How to compare two arrays in java and return an element that isn't included in two list

Question

I need to provide a Java solution that given two list, a & b, that returns a value that is present in one list but not in another.

e.g. lists a = [26, 13, 88, 9]

lists b = [26, 1, 8, 12]

`List` has a `removeAll(Collection)` method, this will remove duplicates . — Arnaud, Jun 14 '17 at 14:51
@Berger - I don't think the OP wants to remove duplicates, just determine which value is in one but not the other — , Jun 14 '17 at 14:52
@QueenSvetlana I think Berger thinks of a new array that has the value of: a.removeAll(b) — Tom Stein, Jun 14 '17 at 14:53
You can make use of HashMap<> also in java, store all the elements of one list in hashmap and then iterate through the other list and check whether they are in hasmap or not — zenwraight, Jun 14 '17 at 14:53
I'm voting to close this question as off-topic because SO is not a codewriting service. — P.J.Meisch, Jun 14 '17 at 15:13
Duplicate question (search first next time?): https://stackoverflow.com/questions/3590677/how-to-do-union-intersect-difference-and-reverse-data-in-java — ldmtwo, Jun 14 '17 at 15:38
@idmtwo I did search and couldn't find an answer. Clearly, I didn't search hard enough! Thank you all for your help. Much appreciated — Mel, Jun 14 '17 at 15:45

ΦXocę 웃 Пepeúpa ツ · Accepted Answer · 2017-06-14T15:12:50.790

6

for those kind of operation it would be better to use collections, the method removeAll() will filter the data containers, from the doc:

Removes from this list all of its elements that are contained in the specified collection (optional operation).

List<Integer> myVarListA = Arrays.asList(26, 13, 88, 9);
List<Integer> myVarListB = Arrays.asList(26, 1, 8, 12);
List<Integer> myVarListAcomplementB = new ArrayList<>(myVarListA);
List<Integer> myVarListBcomplementA = new ArrayList<>(myVarListB);
myVarListAcomplementB.removeAll(myVarListB);
myVarListBcomplementA.removeAll(myVarListA);
System.out.println("elements in A but no in B: " + myVarListAcomplementB);
System.out.println("elements in B but no in A: " + myVarListBcomplementA);
myVarListBcomplementA.addAll(myVarListAcomplementB);
System.out.println("both together: " + myVarListBcomplementA);

edited Jun 14 '17 at 15:12

answered Jun 14 '17 at 14:54

ΦXocę 웃 Пepeúpa ツ

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@FedericoPeraltaSchaffner you are right, i misread something there... but now is ok... thanks anyway for the feedback! – ΦXocę 웃 Пepeúpa ツ Jun 14 '17 at 14:58
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This will return unique values from list A (13, 88, 9), I guess we should also grab unique values from list B (1, 8, 12 ). – Arnaud Jun 14 '17 at 15:01
@Berger do you mean in a reflective way? those that are in B but not in A? – ΦXocę 웃 Пepeúpa ツ Jun 14 '17 at 15:07
Yes exactly, the question doesn't precise if we want unique values from both lists, but probably yes . – Arnaud Jun 14 '17 at 15:08

ldmtwo · Answer 2 · 2017-06-15T03:59:32.917

Simple solution is to calculate the intersection and remove that from the desired list. If you only want what is missing, you can optimize this a little by going a solution like ΦXocę 웃 Пepeúpa ツ.

The great thing about this solution is that you can easily expand this to use 3+ sets/lists. Instead of A-B = diff or A-I(A,B)=diff, you can also do A-I(A,I(B,C)) to find what A is missing from the common set between A, B and C.

  public static <T> HashSet<T> intersection(Collection<T> a, Collection<T> b) {
        HashSet<T> aMinusB = new HashSet<>(a);
        aMinusB.removeAll(b);
        HashSet<T> common = new HashSet<>(a);
        common.removeAll(aMinusB);
        return common;
    }

Let's call the intersection set Collection I = intersection(a,b);.

Now if you want to find what is missing from list A that was in B:

new LinkedList(A).removeAll(I);//ordered and possibly containing duplicates
   OR
new ArrayList(A).removeAll(I);//ordered and possibly containing duplicates. Faster copy time, but slower to remove elements. Experiment with this and LinkedList for speed.
   OR
new LinkedHashSet<T>(a).removeAll(I);//ordered and unique
   OR
new HashSet<T>(a).removeAll(I);//unique and no order

Also, this question is effectively duplicate of How to do union, intersect, difference and reverse data in java

@Idmtwo thank you. I'm new to this so apologies if this question is already on SO — Mel, Jun 14 '17 at 15:40

score 0 · Answer 3 · answered Jun 14 '17 at 14:55

0

You may try this:

a.removeAll(b);

answered Jun 14 '17 at 14:55

Nikita V

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Atti · Answer 4 · 2017-06-14T15:19:33.360

0

Simply parse the second list and add unique elements to the first, delete others.

for (Integer elem : secondList)
  if (firstList.contains(elem))
    firstList.remove(elem);
  else
    firstList.add(elem);

In firstList you will have the values present only in one of the lists.

edited Jun 14 '17 at 15:19

answered Jun 14 '17 at 14:55

Atti

31
6

How to compare two arrays in java and return an element that isn't included in two list

4 Answers4