Sorting arrays based on frequency in Ruby on Rails

Question

I have a nested array of numbers, arranged like this:

ids = [[5,8,10],[8,7,25],[15,30,32],[10,8,7]]

I only need a single array with all of the keys inside, without repeating, so I used this:

ids = ids.flatten.uniq

That produces this:

ids = [5,8,10,7,25,15,30,32]

Since I used .uniq, it eliminates the duplicate values. However, I'd like to order the values based on how often they appear in the sub-arrays, rather than whichever order they happen to be in -- so something like this:

ids = [8,10,7,5,25,15,30,32]

BroiSatse · Accepted Answer · 2015-04-16 16:12:07Z

7

This should do:

ids.flatten.group_by {|i| i}.sort_by {|_, a| -a.count}.map &:first

edited Apr 16, 2015 at 16:12

answered Apr 16, 2015 at 16:02

BroiSatse

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9 Comments

BroiSatse Over a year ago

@QPaysTaxes - Try running the command above step by step to see what is going on. Is there any particular element you are not sure about?

fr3d0 Over a year ago

i tried it but it says there's a syntax error in there some where and since i'm new to rails i don't get the code very much and for the life of me i cant see the error bro

anon Over a year ago

@BroiSatse I know what's going on, but if you just provide code then when someone has a similar but not identical problem, and doesn't get it, this whole answer is useless. If you explain yourself, others can use it as well.

BroiSatse Over a year ago

@fredo - How does the line with this code look like in your code? :) You might need to do map(&:first) in some cases.

BroiSatse Over a year ago

@Humza - Also, your answer has O(n^2) complexity while all the other answers are O(n). :P

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Saurabh · Accepted Answer · 2015-04-16 16:11:35Z

3

ids_flatten = [[5,8,10],[8,7,25],[15,30,32],[10,8,7]].flatten
ids_hist = ids_flatten.group_by{ |v| v }.map{ |k, v| [k, v.size] }
soreted_ids_hist = ids_hist.sort_by{|x| -x[1]}

soreted_ids_hist.map(&:first)
 => [8, 7, 10, 25, 5, 15, 30, 32]

edited Apr 16, 2015 at 16:11

answered Apr 16, 2015 at 16:02

Saurabh

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Comments

humza · Accepted Answer · 2015-04-16 17:00:58Z

3

Slow but clearly understandable (hopefully) to people new to Ruby:

flattened_ids = ids.flatten
unique_ids = flattened_ids.uniq
sorted_ids = unique_ids.sort_by { |e| -flattened_ids.count(e) }

edited Apr 16, 2015 at 17:00

answered Apr 16, 2015 at 16:05

humza

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Comments

Joe Kennedy · Accepted Answer · 2015-04-16 16:07:55Z

0

Probably not the best solution, but:

ids.flatten.each_with_object(Hash.new(0)) { |id, count| count[id] += 1 }.sort_by { |a| -a[1] }.map(&:first)

answered Apr 16, 2015 at 16:07

Joe Kennedy

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