How to refer the index of an element in Array#delete_if

问题

I want to build a custom method Array#drop_every(n) (I know it's monkey patching, I am doing this for a homework), which returns a new array omitting every nth element:

[4, 8, 15, 16, 23, 42].drop_every(2) # [4, 15, 23]

I want to implement it with Array#delete_if, but by referring to the index and not to the element itself, (similar to each_index) something like this:

def drop_every(step)
  self.delete_if { |index| index % step == 0 }
end

How do I do this? I don't insist on using delete_if, I also looked at drop_while and reject, other suggestions are welcome.

回答1:

def drop_every(step)
  reject.with_index { |x,i| (i+1) % step == 0 }
end

[4, 8, 15, 16, 23, 42].reject.with_index{|x,i| (i+1) % 2 ==  0}
# => [4, 15, 23]

[4, 8, 15, 16, 23, 42].reject.with_index{|x,i| (i+1) % 3 ==  0}
# => [4, 8, 16, 23] 

回答2:

You can use with_index method that returns enumerator, filter your collection and then get rid of the indexes.

class Array
  def drop_every(step)
    self.each.with_index.select { |_, index| index % step == 0 }.map(&:first)
  end
end

[4, 8, 15, 16, 23, 42].drop_every(2) # => [4, 15, 23]

回答3:

You could use the values_at method to selectively filter out indices which you want.

class Array
  def drop_every(step)
    self.values_at(*(0...self.size).find_all{ |x| (x+1) % step != 0 })
  end    
end

The answer was accepted while I was typing it. I will post it anyways.

回答4:

def drop_every step
  delete_if.with_index(1){|_, i| i.%(step).zero?}
end

回答5:

class Array
  def drop_every(step)
    self.each_slice(step).flat_map{|slice| slice[0..-2]}
  end
end

p [4, 8, 15, 16, 23, 42].drop_every(2) #=> [4, 15, 23]

回答6:

I'd extend the Enumerable mixin instead:

module Enumerable
  def drop_every(step)
    return to_enum(:drop_every, step) unless block_given?
    each.with_index(1) do |o, i|
      yield o unless i % step == 0
    end
  end
end

(1..10).drop_every(3) { |a| p a }
# outputs below
1
2
4
5
7
8
10

来源：https://stackoverflow.com/questions/19317782/how-to-refer-the-index-of-an-element-in-arraydelete-if