Question

Filter by criterion only

changes.Where(p => Evaluate(p)).Subscribe(p => { // Do something });

But how could you make it so that you get the criterion value and n values after the criterion is fulfilled (and these n values do not have to match the evaluation criterion)?

• e.g. i would like to subscribe to a stream that returns on Evaluate(p) and one value after that (and then starts to evaluate p again)

Answer 1

Please take a look on SkipWhile and Take extension methods of IEnumerable. You can try following code:

changes.SkipWhile(change => Evaluate(change) == false).Take(n).Subscribe(change => { /* do something */ });

Edit

New code to take all matching items from sequence with n items’ tail (without retrieving items repeatedly)

// Let's assume elements in the sequence are of type Change
int i = 0;
Func<Change, bool> evaluateWithTail = change =>
{
    if (i <= 0 || i > n)
    {
        i = Evaluate(change) ? 1 : 0;
    }
    else
    {
        i++;
    }

    return i > 0;
}
// Please note delegate is specified as parameter directly - without lambda expression
changes.Where(evaluateWithTail).Subscribe(change => { /* do something */ });