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

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 2

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);