Sorting an array of hash in Ruby

Question

I have one array of hashes as following:

[{"k1"=>"v1", "k2"=>"75.1%"}, {"k1"=>"v2", "k2"=>"-NA-"}, {"k1"=>"v3", "k2"=>"5.1%"}]

I want to sort this array of hashes based on the value of key "k2". I want to sort it in the decreasing order with "-NA-" coming at top always.

I want sorted array like this:

[{"k1"=>"v2", "k2"=>"-NA-"}, {"k1"=>"v1", "k2"=>"75.1%"}, {"k1"=>"v3", "k2"=>"5.1%"}]

I tried hash.sort_by method, but got incorrect results. How to do it in Ruby?

From what you wrote, it sounds like you want to sort them as string. For example, `"5%"` would come after `"75%"` and before `"15%"` in descending order. Is this correct? — sawa, Sep 28 '13 at 13:08
No. I want to sort them as numbers only. So, order will be : 75, then 15, 5. But NA should always come at top. — user2823083, Sep 28 '13 at 13:25
I see. Then the question was not well stated. Make it clear. — sawa, Sep 28 '13 at 13:26
This was not a duplicate of the linked answer. I had the same question and the other answer is insufficient due to the requirement in this question that a certain value always be sorted to the beginning of the array. — Huliax, Apr 08 '14 at 19:45

sawa · Accepted Answer · 2013-09-28T20:02:07.357

4

a = [
  {"k1"=>"v1", "k2"=>"75.1%"},
  {"k1"=>"v2", "k2"=>"-NA-"},
  {"k1"=>"v3", "k2"=>"5.1%"}
]

na, rest = a.partition{|h| h["k2"] == "-NA-"}
na + rest.sort_by{|h| h["k2"].to_f}.reverse
# =>
[
  {
    "k1" => "v2",
    "k2" => "-NA-"
  },
  {
    "k1" => "v1",
    "k2" => "75.1%"
  },
  {
    "k1" => "v3",
    "k2" => "5.1%"
  }
]

edited Sep 28 '13 at 20:02

answered Sep 28 '13 at 13:28

sawa

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Superb answer, sawa :) I wasn't aware of the partition method. – user2823083 Sep 28 '13 at 13:35
`"5.4&*$#%&+()".to_f` => 5.4. Learning something new every day... – Cary Swoveland Sep 28 '13 at 19:42
My apologies. Somehow I downvoted when I meant to upvote. Was +2 before I voted. – Cary Swoveland Sep 28 '13 at 19:50
1

I corrected my mistake after your edit. – Cary Swoveland Sep 28 '13 at 20:16

steenslag · Answer 2 · 2013-09-28T15:27:52.287

1

ar = [{"k1"=>"v1", "k2"=>"75.1%"}, {"k1"=>"v2", "k2"=>"-NA-"}, {"k1"=>"v3", "k2"=>"5.1%"}]
ar.sort_by{|h|(k2 = h["k2"]) == "-NA-"? -Float::INFINITY : -k2.to_f}
#=>[{"k1"=>"v2", "k2"=>"-NA-"}, {"k1"=>"v1", "k2"=>"75.1%"}, {"k1"=>"v3", "k2"=>"5.1%"}]

Sorting of floats in descending order can be done by ordering them as their negatives. The block will treat "-NA-" as the most negative number: minus Float::INFINITY; all other strings are ordered like their negative float representations would, using the ternary operator (shorthand for if then else).

edited Sep 28 '13 at 15:27

answered Sep 28 '13 at 14:09

steenslag

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Interesting answer. Can you explain it? – AGS Sep 28 '13 at 14:12
1

@AGS added some explanation. – steenslag Sep 28 '13 at 15:28

Cary Swoveland · Answer 3 · 2013-09-28T21:08:22.003

1

Here's another (commonly used) approach (with a from @sawa):

NA = "-NA-"
K2 = "k2" 
a.sort {|a,b| a[K2]==NA ? -1 : b[K2]==NA ? 1 : -a[K2].to_f <=> -b[K2].to_f }

edited Sep 28 '13 at 21:08

answered Sep 28 '13 at 20:59

Cary Swoveland

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Sorting an array of hash in Ruby

3 Answers3