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?

Answer 1

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!

Answer 2

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 }

Answer 3

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.