Question

I was wondering if there was an Array method in Ruby that allows to filter an array based on another array or a bitmask.

Here is an example and a quick implementation for illustration purposes:

class Array
  def filter(f)
    res = []
    if f.is_a? Integer
      (0...self.size).each do |i|
        res << self[i] unless f[i].nil? || 2**i & f == 0
      end
    else
      (0...self.size).each do |i|
        res << self[i] unless f[i].nil? || f[i] == 0
      end
    end
    return res
  end
end

Example:

%w(a b c).filter([1, 0, 1]) ==> ['a', 'c']
%w(a b c).filter(4) ==> ['c']
%w(a b c).filter([1]) ==> ['a']

Thanks!

Answer 1

In ruby 1.9 Fixnum#[] gives you bit values at a particular position, so it will work for both integers and arrays. I’m thinking something like this:

class Array
    def filter f
        select.with_index { |e,i| f[i] == 1 }
    end
end

%w(a b c).filter([1, 0, 1]) #=> ['a', 'c']
%w(a b c).filter(4) #=> ['c']
%w(a b c).filter(5) #=> ['a', c']
%w(a b c).filter([1]) #=> ['a']