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

Using LINQ in one line and with circular search:

Next of

YourList.SkipWhile(x => x != NextOfThisValue).Skip(1).DefaultIfEmpty( YourList[0] ).FirstOrDefault();

Previous of

YourList.TakeWhile(x => x != PrevOfThisValue).DefaultIfEmpty( YourList[YourList.Count-1]).LastOrDefault();

This is a working example (link to the fiddle)

    List<string> fruits = new List<string> {"apple", "banana", "orange", "raspberry", "kiwi"};
    string NextOf = "orange";
    string NextOfIs;

    NextOfIs = fruits.SkipWhile(x => x!=NextOf).Skip(1).DefaultIfEmpty(fruits[0]).FirstOrDefault();
    Console.WriteLine("The next of " + NextOf + " is " + NextOfIs);

    NextOf = "kiwi";
    NextOfIs = fruits.SkipWhile(x => x!=NextOf).Skip(1).DefaultIfEmpty(fruits[0]).FirstOrDefault();
    Console.WriteLine("The next of " + NextOf + " is " + NextOfIs);

    string PrevOf = "orange";
    string PrevOfIs;

    PrevOfIs = fruits.TakeWhile(x => x!=PrevOf).DefaultIfEmpty(fruits[fruits.Count-1]).LastOrDefault();
    Console.WriteLine("The prev of " + PrevOf + " is " + PrevOfIs);

    PrevOf = "apple";
    PrevOfIs = fruits.TakeWhile(x => x!=PrevOf).DefaultIfEmpty(fruits[fruits.Count-1]).LastOrDefault();
    Console.WriteLine("The prev of " + PrevOf + " is " + PrevOfIs);

Answer 2

I have implemented this by Inheriting the .Net List

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

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

        public T MovePrevious
        {
            get { _currentIndex--; return this[CurrentIndex]; }
        }

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

Using this becomes quite easy

 NavigationList<string> n = new NavigationList<string>();
            n.Add("A");
            n.Add("B");
            n.Add("C");
            n.Add("D");
            Assert.AreEqual(n.Current, "A");
            Assert.AreEqual(n.MoveNext, "B");
            Assert.AreEqual(n.MovePrevious, "A");

Answer 3

This can be done using LinkedList<T>

List<int> intList = new List<int> { 1, 3, 5, 7, 9, 13 };

LinkedList<int> intLinkedList = new LinkedList<int>(intList);

Console.WriteLine("Next Value to 9 "+intLinkedList.Find(9).Next.Value);

Console.WriteLine("Next Value to 9 " +intLinkedList.Find(9).Previous.Value);

//Consider using dictionary for frequent use
var intDictionary = intLinkedList.ToDictionary(i => i, i => intLinkedList.Find(i));    

Console.WriteLine("Next Value to 9 " + intDictionary[9].Next.Value);

Console.WriteLine("Next Value to 9 " + intDictionary[9].Previous.Value);

Console.Read();

Answer 4

int index = list.IndexOf(9); // find the index of the given number

// find the index of next and the previous number
// by taking into account that 
// the given number might be the first or the last number in the list
int prev = index > 0 ? index - 1 : -1;

int next = index < list.Count - 1 ? index + 1 : -1;

int nextItem, prevItem;

// if indexes are valid then get the items using indexer 
// otherwise set them to a temporary value, 
// you can also use Nullable<int> instead
nextItem = prev != -1 ? list[prev] : 0;
prevItem = next != -1 ? list[next] : 0;

Answer 5

Approach with ElementOrDefault()

https://dotnetfiddle.net/fxVo6T

int?[] items = { 1, 3, 5, 7, 9, 13  };
for (int i = 0; i < items.Length; i++)
{
    int? previous = items.ElementAtOrDefault(i - 1);
    int? current = items.ElementAtOrDefault(i);
    int? next = items.ElementAtOrDefault(i + 1);
}

Answer 6

        List<int> listInts = new List<int>();
        listInts.AddRange(new int[] { 1, 3, 5, 7, 9, 13 });
        int index = listInts.IndexOf(3); //The index here would be "1"
        index++; //Check first if the index is in the length
        int element = listInts[index]; //element = 5