Linq Expression Get Predicated Items From Last With a Function

Question

Are there any Linq expression exists that gives a predicated list from end of the source list.

i.e: "abc1zxc".ToCharArray().SomeMagicLinq(p=>Char.IsLetter(p));

should give "zxc"

Possible duplicate of [Using Linq to get the last N elements of a collection?](http://stackoverflow.com/questions/3453274/using-linq-to-get-the-last-n-elements-of-a-collection) — 15ee8f99-57ff-4f92-890c-b56153, Sep 22 '16 at 13:45
Not exactly, it gives last N element, but I asked a query(number of element unknown). — eakgul, Sep 22 '16 at 13:47
Ohhh, right, sorry, my bad. This would do it: `"abc1zxc".Reverse().TakeWhile(c => char.IsLetter(c)).Reverse()`. — 15ee8f99-57ff-4f92-890c-b56153, Sep 22 '16 at 13:51

Tim Schmelter · Accepted Answer · 2016-09-22T14:05:49.263

1

You could use this approach:

var lastLetters = "abc1zxc".Reverse().TakeWhile(Char.IsLetter).Reverse();
string lastLettersString = new String(lastLetters.ToArray());

Not the most efficient way but working and readable.

If you really need it as a single (optimized) method you could use this:

public static IEnumerable<TSource> GetLastPart<TSource>(this IEnumerable<TSource> source, Func<TSource, bool> predicate)
 {
    var buffer = source as IList<TSource> ?? source.ToList();
    var reverseList = new List<TSource>();
    for (int i = buffer.Count - 1; i >= 0; i--)
    {
        if (!predicate(buffer[i])) break;
        reverseList.Add(buffer[i]);
    }
    for (int i = reverseList.Count - 1; i >= 0; i--)
    {
        yield return reverseList[i];
    }
}

Then it's more concise:

string lastLetters = new String("abc1zxc".GetLastPart(Char.IsLetter).ToArray());

edited Sep 22 '16 at 14:05

answered Sep 22 '16 at 13:50

Tim Schmelter

450,073
74
686
939

That works, but uses 3 Linq expression. I asked to do it in one :( – eakgul Sep 22 '16 at 13:53
@Kowalski: it doesn't use 3 [query expressions](https://msdn.microsoft.com/en-us/library/bb397676.aspx) but one. It uses three LINQ methods but there is only one expression(the first line). But you are free to create a new method that does this all in one call. – Tim Schmelter Sep 22 '16 at 13:54
@Kowalski. added a one method approach – Tim Schmelter Sep 22 '16 at 14:06

Linq Expression Get Predicated Items From Last With a Function

1 Answers1