Arraylist and recursive implementation

Question

I am trying to outline a java recursive implementation for a function which returns true if all the elements in the list are single digit(ie<10). Can assume the list only contains positive numbers.

Here is what I have tried, but it keeps returning true even there is a 2 digits number in the list.

import java.util.*;

public class Q3b {

static boolean allSingleDigit(ArrayList list) {
    if (list.size() == 0)
        return false;
    else {
        int first = (Integer) list.get(0);
        list.remove(0);

        if (first < 10)
            return true;
        else
            return allSingleDigit(list);
    }

}

public static void main(String[] args) {
    ArrayList<Integer> list = new ArrayList();
    Collections.addAll(list, 4, 25, 3, 2, 3, 1, 3);
    boolean res1 = allSingleDigit(new ArrayList(list));
    System.out.println("List Contains all single number" + res1);
}

}

any suggestions guys ? thanks in advance.

Answer 1

I think you might want to flip the logic slightly so that it returns true only when there are no elements left in the array;

static boolean allSingleDigit(ArrayList list) {
if (list.size() == 0)
    return true;
else {
    int first = (Integer) list.get(0);
    list.remove(0);

    if (first < 10)
        return allSingleDigit(list);
    else
        return false
}

Answer 2

Your program seems fine, only problem is you should return false as soon as first element >= 10 is encountered and return true when list size is 0 . Modify it as shown below:

static boolean allSingleDigit(List<Integer> list) {
  if (list.size() == 0)
    return true;

  int first = list.remove(0);

  if (first == null || first >= 10)
    return false;
  else
    return allSingleDigit(list);

}

You could optimize it and make it more concise by using generic List and using remove() method from List directly. I have also added missing null check.

Answer 3

I think this is the method you wanted.

static boolean allSingleDigit(ArrayList list) {
    if (list.size() == 0)
        return true;
    else {
        int first = (Integer) list.get(0);
        list.remove(0);

        if (first >= 10)
            return false;
        else
            return allSingleDigit(list);
    }

}

I have just changed in your code.