[swift-evolution] [Proposal] Add Binary Search functions to SequenceType

Nate Cook natecook at gmail.com
Wed Mar 16 01:17:32 CDT 2016

> On Mar 15, 2016, at 1:58 PM, Lorenzo Racca via swift-evolution <swift-evolution at swift.org> wrote:
>> On Mar 15, 2016, at 6:49 PM, Haravikk <swift-evolution at haravikk.me <mailto:swift-evolution at haravikk.me>> wrote:
>>> On 15 Mar 2016, at 15:48, Lorenzo Racca <lorenzo.racca at live.it <mailto:lorenzo.racca at live.it>> wrote:
>>> I already knew the impossibility of applying such a predicate as “$0 == 3” and I actually couldn’t quite figure out a solution.
>> I thought so, and I don’t think there is a way to do it, my point was really just that your swift doc comments weren’t clear on that point, then I went off at a bit of a tangent ;)
> No problem! What I am trying to figure out here is how we should implement the lowerBound and upperBound functions. Should they exactly reflect their C++ counterparts? 
> Anyway, it seems all of our implementations have the same problem, that they cannot be univocally called with any predicate whatsoever, (or at least it seemed to me during some tests with the implementations :) ), so I don’t really know how we should act. I am a little blocked.
> Does anyone have ideas on how that could work no matter what predicate is given? Especially, an upperBound() function, which is a little trickier. 

The key is to use a binary predicate (as used in sort and partition) instead of a unary predicate. Then you can use the predicate as is for lowerBound or with the arguments "reversed" for upperBound. The methods would have a similar signature to indexOf—one that just takes a value for comparable collections and one that takes a value and a predicate.

The binary search method can be implemented by finding the lower bound, which is by definition not less than the given value, then using the same predicate to check whether the value is not less than the lower bound. If neither is less than the other, you've found the value.


>>> On Mar 15, 2016, at 6:07 PM, Jeff Hajewski  <jeff.hajewski at gmail.come <mailto:jeff.hajewski at gmail.come>> wrote:
>>> I suspect there is an easy solution here and I'm just having a mental block...
> Jeff, I really do feel you, I’m in the same situation! 
> I think your solution could be applicable though, just in a little more complicated way than C++ did, which is to extract the complement of the predicate and act differently upon that. 
> As of now I don’t have the time to put down some code (time zone sucks) but will try asap.
> Lorenzo

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