Question

I have the need to skip the first element that matches a predicate as a linq query. So far as I can tell the best I can do is something like this:

var list = new [] {1,2,3,4,5,6,7,8,9};
var skippedFirstOdd = false;
var skipFirstEvenNumber = list.SkipWhile(number => 
                                         {
                                             if(number % 2 != 0)
                                             {
                                                 skippedFirst = true;
                                                 return true;
                                             }
                                             else
                                             {
                                                 return false;
                                             }
                                         });

Which works (I think), but isn’t very elegant. Is there a cleaner way to achieve this?

Answer 1

You could write an iterator-block extension method:

public static IEnumerable<T> SkipFirstMatching<T>
      (this IEnumerable<T> source, Func<T, bool> predicate)
{        
    if (source == null)
        throw new ArgumentNullException("source");

    if (predicate == null)
        throw new ArgumentNullException("predicate");

    return SkipFirstMatchingCore(source, predicate);
}

private static IEnumerable<T> SkipFirstMatchingCore<T>
      (IEnumerable<T> source, Func<T, bool> predicate)
{            
    bool itemToSkipSeen = false;

    foreach (T item in source)
    {
        if (!itemToSkipSeen && predicate(item))
            itemToSkipSeen = true;

        else yield return item;
    }
}

And use it as:

var list = new [] { 1,2,3,4,5,6,7,8,9 };
var skipFirstEvenNumber = list.SkipFirstMatching(i => i % 2 == 0);

---

By the way, your current code doesn’t seem correct at all. The variable is called skipFirstEvenNumber, but the query is skipping odd numbers. Secondly, you appear to try to be using a side-effect to get the query to work, but you are only setting the flag variable, not reading it. Consequently, your current code should work just like a normal SkipWhile.

EDIT: Made argument-validation eager.