Dynamically sorting an array of hashes in Ruby

Question

I would like to sort an array of hashes by several dynamic criteria. Let's say I have this array

persons = [
  {
    id: 1,
    first_name: "Bill",
    last_name: "Zamora",
    age: 37
  },
  {
    id: 2,
    first_name: "Alexia",
    last_name: "Reyes",
    age: 70
  },
  {
    id: 3,
    first_name: "Anthony",
    last_name: "Nelson",
    age: 25
  }
]

I know that you can easily sort an array by multiple criteria with the following code

persons.sort_by!{ |p| [p[:age], p[:first_name]] }

However, in this example the number and the order of the fields by which the array is sorted is hard-coded. In my case, this is determined dynamically at runtime. So I do not know by how many fields the array is to be sorted nor what fields are sorted in which order.

I am looking for an elegant solution to sort my array using a configuration object that I don't know before. Such a configuration could look like this:

sort_settings = [
  {
    field: "first_name",
    order: "asc"
  },
  {
    field: "age",
    order: "desc"
  }
]

I am very grateful for any help on this!

The desc part may not be as simple as you think. Just for that functionality, the code could be pretty much complicated. — sawa
– sawa, Commented Aug 15, 2018 at 7:49
It is a better idea to give the field values in your sort_settings as symbols rather than strings given that the keys in your original hashes are symbols. — sawa
– sawa, Commented Aug 15, 2018 at 7:50
For order you could always sort in one direction and then use #reverse to get the opposite. Update: actually that will only help when sorting by multiple fields with different orders. — Kris
– Kris, Commented Aug 15, 2018 at 10:01

Igor Drozdov · Accepted Answer · 2018-08-15 08:12:45Z

It's quite challenging to sort strings in desc order using sort_by, it'd be better to use "lower-level" sort method, which sorts by specified comparator using <=> operator. A quick solution for this looks something like:

persons.sort do |a, b|
  comparator = 0

  sort_settings.each do |s|
    a_field = a[s[:field].to_sym]
    b_field = b[s[:field].to_sym]

    comparator = a_field <=> b_field

    comparator = -comparator if s[:order] == "desc"

    break unless comparator == 0
  end

  comparator
end

The block must implement a comparison between a and b, and return -1, when a follows b, 0 when a and b are equivalent, or +1 if b follows a.

Thus, we iterate over sort_settings and compare specified fields using <=>, which returns 1, 0 or -1. If the specified order is desc, we invert the value. If comparator returns something different to zero, we don't need to continue iterations.

sawa · Accepted Answer · 2018-08-15 08:14:25Z

2

Ignoring the asc/desc functionality and given that the sort keys are given as symbols and are in this format:

sort_settings = [
  :first_name,
  :age,
]

you simply can do:

persons.sort_by{|p| p.values_at(sort_settings)}

answered Aug 15, 2018 at 8:14

sawa

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Comments

Cary Swoveland · Accepted Answer · 2018-08-16 01:02:21Z

Code

def sort_by_settings(persons, sort_settings)
  sort_mult_by_field = sort_settings.each_with_object({}) do |g,h|
    h[g[:field]] = g[:order] == "asc" ? 1 : -1
  end

  longest_string_by_key = persons.each_with_object(Hash.new(0)) do |g,h|
    g.each { |k,v| h[k] = [h[k], g[k].size].max if sort_mult_by_field.key?(k) &&
      v.is_a?(String) }
  end

  sort_by_arr = persons.each_with_object({}) do |g,h|    
    h[g] = sort_mult_by_field.each_with_object([]) do |(f,m),a|
      gv = g[f]
      a <<
      case gv
      when Integer
        m * gv
      when String
        gv.chars.map { |c| m * c.ord }.concat([m * -256]*(longest_string_by_key[f]-gv.size))
      else # rescue...
      end
    end
  end

  persons.sort_by { |g| sort_by_arr[g] }
end

Examples

persons is as defined in the question.

sort_settings = [{field: :first_name, order: "asc"}, {field: :age, order: "desc"}]

sort_by_settings(persons, sort_settings)
  #=> [{:id=>2, :first_name=>"Alexia",  :last_name=>"Reyes",  :age=>70},
  #    {:id=>3, :first_name=>"Anthony", :last_name=>"Nelson", :age=>25},
  #    {:id=>1, :first_name=>"Bill",    :last_name=>"Zamora", :age=>37}]

persons1 = persons + [{ id: 4, first_name: "Alexia", last_name: "Whoosit", age: 71 }]
sort_by_settings(persons1, sort_settings)
  #=> [{:id=>4, :first_name=>"Alexia",  :last_name=>"Whoosit", :age=>71},
  #    {:id=>2, :first_name=>"Alexia",  :last_name=>"Reyes",   :age=>70},
  #    {:id=>3, :first_name=>"Anthony", :last_name=>"Nelson",  :age=>25},
  #    {:id=>1, :first_name=>"Bill",    :last_name=>"Zamora",  :age=>37}]

sort_settings1 = [{field: :first_name, order: "desc"}, {field: :age, order: "asc"}]
sort_by_settings(persons1, sort_settings1)
  #=> [{:id=>1, :first_name=>"Bill",    :last_name=>"Zamora",  :age=>37},
  #    {:id=>3, :first_name=>"Anthony", :last_name=>"Nelson",  :age=>25},
  #    {:id=>2, :first_name=>"Alexia",  :last_name=>"Reyes",   :age=>70}, 
  #    {:id=>4, :first_name=>"Alexia",  :last_name=>"Whoosit", :age=>71}]

Explanation

In the calculations for the first example, the following intermediate values were computed.

sort_mult_by_field
  #=> {:first_name=>1, :age=>-1}

longest_string_by_key
  #=> {:first_name=>7}

sort_by_arr
  #=> {{:id=>1, :first_name=>"Bill",    :last_name=>"Zamora", :age=>37}=>
  #      [[66, 105, 108, 108, -256, -256, -256], -37],
  #    {:id=>2, :first_name=>"Alexia",  :last_name=>"Reyes",  :age=>70}=>
  #      [[65, 108, 101, 120,  105,   97, -256], -70],
  #    {:id=>3, :first_name=>"Anthony", :last_name=>"Nelson", :age=>25}=>
  #      [[65, 110, 116, 104,  111,  110,  121], -25]}

Great job!! The only working correctly with "asc" and "desc"

matthewd · Accepted Answer · 2018-08-15 07:52:21Z

0

You're going to need a method that transforms a given item into a sort key, using the configuration as a guide:

def build_sort_key_for(item, configuration)
  configuration.map { |entry|
    value = item[entry[:field].to_sym]
    value = -value if entry[:order] == "desc" # this will only work on numeric values
    value
  }
end

Then you just call that in your sort_by:

persons.sort_by!{ |p| build_sort_key_for(p, configuration) }

Making "desc" work for strings is a challenge unto itself, and so left to the reader (or a separate question).

answered Aug 15, 2018 at 7:52

matthewd

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