Ruby Sorting [0] – Sorting a Hash


It figures that one of the questions someone asked, following my talk on sorting last month, was something I specifically chose not to cover for the sake of time, and to be able to cover more valuable topics. So, when someone asked whether you can sort a Hash and how, I knew A) that you can, and B) I knew I had barely glanced at it in the rdoc on hash sorting, but didn’t remember how it worked. So I had to simply suggest they go read about it themselves. But I was also curious to go read about it more myself.  So I took my own advice, and here’s what I came up with:

I define a hash:

irb(main):001:0> typical_fish_colors =
irb(main):002:0*    {"clownfish" => ["orange","white","black"],
irb(main):003:1*     "goldfish" => ["orange"],
irb(main):004:1*     "angelfish" => ["black","white"]
irb(main):005:1>    }

Default sorting on a hash – sorts the keys.

Pretty simple I guess. What you may not expect, depending on what you know of hashes, is that you don’t get a Hash object returned from calling .sort, you get an Array.  A nested, three-dimensional Array. This is because arrays are ordered, hashes are not.

irb(main):006:0> typical_fish_colors.sort
=> [["angelfish", ["black", "white"]], ["clownfish", ["orange", "white", "black"]],
   ["goldfish", ["orange"]]]

What you can’t do when sorting a hash is use sort! (sort-bang):

irb(main):007:0> typical_fish_colors.sort!
NoMethodError: undefined method `sort!' for #<Hash:0x2d17ea8>
 from (irb):7

What else you can’t do when sorting a hash by default is use symbols for keys:

irb(main):008:0> typical_fish_colors =
irb(main):009:0*     {:clownfish => ["orange","white","black"],
irb(main):010:1*      :goldfish => ["orange"],
irb(main):011:1*      :angelfish => ["black","white"]
irb(main):012:1>     }

irb(main):013:0> typical_fish_colors.sort
NoMethodError: undefined method `<=>' for :clownfish:Symbol
 from (irb):13:in `<=>'
 from (irb):13:in `sort'
 from (irb):13
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