Having some trouble with some algorithms

when the purchase price is entered all the ouput is correct now except i have two problems.

one like i said above is trying to figure out how to charge an interest of 12 percent anually the other is when the program runs through it doesnt finish when newBalance is zero it ends in the negatives

import javax.swing.*;

import BreezySwing.*;

public class GUICreditPlan extends GBFrame {

private JLabel purchasePriceLabel;

private DoubleField purchasePriceField;

private JButton calculateButton;

private JTextArea output;

private double purchasePrice;

private double downPayment;

private double currentBalance;

private int iterations = 1;

private int month;

private double interest = 0;

private double principal;

private double paymentDue;

private double newBalance;

public GUICreditPlan()

{

String header = Format.justify('l',"Month", 7) +

Format.justify('r',"Current Balance", 16) +

Format.justify('r',"Interest", 10) +

Format.justify('r',"Principal", 11) +

Format.justify('r',"Payment Due", 14) +

Format.justify('r',"New Balance", 14) + "\n";

purchasePriceLabel = addLabel ("Purchase Price", 1,1,1,1);

purchasePriceField = addDoubleField(0.0, 1,2,1,1);

calculateButton = addButton ("Calculate", 2,1,2,1);

output = addTextArea (header , 3,1,5,6);

output.setEnabled(false);

}

public void buttonClicked(JButton buttonObj)

{

purchasePrice = purchasePriceField.getNumber();

downPayment = (purchasePrice * .1);

currentBalance = purchasePrice - downPayment;

do

{

paymentDue = ((purchasePrice - downPayment) * .05);

interest = currentBalance * .01;

principal = currentBalance - (paymentDue - interest);

newBalance = (currentBalance + interest) - paymentDue;

month = iterations;

displayNumbers (month, currentBalance, interest, principal, paymentDue, newBalance);

currentBalance = (currentBalance + interest) - paymentDue;

iterations++;

}

while(currentBalance>0);

}

private void displayNumbers(int month, double currentBalance, double interest, double principal, double paymentDue,double newBalance)

{

String numberLine = Format.justify('l',month, 7) +

Format.justify('r',currentBalance, 16,2) +

Format.justify('r',interest, 10,2) +

Format.justify('r',principal, 11,2) +

Format.justify('r',paymentDue, 14,2) +

Format.justify('r',newBalance, 14,2) + "\n";

output.append (numberLine + "\n");

}

public static void main(String[] args)

{

GUICreditPlan theGUI = new GUICreditPlan();

theGUI.setSize(600,300);

theGUI.setTitle("Credit Plan for the TidBit Computer Store");

theGUI.setVisible(true);

}

}

thanks for the help

cupOfJava2008a at 2007-7-12 17:24:09 >

> thanks that gives the algorithms i think i have them

> now but the problems im having are with the 12

> percent charged each year

I think that 12% a year compounded monthly is simply equivalent to 1% a month. The true yearly interest will be different due to compounding.

> and the output not stopping

> when the balance is zero

It is not set to stop when balance == 0, it is set to stop when this (balance > 0) is false, and that's probably as it should be. Since you are using double, if you try to end when this (balance != 0) is false, you'll likely go on forever. My guess is that the end balance will be negative, but it should be a very small number. How big is it?

petes1234a at 2007-7-12 17:24:09 >

> would this work for the annual interest percent

> charge?

>

> if (month % 12 ==0)

>{

> interest = currentBalance * .12;

>}

>

>else

> {

>interest = currentBalance * .01;

> }

Can you explain what that's supposed to mean? Why are you doing that?

jverda at 2007-7-12 17:24:09 >

there is a 10 percent down payment , an annual interest rate of 12 percent, and monthly payments are 5 percent of the listed purchase price, the amount of interest for a month is equal to balance * rate /12, the amount of principal for a month is equal to the monthly payment minus the interest owed

cupOfJava2008a at 2007-7-21 21:48:22 >

If you do your calcs correctly, you should only have a trivial positive or negative balance at the end. I was able to avoid negative results by using a constant, I called "DELTA", that allows for the trivial positive balances.

Something like this worked

private static final double DELTA = 0.001;

.....

.....

private void runCalculations()

{

getStartingData();

while (newBalance > 0 + DELTA) // the 0 is redundant but left in for clarity

{

doCalc();

showResults();

}

}

Oh yeah, you probably should test newBalance in your while loop, not currentBalance. When newBalance is 0, that's when you quit the loop.

Message was edited by:

petes1234

petes1234a at 2007-7-21 21:48:22 >