGuessRate = newguessRate;
			}
			else
			{
				if(old > 0 && new > 0)
					lowGuessRate = newguessRate;
				else
					highGuessRate = newguessRate;
			}
		}

		oldguessRate = guessRate;
		guessRate = (lowGuessRate + highGuessRate) / 2;
		newguessRate = guessRate;

		//printf("\n Rate: Old %f New %f ... NPV: Old %f New %f ... Low %f High %f", oldguessRate, newguessRate, old, new, lowGuessRate, highGuessRate);
	}

	printf("\n FINAL NPV after %d iterations = %.8f", i, guessRate);
	printf("\n FINAL NPV = %.6f", (guessRate*1200));
	return guessRate;
}

int main()
{
	//Cash flows
	double cf[30], irr = 0.00;

	int numOfFlows = 7;

	cf[0] = -70000;
	cf[1] = 12000;
	cf[2] = 15000;
	cf[3] = 18000;
	cf[4] = 21000;
	cf[5] = 26000;
	numOfFlows = 6;

	irr = computeIRR(cf, numOfFlows);
	printf("\nFinal IRR: %.8f", irr);
}