如何从列表中查找倒数第二个元素？

Question

I have a List<string> like:

 List<String> lsRelation = new List<String>{"99","86","111","105"}.

Now i want to find the Number 111 that is the second to last string.

So I have tried:

String strSecondLast=lsrelation.Last() - 2;

which doesn't work. So how can i find the second to last element of a List using Last() .

Answer 1

Use:

if (lsRelation.Count >= 2)
    secLast = lsRelation[lsRelation.Count - 2];

Answer 2

If you know that's an IList<T> which has an indexer:

string secondLast = null;
if (lsRelation.Count >= 2)
    secondLast = lsRelation[lsRelation.Count - 2];

You could create an extension like:

public static T SecondLast<T>(this IEnumerable<T> items)
{
    if (items == null) throw new ArgumentNullException("items");
    IList<T> list = items as IList<T>;
    if (list != null)
    {
        int count = list.Count;
        if (count > 1)
        {
            return list[count - 2];
        }
        else
            throw new ArgumentException("Sequence must contain at least two elements.", "items");
    }
    else
    {
        try
        {
            return items.Reverse().Skip(1).First();
        } catch (InvalidOperationException)
        {
            throw new ArgumentException("Sequence must contain at least two elements.", "items");
        }
    }
}

Then you can use it in this way:

string secondLast = lsRelation.SecondLast();

Answer 3

Starting with C# 8.0 you can use Index to get access to elements relative to the end of sequence:

if (lsRelation.Count >= 2)
    secLast = lsRelation[^2];

See docs for more information

Answer 4

You can use ElementAt(list.Count - 2) :

List<String> lsRelation = new List<String> { "99", "86", "111", "105" };
Console.WriteLine(lsRelation.ElementAt(lsRelation.Count - 2)); // 111

Answer 5

There are lots of options for doing this. Just to mention one that I've not seen here yet:

List<string> lsRelation = new List<String>{"99","86","111","105"};
String strSecondLast = lsRelation.Skip(lsRelation.Count() - 2).First();

Answer 6

You can't do this using Last() . Try this. You take the length of the list and subtract 2:

if (lsRelation.Count >= 2)
{
    var item = lsRelation[lsRelation.Count - 2];
}

Edit:

Based on the comment, here's an example using the Last() method, which would be ridiculous to use:

if (lsRelation.Count >= 2)
{
    var item = lsRelation.Last(x => x == lsRelation[lsRelation.Count - 2]);
}

Answer 7

Ambiguous question - what if your list has only one element? what if your list is empty? what if your list is null?

the obvious answer to a List would be

var beforeLast = lsRelation[lsRelation.Count -2];

to cover all your edge cases, I would use

string beforeLast = null;
if (lsRelation != null && lsRelation.Count > 1)
   beforeLast = lsRelation[lsRelation.Count - 2];

if it's an IEnumerable<string> ,

string beforeLast = null;
if (lsRelation != null) {
  int l_count = lsRelation.Count();
  if (l_count > 1)
     beforeLast = lsRelation.Skip(l_count - 2).Take(1);
}