Get previous/next item of a given item in a List<>

Question

Says I have this List : 1, 3, 5, 7, 9, 13

For example, given value is : 9, the previous item is 7 and the next item is 13

How can I achieve this using C#?

Answer 1

Following may be helpful

 int NextValue = 0;
 int PreviousValue =0;
 int index = lstOfNo.FindIndex(nd =>nd.Id == 9);

 var Next = lstOfNo.ElementAtOrDefault(index + 1);
 var Previous = lstOfNo.ElementAtOrDefault(index - 1);

 if (Next != null)
     NextValue = Next;


if (Previous != null)
   PreviousValue = Previous;

Answer 2

You can use indexer to get element at desired index. Adding one to index will get you next and subtracting one from index will give you previous element.

int index = 4; 
int prev = list[index-1];
int next = list[index+1];

You will have to check if next and previous index exists other wise you will get IndexOutOfRangeException exception. As List is zero based index so first element will have index 0 and second will have 1 and so on.

if(index - 1 > -1)
   prev = list[index-1];
if(index + 1 < list.Length)
   next = list[index+1];

Answer 3

To make it a kind of Circular list, try this:

public class NavigationList<T> : List<T>
{
    private int _currentIndex = -1;
    public int CurrentIndex
    {
        get
        {
            if (_currentIndex == Count)
                _currentIndex = 0;
            else if (_currentIndex > Count - 1)
                _currentIndex = Count - 1;
            else if (_currentIndex < 0)
                _currentIndex = 0;


            return _currentIndex;
        }

        set { _currentIndex = value; }
    }

    public T MoveNext
    {
        get { _currentIndex++; return this[CurrentIndex]; }
    }

    public T Current
    {
        get { return this[CurrentIndex]; }
    }
}

Answer 4

Also if you want compact solution with circular logic without creating new list you can use following code:

int nextNumber = list[(list.IndexOf(currentNumber) + 1) % list.Count];
int previousNumber = list[(list.IndexOf(currentNumber) - 1 + list.Count) % list.Count];

https://dotnetfiddle.net/PkP2Jy

Answer 5

var index = list.IndexOf(9);
if (index == -1) 
{
   return; // or exception - whater, no element found.
}

int? nextItem = null; //null means that there is no next element.
if (index < list.Count - 1) 
{
   nextItem = list[index + 1];
}

int? prevItem = null;
if (index > 0) 
{
   prevItem = list[index - 1];
}