CODEWITHSUNDEEP
javaformulaarithmetic-expressions
Logan
Logan

Reputation: 21

Trying to convert this formula into an arithmetic expression in Java

formula

I'm trying to take user input in the form of myMonthlyPayment, myAnnualInterestRate, and myPrincipal in order to calculate the number of months needed to pay off debt by using The formula I've attached to this post. What I have in eclipse for the formula right now is:

monthsNeeded = ((Math.log(myMonthlyPayment) - Math.log(myMonthlyPayment) 
                - ((myAnnualInterestRate / 1200.0) * myPrincipal)) 
                  / ((Math.log(myAnnualInterestRate) / 1200.0) + 1.0));

I should be getting an output of 79 months with the inputs I'm using but instead I'm getting -62. I know the formula is correct, I'm almost positive I've made a mistake somewhere in the translation of it into Java. If someone could point it out that would be greatly appreciated!

Upvotes: 1

Views: 494

Answers (2)

Kröw
Kröw

Reputation: 322

I think this is what you're looking for:

monthsNeeded =  (Math.log(myMonthlyPayment) - Math.log(myMonthlyPayment - myAnnualInterestRate / 1200d * myPrincipal)) / Math.log(myAnnualInterestRate / 1200d + 1);

It seems that, in your solution, you weren't calculating your myAnnualInterestRate/1200*myPrincipal inside your second Math.log(...). You had also left some calculations outside of Math.log(...) in the bottom half of your equation.

If you have an equation that does an operation inside a natural log, when you convert that equation to Java code, the operation needs to still be done, inside the natural log:

ln(someNumber + 10)

would be converted to:

Math.log(someNumber + 10),

NOT:

Math.log(someNumber) + 10

Hope this helps and good luck. :)

Upvotes: 0

Roy Shahaf
Roy Shahaf

Reputation: 494

So I've fixed it, with a sample input and output. I didn't put much effort into making this code beautiful but you can see that even separating it into 3 parts using method extraction (although I didn't know how to name them, lacking the domain knowledge) made the code easier to understand.

public class Example {
    public static void main(String[] args) {
        double myMonthlyPayment = 2000;
        double myAnnualInterestRate = 5;
        double myPrincipal = 200000;
        System.out.println(a(myMonthlyPayment));
        System.out.println(b(myPrincipal, myAnnualInterestRate, myMonthlyPayment));
        System.out.println(c(myAnnualInterestRate));
        double monthsNeeded = (a(myMonthlyPayment) - b(myPrincipal, myAnnualInterestRate, myMonthlyPayment))
                / c(myAnnualInterestRate);
        System.out.println(monthsNeeded);
    }

    private static double c(double myAnnualInterestRate) {
        return Math.log((myAnnualInterestRate / 1200.0) + 1);
    }

    private static double b(double myPrinicipal, double myAnnualInterestRate, double myMonthlyPayment) {
        return Math.log(myMonthlyPayment - (myAnnualInterestRate / 1200.0) * myPrinicipal);
    }

    private static double a(double myMonthlyPayment) {
        return Math.log(myMonthlyPayment);
    }
}

Upvotes: 1

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