Recursive Outputs

Question

This is the following code I had for a recursion question

Is anyone able to run me through how the output is 24?

To prove how confused I am, I thought the output would have been 6, 12, 20 ,1

package Examples;

public class QuestionDemo {

public static void main(String[] args) {

    System.out.println(recCall(2));
}

public static int recCall(int num) {

    if (num == 5) {
        return 1;
    } else {
        return num * recCall(++num);
    }

}
}

Walk through why you thought the output would be that. And do you mean you thought it would have 4 outputs? — Carcigenicate, Mar 26 '17 at 12:23
Note that the return of each recurse is being multiplied by the passed number. — Carcigenicate, Mar 26 '17 at 12:25
@simigami 50 rep is easy to get. Ask a few good questions, and you'll get it. Posting non-answers as answers hurts the asker because it makes it less likely someone will post a real answer. — Carcigenicate, Mar 26 '17 at 12:28
I was going along the lines so num == 2 , therefore 2 * recCall(3) and carried on looping printing out, now i realise how wrong I was, I think it has clicked that it will be 2 x 3 x 4 x 1? which then equals 24 — Ashley 'Splash' Elliott, Mar 26 '17 at 12:31
Who went around downvoting half the answers? Please comment. That's rather rude. — Carcigenicate, Mar 26 '17 at 12:41

score 2 · Answer 1 · answered Mar 26 '17 at 12:32

2

You have 4 recursive calls first one when you call recCall(2) then recCall(3), recCall(4) and recCall(5) recCall(5) returns 1 recCall(4) returns 4*1 recCall(3) returns 3*4*1 recCall(2) returns 2*3*4*1 = 24

answered Mar 26 '17 at 12:32

DionizB

1,487
2
10
20

Mustafa Çil · Answer 2 · 2017-03-26T12:41:11.810

recCall(int num):

recall(2)
    |       | 
   2 *    recall(3)
              |       |
              3 *  recall(4)
                      |         |
                      4 *    recall(5)
                                |
                                1           



recall(2) = 24
            |       |
            2 *   12 = 24
                    |      |
                    3 *    4 = 12
                           |       |
                           4 *     1 = 4
                                   |
                                   1

Santiago Varela · Answer 3 · 2017-03-26T15:37:49.550

This recursion takes a bottom-up approach. First going deepest to find the value of the base case, before you can get the value of other recursive calls.

This is why you're getting output = 24:

                                       _
                                      /|\ 
                                     / | \
recCall(4) = 4 * recCall(5) = 4  +  /  |  \ // recCall(5) is your base case.
recCall(3) = 3 * recCall(4) = 12 +     |
-----------------------------------    |
recCall(2) = 2 * recCall(3) = 24       | // This was your first call to the recursive function.

You're not traversing through values, but rather adding them all up.

score 1 · Answer 4 · edited May 23 '17 at 12:25

Its because you are using the recCall(++num), and you are using pre-increment operator which increases value before calling the method.

Please read how ++ pre increment works in java :- How do the post increment (i++) and pre increment (++i) operators work in Java?

So your recursive calls will look like below

 f(2)= 2*f(3)=2*12 = 24.
        f(3)=3*f(4)= 3*4=12
            f(4)=4*f(5) = 4*1
                f(5)= 1

hence it returns 24.

score 0 · Answer 5 · edited Mar 27 '17 at 02:20

0

Yes, if you try to move recall(++num) and put the new number which is ++num so result will be = 2 * 3 * 4 and in 5 it will return 1, so 2 * 3 * 4* 1.

edited Mar 27 '17 at 02:20

Santiago Varela

2,199
16
22

answered Mar 26 '17 at 12:28

mahmoud abdelsattar

11

Recursive Outputs

5 Answers5