What does the "*" mean in this recursive ruby function?

Question

I read this code that is about quicksort with monkey-patching for the Array class.

class Array
  def quicksort
    return [] if empty?

    pivot = delete_at(rand(size))
    left, right = partition(&pivot.method(:>))

    return *left.quicksort, pivot, *right.quicksort
  end
end

I don't know what the star (*) sign seen at the start of *left.quicksort is. Can't we just use left.quicksort?

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score 2 · Accepted Answer · answered Jan 29 '16 at 15:37

The star (in this case) stands for array unpacking. The idea behind it is that you want to get one array with the given elements, instead of array of array, element, array:

left  = [1, 2, 3]
pivot = 4
right = [5, 6, 7]

[left, pivot, right] # => [[1, 2, 3], 4, [5, 6, 7]]

[*left, pivot, *right] # => [1, 2, 3, 4, 5, 6, 7]

score 0 · Answer 2 · answered Jan 29 '16 at 15:39

The * takes a list of arguments and splits them into individual elements. This allows you to return one un-nested array even when the left and right themselves return an array.

Regarding can't we just use left.quicksort did you give it a try?

def a()
  return *[1,2,3], 4, *[5,6]
end

def b()
 return [1,2,3], 4, *[5,6]
end

b()
=> [[1, 2, 3], 4, 5, 6]
a()
=> [1, 2, 3, 4, 5, 6]

score 0 · Answer 3 · answered Jan 29 '16 at 15:40

Without the asterisk, you will have three values returned... the first and last value will be a single array with multiple values

[array1], pivot, [array2]

With the asterisk, the array values will be returned as separate components...

array1_value_1, array1_value_2, array1_value_3, ..., pivot, array2_value_1, array2_value2, array2_value_3, ...

What does the "*" mean in this recursive ruby function?

3 Answers3