Question

I have two arrayLists and I am trying to “subtract” one arrayList from another. For example, if I have one arrayList [1,2,3] and I am trying to subtract [0, 2, 4] the resulting arrayList should be [1,3].

List<Integer> a = new ArrayList<>(Arrays.asList(1, 2, 3));
List<Integer> b = Arrays.asList(0, 2, 4);
subtract(a,b) // should return [1,3]

Here is my code.

//returns a new IntSet after subtracting a from b
// .minus().toString()
ArrayList<Integer> minusArray = new ArrayList<Integer>();

    minusArray.addAll(array1);

    for(int i =0; i< minusArray.size(); i++){
        for(int j = 0; j < array2.size(); j++){
            if(minusArray.get(i).equals(array2.get(j))){
                minusArray.remove(i);
                if(i == 0){
                    ;
                }
                else if(j == 0){
                    ;
                }
                else{
                    i = 0;
                    j = 0;
                }
            }
            else{}
        }
    }

return minusArray;

My code works in some cases, like if arrayList1 = [4,6] and arrayList2 = [6] it will will give me a result of [4]. But if I try something like [1,2,4] and [0,4,8]

I get this exception:

java.lang.IndexOutOfBoundsException: Index: 2, Size: 2
    at java.util.ArrayList.rangeCheck(Unknown Source)
    at java.util.ArrayList.get(Unknown Source)
    at IntSet.minus(IntSet.java:119)
    at IntSetDriver.main(IntSetDriver.java:62)

Here is the code I have come up with. I have done test runs through it and to me I think it should work. The user inputs these arrayLists and they are presorted, I also do not know Hash or big-O.

ArrayList<Integer> minusArray = new ArrayList<Integer>();

    minusArray.addAll(array1);

    for(int i =0; i< minusArray.size(); i++){
        for(int j = 0; j < array2.size(); j++){
            if(minusArray.get(i).equals(array2.get(j))){
                minusArray.remove(i);
            }
            else{}
        }
    }

return minusArray;

Answer 1

Your problem is that in your minusArray.remove(…) call you may shrink the size of the minusArray. To fix this, start at array.size() – 1 and count backwards to 0

Check that – even that won’t fix it. You need to reverse the order of your loops