Sorting an array of arrays in Ruby

Question

I have an array of arrays like so:

irb(main):028:0> device_array => [["name1", "type1", ["A", "N", "N"], ["Attribute", "device_attribute"], 9], ["name2","type2", ["A", "N", "N"], ["Attribute", "device_attribute"], 7]] 

I would like to sort the entire device_array on the 4th element.

I've tried

AllDevicesController.all_devices.sort do | a,b |   for i in 0..(AllDevicesController.all_devices.length - 1) do     a[i][4] <=> b[i][4]   end end 

I've also tried:

AllDevicesController.all_devices.sort do | a,b |   a[][4] <=> b[][4] end 

Both methods have not worked.

I was using this as a reference: http://ariejan.net/2007/01/28/ruby-sort-an-array-of-objects-by-an-attribute/

I imagine I'm missing something rubyish that makes this really easy.

Answer 1

You can't use <=> with nil.

Your code should be something like this:

AllDevicesController.all_devices.sort do |a, b|   a[4].nil? ? -1 : b[4].nil? ? 1 : a[4] <=> b[4] end 

This will put the sub-arrays that have no element of index 4 at the beginning of the result. To do it the other way around, swap -1 with 1.

You could also use sort_by instead of sort. I think this has been introduced in Ruby 1.8.7 (so it might not work if you are using an older version). It goes something like:

AllDevicesController.all_devices.sort_by { |e| e.nil? ? 0 : e[4] } 

This will treat sub-arrays with no 4th element as if it was 0. Change this constant to suit you.

EDIT:

After you adjusted the input, it is now clear you were very close to the right answer. Your code should have been:

AllDevicesController.all_devices.sort do |a, b|   a[4] <=> b[4] end 

Or simple (assuming Ruby 1.8.7 or more):

AllDevicesController.all_devices.sort_by { |e| e[4] } 

In both cases, the variables a and b will contain elements of the original array, this is why you can directly access an element in any position (and you don't need something like a[][4], which is incorrect Ruby syntax).

Answer 2

sort_by

Instead of using spaceship operator (<=>) give a try to sort_by

device_array.sort_by { |el| el[4] } 

Though if you know that the forth element is the last one, you can use el.last too in the block.

Ruby docs: Enumerable#sort_by