3

Here is a method that is supposed to simply create a new ArrayList copying all the elements of parameter ArrayList arrlist, which I think I've done correctly.

public ArrayList<T> copy (ArrayList<T> arrlist) {
        ArrayList<T> um=new ArrayList<T>();
        for (int i=0;i<arrlist.size();i++)
            um.add(arrlist.get(i));
        return um;

However, I'd like to write this exact same method using recursion only with no loops. Here is what I wrote. The copy method uses a recursive helper method.

public ArrayList<T> copy(ArrayList<T> arrlist) {
    return copy(arrlist,0); 
}

private ArrayList<T> copy(ArrayList<T> arrlist, int n) {
    ArrayList<T> um=new ArrayList<T>();
    if (n<arrlist.size())
        um.add(list.get(n));
    return copy(list,n+1); 
}

Except this does not work. Any suggestions or hints?

Improve this question
8
  • What is error there? Commented Oct 21, 2014 at 20:50
  • "doesn't work" in what way? Describe the specific problem you're asking for help with Commented Oct 21, 2014 at 20:50
  • 1
    Is there a base case to this recursion? Or does it just call itself forever? Commented Oct 21, 2014 at 20:50
  • Just about to say the same thing. No base case, can't see this ever terminating. Commented Oct 21, 2014 at 20:50
  • possible duplicate of iterating through arraylists with recursion Commented Oct 21, 2014 at 20:51

5 Answers 5

Reset to default
3

The problem is that you are creating a new ArrayList in every recursion. And you are not returning it anywhere.

What you should do is, in the helper method, also pass the target ArrayList as a parameter, and add to it rather than a new one.

(And of course, don't forget to return when you're done).

Improve this answer
Sign up to request clarification or add additional context in comments.

Comments

1

You are allocating (ArrayList um=new ArrayList();) array list in every recursive call. Also I can not see when the recursive function will stop to call itself

Improve this answer

Comments

0

Try this :

private void copy(ArrayList<T> src, ArrayList<T> src_copy, int n) {
  if (n>=0){
    src_copy.add(src.get(n));
    copy(src,src_copy, n-1); 
   }
 }

Then, use it as below : 

ArrayList<T> src = ... ; //your arrayList
ArrayList<T> src_copy = new ArrayList<T>();
if (src != null && src.size() > 0) {
   copy(src, src_copy, src.size()-1);
 }
Improve this answer

Comments

0

I don't know what error you are getting. but I think this should work.

public ArrayList<T> copy(ArrayList<T> arrlist) {
    ArrayList<T> um = new ArrayList<T>();
    return copy(arrlist, um, 0); 
}

private ArrayList<T> copy(ArrayList<T> arrlist, ArrayList<T> um, int n) {
    if (n < arrlist.size())
        um.add(arrlist.get(n));
    else
        return um;
    return copy(arrlist, um, n+1); 
}
Improve this answer

Comments

-1

See the below basic recursive fibonacci function:

public int fib(int n) {
    if(n <= 1) {
        return n;
    } else {
        return fib(n - 1) + fib(n - 2);
    }
}

This works as a recursive function because it has a base case, a path by which the function will return a result that is not another call to itself. You'll need a base case to complete the recursive call when a condition is met, and return the results.

Improve this answer

Comments

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.