Simple java recursion, unpredictable program behavior

Question

Trivial program to calculate compound interest, I = P(1+n)^y.

public class Interest {

    public static void main(String[] args){

        double sum = calculate(1000,10,3);
        System.out.println(sum);
        //sanity check, using non-recursive formula:
        double amt = 1000*(Math.pow(1 + 0.1, 3));
        System.out.println(amt);


    }

    public static double calculate(double initialAmount, double interest, int years){

        double yearly = initialAmount + initialAmount*(interest/100);

        System.out.println("calculate is called with P = " + initialAmount + ", interest = "+interest+", and years = " + years);
        System.out.println("This year's amount: "+yearly);

        if (years <= 0){
            return initialAmount;
        }
        if (years == 1){
            return yearly;
        }
        else {
            return yearly + calculate(yearly, interest, years - 1);
        }

    }

    }

The output is as follows (notice that YEARLY is calculated correctly but not returned as expected):

debug:
calculate is called with P = 1000.0, interest = 10.0, and years = 3
This year's amount: 1100.0
calculate is called with P = 1100.0, interest = 10.0, and years = 2
This year's amount: 1210.0
calculate is called with P = 1210.0, interest = 10.0, and years = 1
This year's amount: 1331.0
3641.0
1331.0000000000005

When I debug, execution enters if (years == 1) as expected, but then also enters the following else block. How can it enter both branches? Please advise; I've been racking my brains, and restarting the computer and NetBeans didn't help.

I am pretty sure that the else is in one recursive depth ABOVE. please check the years variable. It should be 2 if you are in the else block. — Turing85
– Turing85, Commented Nov 13, 2015 at 17:54
@AndyTurner since this is most likely an exercise on recursion. — Turing85
– Turing85, Commented Nov 13, 2015 at 18:00

janos · Accepted Answer · 2015-11-13 18:14:34Z

1

No, execution does not enter the else block after that if block. That's impossible. If it appears that way, then it seems you are misunderstanding your debugger. Note that after the terminal if, execution will be stepping back through the else statements in the preceding stacks. But it will not enter. Only step back into.

The result is accumulated in the first parameter, there's no need to add the value of yearly on the last line of your method. The logic can also be a bit simplified:

    if (years == 1){
        return yearly;
    }
    return calculate(yearly, interest, years - 1);

edited Nov 13, 2015 at 18:14

answered Nov 13, 2015 at 18:03

janos

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2 Comments

msrd0 Over a year ago

This does not provide an answer to the question. To critique or request clarification from an author, leave a comment below their post. - From Review

janos Over a year ago

@msrd0 Why do you think that? The OP asks how it's possible to enter the else block after the if. I explain that he's misinterpreting the situation, no, execution does not enter the else block after the if, contrary to what OP believes. Then I provide a correction to his broken algorithm. Am I missing something?

Taelsin · Accepted Answer · 2015-11-13 18:17:11Z

1

You're not entering the else block after the years==1 if block, you're simply adding yearly to every recursive call.

Change the return statement in the else block to

return calculate(yearly, interest, years - 1)

from

return yearly + calculate(yearly, interest, years - 1)

edited Nov 13, 2015 at 18:17

answered Nov 13, 2015 at 18:01

Taelsin

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1 Comment

Alice Over a year ago

Thank you, Taelsin! That totally worked. I don't know what I was thinking.

Turing85 · Accepted Answer · 2015-11-13 17:59:04Z

0

The else branch is entered on any call where years > 1, especially for years == 2. Form there, you call calculate with years == 1, which is when you enter the if-statement in question. Now after the return in this if-statement is executed, execution resumes on the previous level of the recursion where it stopped. This is within the else block, after the recursive call. When you check the value of years, you will see that it changes from 1 to 2 after the return in the if is executed.

answered Nov 13, 2015 at 17:59

Turing85

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2 Comments

Alice Over a year ago

I see now how it could enter both branches. It seems there's much I have yet to learn about recursion. Many thanks!

Alice Over a year ago

By "enter both branches" in my comment above I mean "appear to enter" as I'm watching execution in debugger (as Janos pointed out below). I drew a diagram of calls and returns, and it really helped. Great explanation, Turing.

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