Clean way to return array indices matching a condition using LINQ

I want to return all of the indices from an array where a condition is met. I've come up with

myarray
   .Select((p,i) => 
       {              
          if (SomeCondition(p)) return i; 
          else return -1;
       })
   .Where(p => p != -1);

I know I can write an exten开发者_如何学运维sion method or do it with a loop, but I was wondering if there was something built into LINQ that I'm unfamiliar with. Apologies if this is trivial

Personally, I'd go with this:

myarray
    .Select((p, i) => new { Item = p, Index = i })
    .Where(p => SomeCondition(p.Item))
    .Select(p => p.Index);

When I read the text of the question and not the sample:

int[] data = { 1, 2, 5, 6, };

var results = Enumerable.Range(0, data.Length).Where(i => data[i] > 2);

// results = { 2, 3 } 

Do you really need those -1s ?

I would probably write an extension method, but if I chose not to, I would do this:

array
  .Select((item, index) => SomeCondition(item) ? index : -1)
  .Where(index => index != -1);

It's exactly what you are doing already, but less verbose since it uses the ternary if operator instead of the if/else block.