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.

Solution 1

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}

Solution 2

Since value is the last entry, you can do:

metrics.sort_by(&:last)

Solution 3

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.

Solution 4

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?

View more solutions

Comments

[fl00r]

what ruby version do you use? your sort result is very strange

[Mladen Jablanović]

or simply sort_by{|k,v| v}

[Dustin M.]

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.]

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.

[Jacob Mattison]

Yup. Occasionally I hear someone refer to Ruby as "untyped". Oh, no, it's definitely typed. It's just not statically typed. :)

[Hady Elsahar]

this is why i love Ruby ... such problem is pain in a head in other languages. so simple

[engineerDave]

one note is that if your keys are symbols you might have to convert them to strings.

[Marc-André Lafortune]

@engineerDave: Did you mean if your values are symbols? In any case, that's incorrect as Symbols are comparable to one another.

[engineerDave]

@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. gist.github.com/nacengineer/5577868

[Marc-André Lafortune]

@engineerDave: No idea how you get your results. h2.sort works in Ruby 1.9x or 2.0.

[engineerDave]

@Marc-AndréLafortune: nevermind. must have been Gremlins in irb. its working now. thanks!

[Redoman]

don't forget you get an [] back instead of an {}

[Elchin]

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]}) }

[Marc-André Lafortune]

@Elchin: you can use metrics.sort_by{ |k, v| v }.reverse.to_h

[Elchin]

@Marc-AndréLafortune, totally my bad, didn't see #to_h! Thank you!

[Gerry Gleason]

Actually even simpler as: hash.sort_by(&:last) with the same caveat about getting an array of pairs vs. a Hash.

[Tamer Shlash]

This is awesome! any referecne for the pre-definition of &:last?

[lazybios]

Hi @Marc-AndréLafortune, I have a small question why add a underscore at begin of key. This coding style mean what?

[Marc-André Lafortune]

@lazybios See here: stackoverflow.com/questions/8689673/…

[lazybios]

@Marc-AndréLafortune Thank you ! :)

[Beni Cherniavsky-Paskin]

sort_by(&:last) is effectively shorthand for sort_by {|x| x.last} stackoverflow.com/questions/1217088/…

[bragboy]

for reverse sort you may use metrics.sort_by{|k,v| -v}

[Francisco Quintero]

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