Issue #10730 has been updated by Radan Skori.


> 
> I'm neutral to Array#bsearch_index, but how handy is it?  Symmetry/consistency is not a good reason for Ruby design.

Let me then tell you about my use case. There is a sparse array of dates and I want to slice out a part of it that falls within minimum and maximum date. It is then later used to retrieve same values associated with those dates for displaying a chart. In another use case, you have to find a first element that matches the criteria and then take next N elements to display them. Bsearch index would make both concies and also very fast oneliners:

ary.slice((ary.bsearch_index {|e| e>=min} || 0) .. (ary.bsearch_index {|e| e>=max} || ary.size)) 

ary.slice(ary.bsearch_index { |e| predicate(e) }, N)

When implementing it, I knew about bsearch and I actually just kind of assumed I also have bsearch_index and was surprised it's not there. That is what made me go check the ruby source and see if I can actually add the methodin a simple way. 

So the biggest reason why IMHO we need either BOTH or NONE is the principleof least surprise and not symetry or consistency.

But then again, maybe it's just me and not many other people would get the same idea. I'm open to being wrong on that point :)

> 
> http://stackoverflow.com/questions/23481725/using-bsearch-to-find-index-for-inserting-new-element-into-sorted-array
> 
> shows a somewhat reasonable use case, but the insert takes O(n).  Is it okay?  When you have to modify the array that is used for bsearch, rbtree gem might be a better choice; both search and insert take O(log n).
> 

Yes, I agree with you, that is not the best use case. However it shows how that person also thought that exact method will be there.


P.S. Bye the way, thanks for taking the time to discuss this issue. :)


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Feature #10730: Implement Array#bsearch_index
https://bugs.ruby-lang.org/issues/10730#change-50957

* Author: Radan Skori
* Status: Open
* Priority: Normal
* Assignee: 
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We currently have Array#bsearch but no Array#bsearch_index and to me it seems that violates the principle of least surprise, especially when we consider the other combinations of existing methods that find either an element or it°«s index.

For example: the method would be very useful when needing to slice out a part of a sorted array. It would allow very quick location of the indices of the beginning and end of the segment.

A quick google of °»ruby bsearch_index°… reveals that I am not the only one that needed and expected that function to exist:

http://stackoverflow.com/questions/23481725/using-bsearch-to-find-index-for-inserting-new-element-into-sorted-array
http://stackoverflow.com/questions/8672472/is-there-a-built-in-binary-search-in-ruby
https://github.com/tyler/binary_search#usage

The very good thing is that we can get that method almost for free since the current implementation of bsearch internally finds the index and then looks up the actual element.

I have opened a PR on the github mirror that adds bsearch_index in what seems to me the simplest way possible: https://github.com/ruby/ruby/pull/813 . 
I changed the bsearch implementation into bsearch_index and based the bsearch on it. 
Please Note: The diff is deceptively large, if you look carefully you will notice that the change is actually small and the actual binary search algorithm remained completely intact. 

I have kept the behaviour documentation on bsearch and simply referenced itfrom bsearch_index to minimize the documentation changes.



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