How to find single duplicate integer from the list of numbers-CodePudding

In my below program I have to find the single duplicate number. For example (from the series 3, 5, 3, 2, 7, 7, 5, 6) if 3 is repeated then program should exit and return 3. If not then go for next number 5, if it's repeated then should exits and return 5.

I am getting all repeated numbers 3,5,7 but I want to find the single number which is repeated first. How should I get this?

class Program
    {
        private string checkduplicate()
        {
            List<int> list = new List<int>() { 3, 5, 3, 2, 7, 7, 5, 6 };
            IEnumerable<int> duplicates = list.GroupBy(x => x)
                                                    .Where(g => g.Count() > 1)
                                                    .Select(x => x.Key);
            return String.Join(",", duplicates);

            //var list = new List<int>() { 3, 5, 3, 2, 7, 7, 5, 6 };
            //list.GroupBy(n => n).Any(c => c.Count() > 1);

        }
        static void Main(string[] args)
        {
            Program p = new Program();
            Console.WriteLine(p.checkduplicate());
            Console.ReadLine();

        }
    }

CodePudding user response：

There's another option without GroupBy:

private string checkduplicate()
{
    List<int> list = new List<int>() { 3, 5, 3, 2, 7, 7, 5, 6 };
    return list.Where((num, ind) => list.LastIndexOf(num) > ind).FirstOrDefault().ToString();
}

CodePudding user response：

If you want just first item, you don't need the list, since it's always gonna be just one number.

So, instead of IEnumerable<int> duplicates = ... go with int duplicate = ..., and just put FirstOrDefault() after the select in your LINQ.

You can change your return value as well, since there is no need to join anything.

 public class Program
{
public static void Main()
{
        Program p = new Program();
        Console.WriteLine(p.checkduplicate());
        Console.ReadLine();
    
}

 private string checkduplicate()
    {
        List<int> list = new List<int>() { 3, 5, 3, 2, 7, 7, 5, 6 };
        int duplicates = list.GroupBy(x => x)
                                                .Where(g => g.Count() > 1)
                                                .Select(x => x.Key).FirstOrDefault();
        return duplicates.ToString();

        //var list = new List<int>() { 3, 5, 3, 2, 7, 7, 5, 6 };
        //list.GroupBy(n => n).Any(c => c.Count() > 1);

    }
}

CodePudding user response：

Find your duplicates, filter them to > 1 and then grab the first item which has any values in your duplicate list.

private string checkduplicate()
{
    var list = new List<int>() { 3, 5, 3, 2, 7, 7, 5, 6 };
    var duplicates = list
        .GroupBy(x => x)
        .Where(g => g.Count() > 1);

    return list
        .First(item => duplicates.Any(dup => dup.Key == item))
        .ToString();
}

CodePudding user response：

You can use FirstOrDefault() or SingleOrDefault() for this. Modify the checkDuplicate() method as below.

private string checkduplicate()
    {
        List<int> list = new List<int>() { 3, 5, 3, 2, 7, 7, 5, 6 };

        int duplicate = list.GroupBy(x => x)
            .Where(g => g.Count() > 1)
            .Select(x => x.Key).FirstOrDefault();

        return duplicate.ToString();


    }