Sort array using custom sorting preferences?

Question

Can someone show me how to sort a nested array based on custom strings? For example, is there a way to sort:

[['Red','Blue'],['Green','Orange'],['Purple','Yellow']]

by "Orange", "Yellow", and then "Blue"? The end result would look like this:

[['Green','Orange'],['Purple','Yellow'],['Red','Blue']]

It's not sorting by alphabetical order. I'm curious to know if I can define the values to sort by to accomplish the goal above.

Answer 1

sort_by is always very handy for that kind of sorting:

a = [['Red','Blue'],['Green','Orange'],['Purple','Yellow']]
order_array = ['Orange', 'Yellow', 'Blue']

p a.sort_by { |arr| order_array.index(arr[1]) }

# => [["Green", "Orange"], ["Purple", "Yellow"], ["Red", "Blue"]]

Answer 2

This is a task for group_by and values_at:

ORDER = %w[Orange Yellow Blue]
ary = [['Red','Blue'],['Green','Orange'],['Purple','Yellow']]

ary.group_by{ |a| a.last }.values_at(*ORDER)
# => [[["Green", "Orange"]], [["Purple", "Yellow"]], [["Red", "Blue"]]]

Here's what group_by brings to the party:

ary.group_by{ |a| a.last }
# => {"Blue"=>[["Red", "Blue"]],
#     "Orange"=>[["Green", "Orange"]],
#     "Yellow"=>[["Purple", "Yellow"]]}

Once you have the hash of values used to group each array, then values_at makes it easy to extract them in the right order.

This is an extremely fast and efficient way to do the task as it will barely slow down as ary grows due to the fact that there is no real sorting going on, it's just grouping by a value, then extracting from the hash in a given order.

If you want the exact same array-of-arrays as in your example, flatten the result once:

ary.group_by{ |a| a.last }.values_at(*ORDER).flatten(1)
# => [["Green", "Orange"], ["Purple", "Yellow"], ["Red", "Blue"]]

You won't want to do that if there are going to be multiple "Orange", "Yellow" or "Blue" elements as the result won't be very usable.