Is there a built-in binary-search In Ruby?

Question

I am looking for a built-in Ruby method that has the same functionality as index but uses a binary search algorithm, and thus requires a pre-sorted array.

I know I could write my own implementation, but according to "Ruby#index Method VS Binary Search", the built-in simple iterative search used by index is faster than a pure-Ruby version of binary search, since the built-in method is written in C.

Does Ruby provide any built-in methods that do binary search?

No need to write your own: [tyler/binary_search](https://github.com/tyler/binary_search). The author has also taken the time to run some benchmarks. — sczizzo, Dec 29 '11 at 19:33
Hi sczizzo, I am new to ruby so this is a pretty newb question, but how do I add this functionality to my ruby installation? Is it just a matter of running the rakefile? Thanks. — Jonah, Dec 29 '11 at 19:53
Might be easier to use the `bsearch` gem, as Marc-André suggested. Then it's pretty much as simple as `gem install bsearch` on the command line, and `require 'bsearch'` in your Ruby. You might want to [look at the documentation for usage](http://rubydoc.info/gems/bsearch/1.5.0/frames). — sczizzo, Dec 29 '11 at 20:15
It should be noted that even if it is written in ruby any implementation of a binary search will eventually outperform any implementation of linear search, no matter how optimized, for large enough arrays. — sepp2k, Dec 29 '11 at 20:31

score 62 · Accepted Answer · edited Jan 26 '15 at 11:21

62

Ruby 2.0 introduced Array#bsearch and Range#bsearch.

For Ruby 1.9, you should look into the bsearch and binary_search gems. Other possibility is to use a different collection than an array, like using rbtree

bsearch is in my backports gem, but this is a pure Ruby version, so quite a bit slower. Note that a pure Ruby binary search will still be faster than a linear builtin search like index or include? for big enough arrays/ranges (or expensive comparisons), since it's not the same order of complexity O(log n) vs O(n).

To play with it today, you can require 'backports/2.0.0/array/bsearch' or require 'backports/2.0.0/range/bsearch'.

Good luck!

edited Jan 26 '15 at 11:21

Dennis

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answered Dec 29 '11 at 19:32

Marc-André Lafortune

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Hey Marc, Thanks for the answer. Are there any 3rd party libraries (written in C) that I could use? – Jonah Dec 29 '11 at 19:33
And @sczizzo linked to another gem too. – Marc-André Lafortune Dec 29 '11 at 19:52
Note that Array#bsearch has now been merged into Ruby: https://bugs.ruby-lang.org/issues/4766#change-32923 – ndbroadbent Nov 15 '12 at 09:44
Will this be faster than "include?" which seems to be a linear search? – Rahul Sep 02 '13 at 09:32
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@Rahul: Yes, `Array#include?` is linear so will be slower. – Marc-André Lafortune Sep 02 '13 at 15:32
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Is there a way to return the index instead of the value? – Translunar Aug 10 '15 at 20:30
That's a different question... Post separately and comment back here? – Marc-André Lafortune Aug 10 '15 at 23:24
I just tried this on Ruby 2.2.2 and would like to note it doesn't work with ranges of characters. Ex. ('a'..'z').bsearch {|letter| letter == 'b'} fails w/ "TypeError: can't do binary search for String" – frankpinto Dec 14 '16 at 20:01
@frankpinto You actually can, but you need to convert that range to an array first, and use >= rather than ==. This works: ('a'..'z').to_a.bsearch { |letter| letter >= 'b' } – Michael Feb 25 '19 at 15:57

score 12 · Answer 2 · answered Jul 01 '17 at 23:11

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A lot has changed since 2011, in Ruby 2.3, you can use bsearch_index

https://ruby-doc.org/core-2.3.0/Array.html#method-i-bsearch_index

array.bsearch_index { |val| query <=> val }

answered Jul 01 '17 at 23:11

Yoav Epstein

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What's wrong with this? `arr = ['a', 'b', 'c', 'd','e']` `arr.sort.bsearch { |x| 'a' == x }` – A H K Jan 24 '18 at 14:21
@AHK try arr.sort.bsearch{ |x| 'a' <=> x } – Dan Dec 27 '18 at 03:21

score 3 · Answer 3 · answered May 07 '18 at 13:28

I use bsearch. This is how it works:

array = ['one', 'two', 'three', 'four', 'five']

search = array.sort.bsearch { |value| 'four' <=> value }

Note: binary search needs a sorted array; this adds a li'l overhead but it's fine, compared to the speed of the search.

search will return the value four of the array, else nil if it doesn't find the value.

Is there a built-in binary-search In Ruby?

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