Ruby: Sort array of objects based on array of integers

Question

This seems like its fairly simple, and should have been asked before, but everything I find on Stack Overflow doesn't seem to work. I have an array of 4 objects, and I'd like to re-order it in a particular order. So, it looks like this:

array = [Obj1, Obj2, Obj3, Obj4]

I have another array of integers which represent the desired order of the indices:

desired_order = [2,3,0,1]

So what I would like to see after ordering array properly is:

array = [Obj3, Obj4, Obj1, Obj2]

I've already figured sort_by is the method to use, but I can't seem to come up with the proper syntax. Any help is greatly appreciated!

possible duplicate of How to sort an array in Ruby to a particular order? — the Tin Man
– the Tin Man, Commented Dec 14, 2012 at 0:59
@theTinMan. Doesn't seem the same problem, here you already have the indexes. — tokland
– tokland, Commented Dec 14, 2012 at 8:08
I don't know what your task is, but it looks like array.rotate.rotate could help in this case. Maybe also in other cases? :-P — musicmatze
– musicmatze, Commented Dec 14, 2012 at 11:34
Yeah, this was an incredibly rare edge-case on dealing with an out of date CMS. I have no ability to alter how the data was entered, and it has to be rendered in a particular order. Only this one particular item in the entire system is in the wrong order... =\ — David Savage
– David Savage, Commented Dec 14, 2012 at 22:54

tokland · Accepted Answer · 2012-12-13 23:16:41Z

7

Array#values_at does exactly what you need:

array.values_at(*desired_order)

edited Dec 13, 2012 at 23:16

answered Dec 13, 2012 at 22:52

tokland

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5 Comments

David Savage Over a year ago

Any idea how this compares benchmark-wise to map? In this case it won't matter since I only have 4 items, but for future generations' sake...

tokland Over a year ago

Why a map would be better with more elements? it's exactly the same operation, only that the abstraction already exists in the core (and probably implemented in C)

David Savage Over a year ago

Wasn't claiming it was, I was asking the question..Didn't know this is using the same operation as map.

tokland Over a year ago

Yeah, I know, the thing is I don't understand why you are asking. Imagine that you have a million elements to retrieve, you still have an array of a million elements to map, don't you? so there is no difference conceptually. However, since the map is done at Ruby level, and values_at at C level, values_at should be faster. You can do the benchmarking but I'd very surprised if that's not the case.

David Savage Over a year ago

Could you edit this a bit? Looks like values_at requires a comma separated list of integers, whereas desired_order in my above question is an array of integers, so as written with my code above this would throw an error.

sawa · Accepted Answer · 2012-12-13 22:42:33Z

5

desired_order.map{|i| array[i]}

answered Dec 13, 2012 at 22:42

sawa

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3 Comments

David Savage Over a year ago

Ah ha! Thank you both, guess sort_by was not the correct method after all :)

sawa Over a year ago

You can also do it with sort_by, but it is not as straightforward: array.sort_by.with_index{|_, i| desired_order.index(i)} and is much less efficient.

steenslag Over a year ago

@Tokland's values_at is better.

Sergio Tulentsev · Accepted Answer · 2012-12-13 22:43:22Z

1

If you have indexes already, then you can just map them to objects:

array = %w[obj1 obj2 obj3 obj4]
desired_order = [2,3,0,1]

desired_order.map{|idx| array[idx]} # => ["obj3", "obj4", "obj1", "obj2"]

answered Dec 13, 2012 at 22:43

Sergio Tulentsev

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