What is the Ruby <=> (spaceship) operator?

Question

What is the Ruby <=> (spaceship) operator? Is the operator implemented by any other languages?

Answer 1

Since this operator reduces comparisons to an integer expression, it provides the most general purpose way to sort ascending or descending based on multiple columns/attributes.

For example, if I have an array of objects I can do things like this:

# `sort!` modifies array in place, avoids duplicating if it's large...

# Sort by zip code, ascending
my_objects.sort! { |a, b| a.zip <=> b.zip }

# Sort by zip code, descending
my_objects.sort! { |a, b| b.zip <=> a.zip }
# ...same as...
my_objects.sort! { |a, b| -1 * (a.zip <=> b.zip) }

# Sort by last name, then first
my_objects.sort! { |a, b| 2 * (a.last <=> b.last) + (a.first <=> b.first) }

# Sort by zip, then age descending, then last name, then first
# [Notice powers of 2 make it work for > 2 columns.]
my_objects.sort! do |a, b|
      8 * (a.zip   <=> b.zip) +
     -4 * (a.age   <=> b.age) +
      2 * (a.last  <=> b.last) +
          (a.first <=> b.first)
end

This basic pattern can be generalized to sort by any number of columns, in any permutation of ascending/descending on each.