Delete array elements while looping in ruby

Question

I am trying to delete all elements from an array. For example:

@array = [1,2,3,4,5]
@array.each do |element|
  @array.delete(element)
end

Now, I understand why the code above does not work. However, I am tasked with deleting all elements of an array while using this delete_entry method:

def delete_entry(entry)
   @array.delete(entry)
end

I have read that removing elements in the midst of iteration is disallowed by design in ruby. Any ideas as to how I would go about deleting all elements from an array while using the delete_entry method in my implementation?

[`Array#delete_if`](http://ruby-doc.org/core-2.2.0/Array.html#method-i-delete_if)? — D-side, Jun 14 '15 at 14:46
That was my initial thought: `@array.delete_if { true }`. But I am required to use the `delete_entry` method. — country_dev, Jun 14 '15 at 14:48
I think it would be more accurate to say, "removing elements of a receiver while enumerating over them is dicouraged..." — Cary Swoveland, Jun 14 '15 at 17:02

Cary Swoveland · Answer 1 · 2015-06-14T23:34:31.317

3

def delete_entry(entry)
  @array.delete(entry)
end

@array = [1,2,3,4,5]

delete_entry(@array.first) until @array.empty?
@array #=> []

I think this reads better than using an enumerator and requires fewer operations when @array contains duplicate values.

edited Jun 14 '15 at 23:34

answered Jun 14 '15 at 16:18

Cary Swoveland

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`[nil, nil, false, false]`? – 7stud Jun 14 '15 at 21:51

score 2 · Answer 2 · answered Jun 14 '15 at 15:20

2

Loop through the array in reverse:

@array = [1,2,3,4,5]
@array.reverse_each do |element|
  @array.delete(element)
end
p @array # => []

answered Jun 14 '15 at 15:20

steenslag

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*Builds a temporary array and traverses that array in reverse order.* – 7stud Jun 14 '15 at 15:38
@7stud, [Array#reverse_each](http://ruby-doc.org/core-2.2.0/Array.html#method-i-reverse_each) is an enumerator, so no temporary array is constructed. When given a block, it returns the receiver, just as `Array.each` does. – Cary Swoveland Jun 14 '15 at 16:34
@Cary Swoveland, Yeah, I quoted the Enumerable docs--I didn't realize Array had a reverse_each(). – 7stud Jun 14 '15 at 17:02

score 1 · Answer 3 · answered Jun 14 '15 at 14:56

1

@array = [1,2,3,4,5]
@_array = @array.clone
@_array.each do |element|
  delete_entry(element)
end

answered Jun 14 '15 at 14:56

Prathan Thananart

4,007
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score 1 · Accepted Answer · edited May 23 '17 at 12:14

1

class Dog
  attr_reader :array

  def initialize
    @array = [1, 2, 3]
  end

  def delete_entry(entry)
     @array.delete(entry)
  end
end


d = Dog.new

d.array.length.times do 
  d.delete_entry(d.array[0])
end

p d.array

--output:--
[]

I have read that removing elements in the midst of iteration is disallowed by design in ruby.

Ridiculous. It just won't work as you may expect. Here's why:

deleting elements from an array while iterating over the array

edited May 23 '17 at 12:14

Community

1
1

answered Jun 14 '15 at 15:02

7stud

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2

What is the point of class creation? your approach as simple as these 2 lines: `@array = [1,2,3,4,5]; @array.size.times { |element| @array.delete(@array[0]) }` – Rustam Gasanov Jun 14 '15 at 15:20
@RustamA.Gasanov, What is the point of using an `@variable` at the top level in your code? – 7stud Jun 14 '15 at 15:37
in order not to create mysterious `Dog`s and just show the approach which op can apply to his code? If you don't like instance variables on top level what is stopping you from writing `array = [1,2,3,4,5]; array.size.times { |element| array.delete(array[0]) }` I just don't understand how `Dog` is related with question and artificial code complication – Rustam Gasanov Jun 14 '15 at 15:39
Do you ask what is the point of writing 2 lines of code instead of 20? :) – Rustam Gasanov Jun 14 '15 at 15:44
@Rustam A. Gasanov, ***If you don't like instance variables on top level what is stopping you from writing array = [1,2,3,4,5];*** Because that wasn't the op's code. Your example does the same thing to the main object and the Object class(ignoring private access) that my code does to a dog instance and the Dog class--except that you have exposed a global method to all objects instead of limiting its scope to the Dog class. OKAY?! The op's example seemed to come from inside a class, so I mimicked that class with the Dog class. – 7stud Jun 14 '15 at 16:16

Delete array elements while looping in ruby

4 Answers4