Get index of array element faster than O(n)

Question

Given I have a HUGE array, and a value from it. I want to get index of the value in array. Is there any other way, rather then call Array#index to get it? The p

Answer 1

If your array has a natural order use binary search.

Use binary search.

Binary search has O(log n) access time.

Here are the steps on how to use binary search,

• What is the ordering of you array? For example, is it sorted by name?
• Use bsearch to find elements or indices

Code example

# assume array is sorted by name!

array.bsearch { |each| "Jamie" <=> each.name } # returns element
(0..array.size).bsearch { |n| "Jamie" <=> array[n].name } # returns index

Answer 2

If it's a sorted array you could use a Binary search algorithm (O(log n)). For example, extending the Array-class with this functionality:

class Array
  def b_search(e, l = 0, u = length - 1)
    return if lower_index > upper_index

    midpoint_index = (lower_index + upper_index) / 2
    return midpoint_index if self[midpoint_index] == value

    if value < self[midpoint_index]
      b_search(value, lower_index, upper_index - 1)
    else
      b_search(value, lower_index + 1, upper_index)
    end
  end
end