CODEWITHSUNDEEP
c#
ptf
ptf

Reputation: 3370

Lazily filtering list

I have a list of items, and I want to select 3 items from that list that fulfills a predicate.

I've tried this: (examples)

list.Where(x => x == 2).Take(3)

list.Take(3).Where(x => x == 2)

But of course they don't work.

Is there a mechanism that only selects elements that meets a predicate until the .Take(3) is fulfilled?

EDIT: The problem with the first example is that it first selects EVERY element that matches, then takes the first 3. I only want it to select until it has taken 3. I suppose it only really matters on huge collections, but still. Or that's how I understand it anyway.

This happens: (edit: I was wrong about this)

[1, 2, 2, 2, 3, 4, 5, 2].Where(x => x == 2) => [2, 2, 2, 2].Take(3) => [2, 2, 2]

I want something like: [1, 2, 2, 2, 3, 4, 5, 2].Where(x => x == 2, 3) => [2, 2, 2], where the 3 is the number of elements it matches before it stops filtering.

Upvotes: 2

Views: 73

Answers (1)

Tim Schmelter
Tim Schmelter

Reputation: 460208

I assume that you've not understood linq's deferred execution. Your first approach does exactly what you want:

list.Where(x => x == 2).Take(3)

It checks every item but just until it has found three items matching the predicate.

Have a look at Eric Lipperts answer on my question since he explains it very well: https://stackoverflow.com/a/10110269/284240

This is also worth reading: http://blogs.msdn.com/b/charlie/archive/2007/12/09/deferred-execution.aspx

Upvotes: 8

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