How to sort a Ruby Hash by number value?

Question

I have a counter hash that I am trying to sort by count. The problem I am running into is that the default Hash.sort function sorts numbers like strings rather than by number size.

i.e. Given Hash:

metrics = {"sitea.com" => 745, "siteb.com" => 9, "sitec.com" => 10 }

Running this code:

metrics.sort {|a1,a2| a2[1]<=>a1[1]}

will return a sorted array:

[ 'siteb.com', 9, 'sitea.com', 745, 'sitec.com', 10]

Even though 745 is a larger number than 9, 9 will appear first in the list. When trying to show who has the top count, this is making my life difficult. :)

Any ideas on how to sort a hash (or an array even) by number value size?

I appreciate any help.

what ruby version do you use? your sort result is very strange — fl00r, Mar 29 '10 at 19:39

Marc-André Lafortune · Accepted Answer · 2015-01-16T17:10:44.883

313

No idea how you got your results, since it would not sort by string value... You should reverse a1 and a2 in your example

Best way in any case (as per Mladen) is:

metrics = {"sitea.com" => 745, "siteb.com" => 9, "sitec.com" => 10 }
metrics.sort_by {|_key, value| value}
  # ==> [["siteb.com", 9], ["sitec.com", 10], ["sitea.com", 745]]

If you need a hash as a result, you can use to_h (in Ruby 2.0+)

metrics.sort_by {|_key, value| value}.to_h
  # ==> {"siteb.com" => 9, "sitec.com" => 10, "sitea.com", 745}

edited Jan 16 '15 at 17:10

answered Mar 29 '10 at 18:55

Marc-André Lafortune

78,216
16
166
166

91

or simply `sort_by{|k,v| v}` – Mladen Jablanović Mar 29 '10 at 19:01
My number was returning as a string, that fixed it.. I had a2 and a1 in that order because I wanted the results to sort decending.. thanks for your feedback though. – Dustin M. Mar 29 '10 at 22:18
one note is that if your keys are symbols you might have to convert them to strings. – engineerDave May 14 '13 at 14:02
@engineerDave: Did you mean if your values are symbols? In any case, that's incorrect as `Symbol`s are comparable to one another. – Marc-André Lafortune May 14 '13 at 14:46
@Marc-AndréLafortune: Not if they can't be coerced to strings? I'm a little stymied on this one. :) Welcoming any clarification. Perhaps a visual example can clarify. https://gist.github.com/nacengineer/5577868 – engineerDave May 14 '13 at 17:43
@engineerDave: No idea how you get your results. `h2.sort` works in Ruby 1.9x or 2.0. – Marc-André Lafortune May 14 '13 at 17:50
@Marc-AndréLafortune: nevermind. must have been Gremlins in irb. its working now. thanks! – engineerDave May 14 '13 at 19:17
don't forget you get an [] back instead of an {} – Redoman May 27 '13 at 01:39
if you want to revert it to hash for, I found the following working: `metrics.sort_by{ |k, v| v }.inject(Hash.new){ |h, a| h.merge!({a[0] => a[1]}) }` – Elchin Jan 13 '15 at 04:51
21

@Elchin: you can use `metrics.sort_by{ |k, v| v }.reverse.to_h` – Marc-André Lafortune Jan 13 '15 at 08:04
4

Actually even simpler as: hash.sort_by(&:last) with the same caveat about getting an array of pairs vs. a Hash. – Gerry Gleason May 18 '15 at 13:55
Hi @Marc-AndréLafortune, I have a small question why add a underscore at begin of key. This coding style mean what? – lazybios Sep 17 '15 at 08:12
@lazybios See here: http://stackoverflow.com/questions/8689673/marking-an-unused-block-variable – Marc-André Lafortune Sep 17 '15 at 15:04
2

for reverse sort you may use `metrics.sort_by{|k,v| -v}` – bragboy Nov 02 '17 at 06:28
Awesome. I came up with this way which is ugly, IMO `metrics.invert.sort.reverse.to_h.invert` Using the `#invert` method of `Hash` class – Francisco Quintero Mar 20 '20 at 16:32

shock_one · Answer 2 · 2014-12-04T09:30:23.327

102

Since value is the last entry, you can do:

metrics.sort_by(&:last)

edited Dec 04 '14 at 09:30

answered Apr 07 '13 at 10:59

shock_one

5,845
3
28
39

5

This is awesome! any referecne for the pre-definition of `&:last`? – Tamer Shlash Sep 07 '15 at 12:59
3

`sort_by(&:last)` is effectively shorthand for `sort_by {|x| x.last}` https://stackoverflow.com/questions/1217088/what-does-mapname-mean-in-ruby – Beni Cherniavsky-Paskin Aug 07 '17 at 16:06

score 15 · Answer 3 · answered Mar 30 '10 at 09:10

15

Already answered but still. Change your code to:

metrics.sort {|a1,a2| a2[1].to_i <=> a1[1].to_i }

Converted to strings along the way or not, this will do the job.

answered Mar 30 '10 at 09:10

dimitarvp

2,316
2
20
29

score 10 · Answer 4 · answered Mar 29 '10 at 19:01

10

That's not the behavior I'm seeing:

irb(main):001:0> metrics = {"sitea.com" => 745, "siteb.com" => 9, "sitec.com" =>
 10 }
=> {"siteb.com"=>9, "sitec.com"=>10, "sitea.com"=>745}
irb(main):002:0> metrics.sort {|a1,a2| a2[1]<=>a1[1]}
=> [["sitea.com", 745], ["sitec.com", 10], ["siteb.com", 9]]

Is it possible that somewhere along the line your numbers are being converted to strings? Is there more code you're not posting?

answered Mar 29 '10 at 19:01

Jacob Mattison

50,258
9
107
126

Ahh your right it looks like the result in my code was returning it as a string. Pesky data types. :) Sometimes I am just too close to the problem. Thanks. – Dustin M. Mar 29 '10 at 22:18
2

Yup. Occasionally I hear someone refer to Ruby as "untyped". Oh, no, it's definitely typed. It's just not statically typed. :) – Jacob Mattison Mar 30 '10 at 01:05

How to sort a Ruby Hash by number value?

4 Answers4

Linked