Chris Holtz

More Than You Care to Know About the Uniq() Method

The other day I was digging through the ruby documentation and stumbled on the uniq method. I was startled to find that you can pass in a block as a parameter. My instinct was to back away slowly, not unlike a cat looking at a bathtub full of water.

After all, why on Earth would one want to filter uniqueness based on some criteria other than an exact match? Undaunted, I figured it would be fun to explore this method a bit more… find out what makes it tick.

Basic Usage

This is a pretty common method; provided you aren’t completely new to Ruby, you’ve probably seen it plenty:

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  > values = [1,1,2,2,3,3,4,4]
  > values.uniq
  => [1,2,3,4]

Or perhaps you’ve used it with strings:

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  > values = ["a", "a", "b", "b", "c", "c"]
  > values.uniq
  => ["a", "b", "c"]

Why not mix things up?

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  > [1, 1, "a", "a", true, true].uniq
  => [1, "a", true]

This raises the question – are there any data types that uniq cannot process?

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  > [String, String,\
  >   {:a => 1}, {:a => 1},\
  >   Fixnum, Fixnum,\
  >   Hash.new, Hash.new,\
  >   Object.new, Object.new].uniq
  => [String, {:a=>1}, Fixnum, {}, #<Object:0x89c838c>, #<Object:0x89c8378>]

This is quite interesting for a couple reasons. First, notice that uniq works on classes – this makes sense because Ruby classes are first class (pardon the pun) objects. Custom-defined classes work here as well as native classes.

Second, multiple instances of the same classes (Object.new) are treated distinctly and do not reduce to a single object in the returned array. However, multiple instances of the Hash class are reduced, provided they contain the same data:

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  > [ {:a => 1}, {:a => 1}, {:a => 2}].uniq
  => [{:a=>1}, {:a=>2}]

How about nested data?

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  > [ {:a => {:b => 1}}, {:a => { :b => 1 }}].uniq
  => [{:a=>{:b=>1}}]

Nested hashes reduce to a single element, but only (as we would expect) if the data is identical in each element in the source array:

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  > [ {:a => {:b => Object.new}}, {:a => { :b => Object.new }}].uniq
  => [{:a=>{:b=>#<Object:0x88bbee4>}}, {:a=>{:b=>#<Object:0x88bbea8>}}]

Because multiple instances of Object.new are treated as distinct elements by the uniq method, data structures that contain them are treated as distinct as well.

Using Blocks

Back to the original point of this article… you can pass a block as a parameter to the uniq method. I was at something of a loss as to why this would be useful – it is doesn’t really return a unique set of data. To get a better understanding of this, let’s try it out:

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  > data = [{:a => 1, :b => 1},\
            {:a => 1, :b => 2},\
            {:a => 2, :b => 1},\
            {:a => 2, :b => 2}]

  > data.uniq{|d| d[:a]}
  => [{:a=>1, :b=>1}, {:a=>2, :b=>1}]

Interesting… based on the block criteria, it appears the uniq returns the first match – in this case, {:a=>1, :b=>1} and {:a=>2, :b=>1}. Change the order of the elements in the array, let’s see what happens:

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  > data = [{:a => 1, :b => 2},\
            {:a => 1, :b => 1},\
            {:a => 2, :b => 2},\
            {:a => 2, :b => 1}]

  > data.uniq{|d| d[:a]}
  => [{:a=>1, :b=>2}, {:a=>2, :b=>2}]

Uniq continues to return the first match based on the block provided. We can duplicate the uniq-with-a-block behavior by grouping the array using the same block. Then skim off all results in the group except for the first match:

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  > data = [{:a => 1, :b => 1},\
            {:a => 1, :b => 2},\
            {:a => 1, :b => 3},\
            {:a => 2, :b => 1},\
            {:a => 2, :b => 2},\
            {:a => 2, :b => 3}]

  > data.group_by{ |d| d[:a] }.map{ |d| d[1] }.map{ |d| d.first }
  => [{:a=>1, :b=>1}, {:a=>2, :b=>1}]

  > data.uniq{ |d| d[:a] }
  => [{:a=>1, :b=>1}, {:a=>2, :b=>1}]

As you can see, the results are the same. That’s how this began to make sense to me. Uniq with a block is just a group_by that returns the first match of each group.

I’m not sure I’ll find a use for passing a block into uniq, but at the very least it was a fun exercise. If you can think of use cases for this, let me know.