ruby sorting hashes inside array

Question

I have an array of hashes that I would like to sort based on the :reference values. But some of the elements within the array do not have a :reference key and therefore cannot be sorted. Is there a way to ignore this field and just sort the hash elements that contain this key?

I've tried the following approach but I'm getting an argument error

ArgumentError: comparison of NilClass with String failed
  sort_by at org/jruby/RubyEnumerable.java:503



arr1 = [{:reference=> "F123",
            :name=> "test4"
          },
          {
            :reference=> "ZA4",
            :name=> "test3"
          },
          {
            :reference=> "A43",
            :name=> "test2"
          },
          {
            :name=> "test1"
          },
          {
            :name=> "homework1"
          }]

arr1 = arr1.sort_by { |hash| hash[:reference] }

puts arr1

The correct output should look like this :

=> arr1= [
        {:reference=>"A43", :name=>"test2"},
        {:reference=>"F123", :name=>"test4"},
        {:reference=>"ZA4", :name=>"test3"},
        {:name=> "test1"},
        {:name=> "homework1"}
      ]

tadman · Accepted Answer · 2016-10-12 18:07:11Z

3

You can only sort on values that can be compared, so if you've got values of different types it's best to convert them to the same type first. A simple work-around is to convert to string:

arr1.sort_by { |hash| hash[:reference].to_s }

You can also assign a default:

arr1.sort_by { |hash| hash[:reference] || '' }

Edit: If you want the nil values sorted last:

arr1.sort_by { |hash| [ hash[:reference] ? 0 : 1, hash[:reference].to_s ] }

edited Oct 12, 2016 at 18:07

answered Oct 12, 2016 at 17:45

tadman

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5 Comments

danynl Over a year ago

thank you. Is there a way to sort the array so that the hashes without the :reference keys end up at the back of the array?

tadman Over a year ago

I've added a method to push those to the end by adding a second element to their sorting to force them back. An array is a convenient way of doing this since it's sorted based on first element then second element.

Cary Swoveland Over a year ago

You don't need .to_s in your last line. Strings will never be compared to nil and nil <=> nil #=> 0.

tadman Over a year ago

@CarySwoveland That's a bit of paranoia that some crazy stuff like false might show up in there, but you're basically right that it's extraneous.

Cary Swoveland Over a year ago

If false an exception is raised in any event, which is probably what you want. Note that if false the first element of the sort_by array will be 1, so you will comparing false.to_s #=> "false" with nil, raising an exception. If you drop to_s, false will be compared with nil, again raising an exception (which is good). So not only does to_s not help, it's a distraction.

Stefan · Accepted Answer · 2016-10-12 20:10:07Z

1

If you don't mind temporary variables, you could split the array into two arrays, one containing hashes with :reference and the other those without:

with_ref, without_ref = arr1.partition { |h| h[:reference] }

Then sort the first one:

with_ref.sort_by! { |h| h[:reference] }

And finally concatenate both arrays:

arr1 = with_ref + without_ref

answered Oct 12, 2016 at 20:10

Stefan

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