Java Beginner - Mistake on a simple code

Question

is my first time here so i dont know exactly how it works, so sorry for the mistakes.

What's the result from this function, when we give as "aktueller Parameter" the number 3?

(The original text:

Welches Ergebnis liefert diese Methode, wenn bei einem Aufruf als aktueller Parameter der Wert 3 übergeben wird?

Im studying in German, so i dont really know the english terms :/ )

public int m(int p)
{
   int result;
   if (p == 0)
   {
    result = 0;
   }
   else
   {
    result = 3*p + m(p-1);
   }
    return result;
   }

I have already tried it and the answer is 18, but when im trying to do it without any program the answer i find is 15:

result = 3 * 3 + 3(3-1);

Can someone please explain me why is 18 and not 15? Im assuming that i am making something wrong.

Thank you in advance.

You're mistaking m and p. It's a recursive code, evaluates to 3*3 + m(2), the second part becomes 3*2 + m(1), and m(1) = 3, total is 9+6+3 = 18. — AntonH
– AntonH, Commented Feb 26, 2014 at 17:12
I think "aktueller Parameter" == "actual parameter" is close enough. — ajb
– ajb, Commented Feb 26, 2014 at 17:25

rgettman · Accepted Answer · 2014-02-26 17:15:31Z

9

Let's break down this recursive call:

With m(3), p isn't 0, so we return 3*3 + m(2);.

3*3 + (m(2))

With m(2), p isn't 0, so we return 3*2 + m(1);.

3*3 + (3*2 + m(1))

With m(1), p isn't 0, so we return 3*1 + m(0);.

3*3 + (3*2 + (3*1 + m(0))

With m(0), p is 0, so we return 0. Then the recursive call stack unwinds.

3*3 + (3*2 + (3*1 + (0)) =
9 + (6 + (3 + 0)) = 
9 + (6 + 3) = 
9 + 9 = 
18

answered Feb 26, 2014 at 17:15

rgettman

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2 Comments

syiannop Over a year ago

Thank you very much!That was really helpful!! :D (I cant vote up or accept the answer)

rgettman Over a year ago

You gain the privilege of voting up when you reach 15 reputation, and you are always allowed to accept one answer that you feel helped you the most to solve your problem after 15 minutes.

Embattled Swag · Accepted Answer · 2014-02-26 17:15:37Z

2

When you see m(p-1), that means that you're calling the function m again from inside itself, which is called recursion.

Essentially, the arithmetic it's doing is 3*3+3*2+3*1 = 18.

answered Feb 26, 2014 at 17:15

Embattled Swag

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Kick · Accepted Answer · 2014-02-26 17:17:28Z

0

Due to recursive code.Please c the strack trace

When you pass the p value as 3,the function interact as below :

 result = 3*p + m(p-1);        // 3*3 + m(2)  = 9
                               // 3*2 + m(1)  = 6
                               // 3*1 + m(0)  = 3 
                                             -------
                                               18

answered Feb 26, 2014 at 17:17

Kick

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B.K. · Accepted Answer · 2014-02-26 17:31:51Z

                           // Original argument is 3
3*3 + m(3-1)               // Step #1  New argument is 3 - 1
3*2 + m(2-1)               // Step #2  New argument is 2 - 1
3*1 + m(1-1)               // Step #3  New argument is 1 - 1

                           // p == 0   Return 0

3*1 + 0 = 3                // Step #4  Return 3
3*2 + 3 = 6 + 3 = 9        // Step #5  Return 9
3*3 + 9 = 9 + 9 = 18       // Step #6  Return 18, which is the final value

Here is how I like to work on recursive functions, and it a personal preference. Start at the top, and put together the known values, then write the function on the side, in this case we simply add it. Then, on the next line, write the known values again (this time, one of them is one less (2, rather than 3), and write the function with the new arguments to the right of it (we're once again adding), and so on. I work from top to bottom.

I then show the results on the right side and work back up. So, I don't write the results right away.

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