How to custom sort with ruby

Question

I've got an array of objects which I pull from the database. But I can sort them only in ascending or descending order from database, however I need them in custom order.

Let's say I have an array of objects from db :

arr =  [obj1,obj2,obj3]

where obj1 has id 1, obj2 has id 2 and obj3 has id 3

but my sort order would be 3,1,2 or I'd have some array of ids which would dictate the order i.e [3,1,2]

So the order of custom sorting would be :

arr =  [obj3,obj1,obj2]

I've tried :

arr.sort_by{|a,b| [3,1,2]}

I've been reading some tutorials and links about sorting and it's mostly simple sorting. So how would one achieve the custom sorting described above?

Chuck · Accepted Answer · 2013-08-01 23:02:17Z

4

You're close. [3,1,2] specifies an ordering, but it doesn't tell the block how to relate it to your objects. You want something like:

arr.sort_by {|obj| [3,1,2].index(obj.id) }

So the comparison will order your objects sequentially by the position of their id in the array.

Or, to use the more explicit sort (which you seem to have sort_by slightly confused with):

arr.sort do |a,b|
  ordering = [3,1,2]
  ordering.index(a.id) <=> ordering.index(b.id)
end

answered Aug 1, 2013 at 23:02

Chuck

238k31 gold badges306 silver badges392 bronze badges

Sign up to request clarification or add additional context in comments.

3 Comments

yfeldblum Over a year ago

The performance of this sort is likely to be O(n^2 log n), which is very very bad performance.

Chuck Over a year ago

@yfeldblum: That is true and a good point, but for a case like this where you have a handful of elements you're ordering "by hand," the asymptotic behavior of the algorithm is not important — to wit, n is a constant when you'll only ever work with three elements, meaning any algorithm turns into O(1). Constant factors like computing and allocating a hash are likely to dominate your real-world performance with such small datasets.

yfeldblum Over a year ago

That is absolutely true with small datasets such as the one given in the questioner's example. My concern is twofold: (1) what if the dataset is larger than the example given and large enough to make this a real issue? and (2) it's good to provide examples of techniques that are fast in the larger cases so that other people can see what issues there are to think about and how to solve them.

yfeldblum · Accepted Answer · 2013-08-02 00:49:14Z

2

This is like @Chuck's answer, but with O(n log n) performance.

# the fixed ordering
ordering = [3, 1, 2]

# a map from the object to its position in the ordering
ordering_index = Hash[ordering.map(&:id).each_with_index.to_a]

# a fast version of the block
arr.sort_by{|obj| ordering_index[obj.id]}

answered Aug 2, 2013 at 0:49

yfeldblum

65.5k12 gold badges132 silver badges169 bronze badges

Collectives™ on Stack Overflow

How to custom sort with ruby

2 Answers 2

3 Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

3 Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related