bin_find.rb

def find(arr, value)
  l = 0
  r = arr.size - 1
  loop do
    return nil if l == r and arr[l] != value
    
    m = (l+r)/2

    if arr[m] < value
      l = m+1
    elsif arr[m] > value
      r = m-1
    else
      return m
    end
  end
end