Getting an array of keys from a hash ordered by the hash's values

Question

I have a hash which is something like this

{"red" => 3, "blue" => 1, "yellow" => 3, "green" => 1, "black" => 4}

I want to sort and display the top three colors in the order of the hash's values i.e

["black", "red", "yellow"]

I tried to do something like sort_by { |x,y| h[x] <=> h[y] } and max_by { |x,y| h[x] }, but I only get ["black"]. How do you get the top three occurrences?

See answers: http://stackoverflow.com/questions/4264133/descending-sort-by-value-of-a-hash-in-ruby — Ted Shaw, Aug 22 '12 at 04:18

Danil Speransky · Answer 1 · 2012-08-22T05:01:29.707

8

As variant:

h = {"green"=>1, "red"=>3, "yellow"=>3, "blue"=>1, "black"=>4}
h.sort_by{ |color, n| -n }.map(&:first).take(3) #=> ["black", "red", "yellow"]

edited Aug 22 '12 at 05:01

answered Aug 22 '12 at 04:15

Danil Speransky

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I like the use of `sort_by`. A bit more clever than my solution. – Blake Taylor Aug 22 '12 at 04:19
You could also `.map(&:first).take(3)` to cut down on the block noise. – mu is too short Aug 22 '12 at 05:00

score 3 · Answer 2 · answered Aug 22 '12 at 04:13

3

Here's one way to do it.

colors = {"red"=>3, "blue"=>1, "yellow"=>3, "green"=>1, "black"=>4}
colors.sort { |a, b| b[1] <=> a[1] }.map { |a| a[0] }.take(3)

answered Aug 22 '12 at 04:13

Blake Taylor

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Does that gel with *Ruby hashes are unordered*? – alex Aug 22 '12 at 04:14
Yes, sort will actually return an array of arrays. So yes. – Blake Taylor Aug 22 '12 at 04:16
I see, that makes a lot of sense :) – alex Aug 22 '12 at 04:19
b[1] <=> a[1] is pretty perl style for reverse sort, anyway, I like it. – Ted Shaw Aug 22 '12 at 04:24

score 2 · Answer 3 · answered Aug 22 '12 at 04:08

2

This seems to do it.

sortedColours = colours.keys
 .sort {|keyA, keyB| colours[keyB] - colours[keyA] }

Riddle.

If you want the top 3, just slice off the first 3.

firstThreeColours = sortedColours[0,3]

answered Aug 22 '12 at 04:08

alex

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`.take(3)` will give you first 3 elements. – xdazz Aug 22 '12 at 04:12
@xdazz Is that recommended over `[0,3]` ? – alex Aug 22 '12 at 04:12

Getting an array of keys from a hash ordered by the hash's values

3 Answers3