Getting index from unqiue (stand-alone) elements in C#

simple iterate over the items and check for that condition

char[] items = { 'A', 'A', 'A', 'B', 'B', 'A', 'B', 'B', 'C', 'B' };
for (int i = 0; i < items.Length; i++)
{
    if (i == 0)
    {
        // in case of the first element you only have to validate against the next
        if (items[i] != items[i + 1])
            Console.WriteLine(i + 1);
    }
    else if (i == items.Length - 1)
    {
        // in case of the last element you only have to validate against the previous       
        if (items[i] != items[i - 1])
            Console.WriteLine(i + 1);
    }
    else
    {
        // validate against previous and next element
        if (items[i] != items[i - 1] && items[i] != items[i + 1])
            Console.WriteLine(i + 1);
    }
}

https://dotnetfiddle.net/kWmqu7

Here is one solution I came up with:

So you have your example list like that:

var list = new List<string>
{
    "A", // 1
    "A", // 2
    "A", // 3
    "B", // 4
    "B", // 5
    "A", // 6
    "B", // 7
    "B", // 8
    "C", // 9
    "B"  // 10
};

and then you call a method called GetSingleListPositions and receive List<int> representing your desired positions.

private static List<int> GetSingleListPositions(IList<string> list)
{
    var uniquePositions = new List<int>();
    var occurence = new List<string>();

    for (int i = list.Count - 1; i >= 0; i--)
    {
        if (!occurence.Contains(list[i]))
        {
            occurence.Add(list[i]);
            uniquePositions.Add(++i);
        }
    }

    uniquePositions.Reverse();
    return uniquePositions;
}

You call it like this:

var result = GetSingleListPositions(list);
Console.WriteLine(string.Join(' ', result));

As a result, I receive this:

6 9 10

Hope this helps, Cheers

Here's a solution that works in O(N) time even if the input sequence does not support indexing. It also works for any type that implements a proper Equals():

public static IEnumerable<int> IndicesOfStandaloneElements<T>(IEnumerable<T> elements)
{
    int index = 0;

    T previous = default;
    T current  = default;

    var comparer = EqualityComparer<T>.Default;

    foreach (var next in elements)
    {
        if (index == 1)
        {
            if (!comparer.Equals(current, next))
                yield return 0;
        }
        else if (index > 1)
        {
            if (!comparer.Equals(current, previous) && !comparer.Equals(current, next))
                yield return index - 1;
        }

        previous = current;
        current  = next;

        ++index;
    }

    if (index > 0 && !comparer.Equals(current, previous))
        yield return index - 1;
}

Assuming that you want last index of the elements. Try below code.

    string[] abc = new string[]{"A","A","A","B","B","A","B","B","C","B"};

    var distinctValues = abc.Distinct().Select(x => x);
    foreach (var d in distinctValues)
    {
        Console.WriteLine(Array.LastIndexOf(abc.ToArray(), d));
    }

//Result
5
9
8

using System.Linq;
using System;

Add your namespace and class

**variables needed**
string[] printer = {"A", "A", "A", "B", "B","A","B","B","C","B"};
int[] terms = new int[10];
int check=0;
 int index1=0;
**method to check the value present in array and add index if not present in array for your output**

for (int i = i; i <= printer.Length; i++)
{
   int test=findIndex(i,printer.Length,printer[i]);
if(test==0)
{
  addIndex(i);
}
}
**method to addindex delare**

public addIndex(int num)
{
    terms[index1] = value;
    index1=index1+1;

}
**method to findIndex**

public int findIndex(int num1, int num2,string test) {
for (int i = num1; i < num2; i++)
{
    if (string.Compare(printer[i],test)==0 )
        {
            return 1;
       }

}
    return 0;

}