How to insert a new element in between all elements of a Ruby array?

Question

I have an Array and want to insert a new element in between all elements, someway like the join method. For example, I have

[1, [], "333"]

and what I need is

[1, {}, [], {}, "333"]

Note a new empty hash was inserted in between all elements.

Edit: Currently what I have is:

irb(main):028:0> a = [1, [], "333"]
=> [1, [], "333"]
irb(main):029:0> a = a.inject([]){|x, y| x << y; x << {}; x}
=> [1, {}, [], {}, "333", {}]
irb(main):030:0> a.pop
=> {}
irb(main):031:0> a
=> [1, {}, [], {}, "333"]
irb(main):032:0>

I want to know the best way.

x.push(y, {}) is shorter, both << and push return array, so you don't need ;x, inject is slow — Victor Moroz
– Victor Moroz, Commented Feb 23, 2012 at 22:31

tokland · Accepted Answer · 2016-08-21 21:21:01Z

18

[1, 2, 3].flat_map { |x| [x, :a] }[0...-1]
#=> [1, :a, 2, :a, 3]

FYI, that function is called intersperse (at least in Haskell).

[Update] If you want to avoid the slice (that created a copy of the array):

[1, 2, 3].flat_map { |x| [x, :a] }.tap(&:pop)
#=> [1, :a, 2, :a, 3]

edited Aug 21, 2016 at 21:21

answered Feb 23, 2012 at 22:30

tokland

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3 Comments

Victor Moroz Over a year ago

I wouldn't do [0..-1] in Haskell (and probably in Ruby too)

Sean Hill Over a year ago

@VictorMoroz, what would you do instead?

tokland Over a year ago

@VictorMoroz: Added an inplace snippet. Ruby has no abstraction for this, so I've used tap+pop.

ejoubaud · Accepted Answer · 2013-01-23 07:45:25Z

2

Another similar solution uses #product :

[1, 2, 3].product([{}]).flatten(1)[0...-1]
# => [ 1, {}, 2, {}, 3 ]

answered Jan 23, 2013 at 7:45

ejoubaud

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Comments

joslinm · Accepted Answer · 2016-12-01 15:44:37Z

2

One approach is to zip another array of desired elements and then flatten it with depth = 1:

> arr = [1, [], "333"]
> element = {}
> interspersed = arr.zip([element] * (arr.size - 1)).flatten(1).compact
> # [1, {}, [], {}, "333" ]

You can extend Array to make this behavior more accessible.

class Array
  def intersperse(elem)
    self.zip([elem] * (self.size - 1)).flatten(1).compact
  end
end

e.g.,

[43] pry(main)> [1,2,3].intersperse('a')
=> [1, "a", 2, "a", 3]

answered Dec 1, 2016 at 15:44

joslinm

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Comments

Victor Moroz · Accepted Answer · 2012-02-23 22:26:39Z

1

a = [1,2,3]
h, *t = a
r = [h]
t.each do |e|
  r.push({}, e)
end
r #=> [1, {}, 2, {}, 3]

answered Feb 23, 2012 at 22:26

Victor Moroz

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Comments

Sean Hill · Accepted Answer · 2012-02-23 22:33:29Z

1

You could do something like:

a = [1, [], "333"]
new_a = a.collect {|e| [e, {}]}.flatten(1)
=> [1, {}, [], {}, "333", {}]

You need to do .flatten(1) because it will flatten your blank array without it.

Or as @David Grayson says in the comment, you can do a flat_map which will do the same thing.

a.flat_map {|e| [e, {}]}
=> [1, {}, [], {}, "333", {}]

@tokland has the correct answer if the last {} is not necessary. You return a slice from 0 to length - 1 or [0..-1].

edited Feb 23, 2012 at 22:33

answered Feb 23, 2012 at 22:27

Sean Hill

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3 Comments

David Grayson Over a year ago

Shorter version: a.flat_map { |e| [e, {}] }

Sean Hill Over a year ago

Oh, nice! And it doesn't flatten the blank array. Good call.

Victor Moroz Over a year ago

only last {} was not required

pguardiario · Accepted Answer · 2012-02-23 23:25:01Z

1

Another one that's similar to Tokland's:

xs.inject([]){|x,y| x << y << {}}[0...-1]

answered Feb 23, 2012 at 23:25

pguardiario

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dubadub · Accepted Answer · 2017-09-19 18:38:25Z

1

[1, 2, 3, 4, 5].inject { |memo, el| Array(memo) << {} << el }
#=> [1, {}, 2, {}, 3, {}, 4, {}, 5]

inject will use the first element to start with, so you don't need to mess with indices.

answered Sep 19, 2017 at 18:38

dubadub

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Comments

Just a learner · Accepted Answer · 2012-02-23 23:19:56Z

irb(main):054:0* [1, 2, 3, 4, 5, 6, 7, 8, 9].each_slice(1).flat_map {|e| e << "XXX"}[0...-1]
=> [1, "XXX", 2, "XXX", 3, "XXX", 4, "XXX", 5, "XXX", 6, "XXX", 7, "XXX", 8, "XXX", 9]
irb(main):055:0> [1, 2, 3, 4, 5, 6, 7, 8, 9].each_slice(2).flat_map {|e| e << "XXX"}[0...-1]
=> [1, 2, "XXX", 3, 4, "XXX", 5, 6, "XXX", 7, 8, "XXX", 9]
irb(main):056:0> [1, 2, 3, 4, 5, 6, 7, 8, 9].each_slice(3).flat_map {|e| e << "XXX"}[0...-1]
=> [1, 2, 3, "XXX", 4, 5, 6, "XXX", 7, 8, 9]
irb(main):057:0> [1, 2, 3, 4, 5, 6, 7, 8, 9].each_slice(4).flat_map {|e| e << "XXX"}[0...-1]
=> [1, 2, 3, 4, "XXX", 5, 6, 7, 8, "XXX", 9]
irb(main):058:0> [1, 2, 3, 4, 5, 6, 7, 8, 9].each_slice(5).flat_map {|e| e << "XXX"}[0...-1]
=> [1, 2, 3, 4, 5, "XXX", 6, 7, 8, 9]
irb(main):059:0> [1, 2, 3, 4, 5, 6, 7, 8, 9].each_slice(6).flat_map {|e| e << "XXX"}[0...-1]
=> [1, 2, 3, 4, 5, 6, "XXX", 7, 8, 9]
irb(main):060:0> [1, 2, 3, 4, 5, 6, 7, 8, 9].each_slice(7).flat_map {|e| e << "XXX"}[0...-1]
=> [1, 2, 3, 4, 5, 6, 7, "XXX", 8, 9]
irb(main):061:0> [1, 2, 3, 4, 5, 6, 7, 8, 9].each_slice(8).flat_map {|e| e << "XXX"}[0...-1]
=> [1, 2, 3, 4, 5, 6, 7, 8, "XXX", 9]
irb(main):062:0> [1, 2, 3, 4, 5, 6, 7, 8, 9].each_slice(9).flat_map {|e| e << "XXX"}[0...-1]
=> [1, 2, 3, 4, 5, 6, 7, 8, 9]
irb(main):063:0>

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