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

Question

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.

make sure to see the updated answer as one before did not work for step > 2 — tihom
– tihom, Commented Oct 11, 2013 at 12:37

Sergio Tulentsev · Accepted Answer · 2013-10-11 12:12:25Z

3

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]

answered Oct 11, 2013 at 12:12

Sergio Tulentsev

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tihom · Accepted Answer · 2013-10-11 12:34:01Z

2

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]

edited Oct 11, 2013 at 12:34

answered Oct 11, 2013 at 12:13

tihom

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2 Comments

Alexander Popov Over a year ago

I like the short conciseness of your solution, but the method returns exactly the opposite of what I want - it returns every nth element and I want it to omit every nth element.

tihom Over a year ago

@AlexPopov changed by solution as finally got what you meant...the previous one did not work for steps more than 2

bsd · Accepted Answer · 2013-10-11 12:45:03Z

2

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.

answered Oct 11, 2013 at 12:45

bsd

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3 Comments

tihom Over a year ago

use self.size -1 else it will return nil for last element in some cases.

bsd Over a year ago

@tihom I used ...(half open interval)

tihom Over a year ago

nice..did not know that syntax...my bad

sawa · Accepted Answer · 2013-10-11 13:05:14Z

1

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

answered Oct 11, 2013 at 13:05

sawa

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Comments

steenslag · Accepted Answer · 2013-10-11 14:04:53Z

1

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]

answered Oct 11, 2013 at 14:04

steenslag

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Comments

Stefan · Accepted Answer · 2013-10-14 08:44:15Z

1

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

answered Oct 14, 2013 at 8:44

Stefan

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