Ruby equilibrium index - odd usage of inject

Question

I spent a bit of time today tackling the equilibrium index problem (described here)

After writing my own solution (which performed badly with large numbers), I decided to find one that would be a perfect score. I found this (which Codility scores as 100/100):

  def equi(a)
    left, right = 0, a.inject(0, &:+)
    indices = []
    a.each_with_index do |val, i|
      right -= val

      indices << i if right == left

      left += val
    end
    indices
  end

What I don't understand is the piece of parallel assignment and use of inject at the top of the method. Is anyone able to describe what this is doing?

Many thanks! Stu

score 2 · Accepted Answer · edited Jun 23 '12 at 11:33

2

It assigns 0 to left and the sum of a's elements to right. The 0 argument is there because otherwise an empty array would return nil. The shorthand used for summing is Symbol#to_proc and is unnecessary because inject directly takes a symbol as its argument - inject(0, :+).

edited Jun 23 '12 at 11:33

KL-7

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answered Jun 23 '12 at 11:27

Michael Kohl

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Thanks a lot for the explanation @Michael Kohl. I need to read up on parallel assignment. One thing that made this extra weird is that, when I stuck a debugger after the assignment line, the values of 'left' and 'right' are both 0. Shouldn't I expect 'right' to be the sum of the vals in the array at that stage? Or is it evaluated later on? – Stu Jun 23 '12 at 11:46
Parallel assignment isn't exactly special. `right` should only be 0 if `a` is empty though. – Michael Kohl Jun 23 '12 at 12:04

score 0 · Answer 2 · answered Jun 23 '12 at 11:25

0

It's just a sum of the array:

>> ar = (1..10).to_a
=> [1, 2, 3, 4, 5, 6, 7, 8, 9, 10] 
>> ar.inject(0, &:+)
=> 55 
# or a shorter version
>> ar.inject(&:+)
=> 55

You should really read the doc on the Enumerable#inject method. It explains everything.

answered Jun 23 '12 at 11:25

KL-7

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The shorter version returns `nil` if `ar == []`. – Michael Kohl Jun 23 '12 at 11:29
Appreciate the answer on the inject part. I had read about inject, but it turns out it was the parallel assignment that made it most confusing – Stu Jun 23 '12 at 11:36

Ruby equilibrium index - odd usage of inject

2 Answers2