Sorting in lexicographical order

Question

So I want to sort my array of coordinates in lexicographical order. But I'm not sure how to do that. Each element in the array is a Coordinate object, with Fixnum fields #x and #y.

I'm new to Ruby and don't necessarily understand the sort enumeration. Would it be something like this?

coordinate_array.sort! { |a,b| a.x <==> b.x && a.y <==> b.y }

Answer 1

First the spaceship operator is <=> not <==>

Secondly you're not combining the 2 comparisons correctly: the result of the comparison will be -1,0,or 1. These are all truthy values and true && foo is just foo, so your code would just sort by the y values

You could write this as

x_ordering = a.x <=> b.x
x_ordering == 0 ? a.y <=> b.y : x_ordering

However array already implements <=> so you could just do

array.sort! { |a,b| [a.x, a.y] <=> [b.x, b.y]}

Which is a little terser and clearer at expense of creating 2 arrays in each comparison

You could even do

 array.sort_by! { |a| [a.x, a.y] }

Which is even clearer, but with a slightly different memory profile. This creates an array with the original values replaced by the values returned by the block and uses that to sort the original array.

I'd usually use the latter version unless I had a compelling reason to do otherwise.

Answer 2

Frederick Cheung's answer already describes how to sort by custom attributes.

Another option is to provide a default sort order by implementing Coordinate#<=>:

class Coordinate
  # ...

  def to_a
    [x, y]
  end

  def <=>(other)
    return unless other.is_a? Coordinate

    to_a <=> other.to_a
  end
end

And just call:

coordinate_array.sort!

You can also include the Comparable mixin which ...

(...) uses <=> to implement the conventional comparison operators (<, <=, ==, >=, and >) and the method between?.