Logic for pairing up two sets of values in ArrayList with duplication

Question

I'm baffled on the logic of programming this. I'm sure it's extremely simple.

So, I have an ArrayList<String> with values A, B, C, D, E, F, G. Indexed in that order.

I would like to randomly select a unique letter for each but have no duplication.

The output would look similar to this.

A:B
B:A
C:E
D:F
E:C
F:D

I’ve tried shuffling the ArrayList then moving the index by two, but couldn’t think of a way to prevent duplication, and also randomly selecting indexes then pairing them up but this only works with even numbers.

can you please provide the complete problem statement or elaborate it? — Shubham Srivastava
– Shubham Srivastava, Commented May 1, 2020 at 12:08
I've simplified the problem quite a bit. I just cannot think of the logic for doing this. I'm creating an android app which needs to pair up two users in a limited group of users. — Jack Reilly
– Jack Reilly, Commented May 1, 2020 at 12:11
You said "...but have duplication" (twice), but did you really mean "...but not have duplication"? — Kevin Anderson
– Kevin Anderson, Commented May 1, 2020 at 12:12
so you want to create random pairs of letters, without duplicating those pairs? — Shubham Srivastava
– Shubham Srivastava, Commented May 1, 2020 at 12:12

cegredev · Accepted Answer · 2020-05-01 12:38:08Z

1

This should be all you need:

ArrayList<String> list = // your list containing the values

ArrayList<String> shuffled = new ArrayList<>(list);
Collections.shuffle(shuffled);

This will give you a second ArrayList<String> that contains all the values of list in random order without duplicates.

edited May 1, 2020 at 12:38

answered May 1, 2020 at 12:32

cegredev

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1 Comment

Kevin Anderson Over a year ago

If Collections.shuffle itself is off-limits for you, just implement the algorithm for yourself, as conveniently described in the javadoc.

Shubham Srivastava · Accepted Answer · 2020-05-01 12:39:03Z

class Solution
{
    public static class Pair
    {
        public String a;
        public String b;

        public Pair(String a,String b)
        {
            this.a=a;
            this.b=b;
        }

        @Override
        public boolean equals(Object o)
        {
            if((this.a.equals(o.a))&&(this.b.equals(o.b)))
            {
                return true;
            }
            if((this.b.equals(o.a))&&(this.a.equals(o.b)))
            {
                return true;
            }
            return false;
        }

        @Override
        public String toString()
        {
            return a+":"+b;
        }
    }
    public static void main(String args[])
    {
        ArrayList<String> list=new ArrayList<String>();
        ......
        .....
        ......
        //list now contains {A,B,C,D,E,F,G}
        Set<Pair> letterset=new HashSet<Pair>();
        for(int i=0;i<list.size();i++)
        {
            for(int j=0;j<list.size();j++)
            {
                Pair pair=new Pair(list.get(i),list.get(j));
                letterset.add(pair);
            }
        }
        Iterator<Pair> iter=letterset.iterator();
        while(iter.hasNext())
        {
            System.out.println(iter.next());
        }
    }
}

Here 1. Pair: it is a class which holds two letters a and b, if two objects same set they are considered to be equal as implemented in the equals() function
2. letterset: set has been creates to remove the duplicate objects. 3. nested loop created to create pairs

I hope this is what you wanted.

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