Recursive thinking

[Steve Haggan]

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

import java.util.*;

public class ListMethods
{
  public static ArrayList <Integer> makeList (int n)
  {
		 ArrayList<Integer> tempList = new ArrayList<Integer>();
		
		if(n<=0)//the smallest list we can make
		{	
			tempList.isEmpty();
		}
		else
		{
			tempList = makeList(n-1);
			tempList.add(n);
			//return tempList;
	 	}
			return tempList;
	 }	 
		
		 	public static ArrayList <Integer> deepClone (ArrayList <Integer> tList)
		 			 
			 {
				 ArrayList<Integer> list = new ArrayList <Integer>();
				 for (Integer i : tList)
				 {
					 list.add(new Integer (i));
				 }
				 return list;
			 }
		 
		 public static ArrayList <Integer> reverseList (ArrayList<Integer> tList)
		 {
			 ArrayList<Integer>list = ListMethods.deepClone(tList);
			
			 if(list.size()<=1)
			 {
				 return list;//return the list as is - no need to reverse
			 }
			 else
			 {
				//ArrayList<Integer> newList = new ArrayList<Integer>();
				Integer first = list.get(0);
				list.remove(0);
				list = reverseList(list);
				list.add(first);
											 
			 }
			 return list;
		 }
		 
		 public static ArrayList <Integer> deepClone1 (ArrayList <Integer>newList)
		 {
			 ArrayList <Integer> list = new ArrayList <Integer>();
			 for (Integer i : newList)
			 {
				 list.add(new Integer (i));
			 }
			 return list;
		 }
		
		 public static ArrayList <Integer> even (ArrayList <Integer>newlist)
		 {
			 ArrayList<Integer>eList = ListMethods.deepClone(newlist);
			 
			 if (eList.size()<=1)
			 {
				 return eList;
			 }
			 else
			 {
				
				 int even = eList.remove(0);
				 eList.remove(0);
								 				 
				 eList = ListMethods.even(eList);
				 eList.add(even);
				 
			 }
			 return eList;
					 
		 }

[Seetharaman Venkatasamy]

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]

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]

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

0     1     2     3     4     5
↓     ↓     ↓     ↓     ↓     ↓
1st   2nd   3rd   4th   5th   6th

… and examine the structure of the List as you go through that even method.
There is something wrong about

eList = ...;
if (eList.size <= 1)
{
   return eList;
}
else
{
 ...
} return eList;

You can simply write

eList =...;
if (eList.size() > 1)
{
 ...
}
return eList;