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Recursive thinking

Steve Haggan
Greenhorn

Joined: Nov 03, 2012
Posts: 12
I have a problem where I need to make an Arraylist of numbers recursively.
I then need use recursion to create another ArrayList of the elements at the even indexes of the arraylist.
My output appears to be at the mercy of my input.
If i put in an even number lets say 10 - the output is 2,4,6,8,10 (and i know these are not the even indexes of the array!)
However if I put an odd number in - 9, the output is 1,3,5,7,9 (which is the correct even indexes of the array.)
Im sure its something simple but i cannot see where I have gone wrong.
Code attached
Seetharaman Venkatasamy
Ranch Hand

Joined: Jan 28, 2008
Posts: 5575

Steve Haggan wrote:
If i put in an even number lets say 10 - the output is 2,4,6,8,10 (and i know these are not the even indexes of the array!)
However if I put an odd number in - 9, the output is 1,3,5,7,9 (which is the correct even indexes of the array.)

I dont understand your code. *just guessing*, your logic can get some light here..

1. just think if i pass n to method say print(n) then it should print 1 2... n recursively

2. now how can I trace even or odd so that i can print all even and all odd , *is this necessary* ? just subtract 2 while traveling recursively ...

Steve Haggan
Greenhorn

Joined: Nov 03, 2012
Posts: 12
Seetharaman Venkatasamy wrote:
Steve Haggan wrote:
If i put in an even number lets say 10 - the output is 2,4,6,8,10 (and i know these are not the even indexes of the array!)
However if I put an odd number in - 9, the output is 1,3,5,7,9 (which is the correct even indexes of the array.)

I dont understand your code. *just guessing*, your logic can get some light here..

1. just think if i pass n to method say print(n) then it should print 1 2... n recursively

2. now how can I trace even or odd so that i can print all even and all odd , *is this necessary* ? just subtract 2 while traveling recursively ...


Im not sure if I understand what you are saying, or perhaps I have explained it badly!
My task is to print out all the even element numbers
ie if n was 6 the initial array is 1,2,3,4,5,6 thus ArrayList (0) as an even number would print out 1, ArrayList (2) as an even number would print 3 and arrayList (4) as an even number will print out 5.
However with my code if n = 6 the out put is 2,4,6
But if my input is changed to 5 it prints out 1,3,5
I dont understand how the value of n can have an impact upon my recursive method to find all the values at an even element of the array.
Campbell Ritchie
Sheriff

Joined: Oct 13, 2005
Posts: 41098
    
  29
There is something wrong about your approach, and I can’t quite put my finger on it. There are a few specific things: for example, you have completely misunderstood what the isEmpty() method does. You do not need to use that technique to copy a List; you can use one of its constructors.
Why have you got all those static methods? Are you actually using object‑orientation.
I am finding the point of the exercise hard to understand (like Seetharaman). Are you supposed to remove odd/even‑placed elements from the List? And your method to remove alternate elements is really difficult to understand (even without the recursion).
I suggest you draw the structure of a List, like this… and examine the structure of the List as you go through that even method.
There is something wrong aboutYou can simply write
 
Don't get me started about those stupid light bulbs.
 
subject: Recursive thinking