Composite Data Types
Three-way comparisons have the elegant property of being simple to compose to build lexicographic comparisons of non-primitive data types, unlike two-way comparisons.
Here is a composition example in Perl.
sub compare($$) { my ($a, $b) = @_; return $a->{unit} cmp $b->{unit} || $a->{rank} <=> $b->{rank} || $a->{name} cmp $b->{name}; }Note that cmp is for strings as <=> is for numbers. Two-way equivalents tend to be less compact but not necessarily less legible. The above takes advantage of short-circuit evaluation of the || operator, and the fact that 0 is considered false in Perl. As a result, if the first comparison is equal (thus evaluates to 0), it will "fall through" to the second comparison, and so on., until it finds one that is non-zero, or until it reaches the end.
In some languages, including Python, Ruby, Haskell, etc., comparison of lists are done lexicographically, which means that it is possible to build a chain of comparisons like the above example by putting the values into lists in the order desired; for example in Ruby:
<=>Read more about this topic: Three-way Comparison, High-level Languages
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