Erlang and OTP in Action.pdf - Synrc

30
That was the last in our list of built-in data types. We will now discuss something that
unites them all: the comparison operators.
2.2.9 – Comparing terms
The different data types in Erlang have one thing in common: they can all be compared and
ordered, using built-in operators like , and ==. The normal orderings on numbers of
course hold, so that 1 < 2 and 3.14 > 3 and so on, and atoms and strings (as well as any
other lists) and tuples are ordered lexicographically, so that 'abacus' < 'abba', "zzz"
> "zzy", [1,2,3] > [1,2,2,1], and {fred,baker,42} < {fred,cook,18}.
So far, it all seems pretty normal, but on top of that you also have an ordering between
values of different types, so that for example 42 < 'aardvark', [1,2,3] > {1,2,3},
and 'abc' < "abc". That is, all numbers come before all atoms, all tuples come before all
lists, and all atoms come before all tuples and lists (strings are really lists, remember).
You don’t have to memorize which data types come before which in this order. The
important thing to know is that you can compare any two terms for order, and you will get
the same result always. In particular, if you sort a list of various terms of different types (a
list of mixed numbers, strings, atoms, tuples, …) by using the standard library function
lists:sort(…), you will always get a properly sorted list as result, where all the numbers
come first, then all atoms, and so on. You can try it out in the shell: for example,
lists:sort([b,3,a,"z",1,c,"x",2.5,"y"]).
LESS-THAN/GREATER-THAN OR EQUALS
One of those little differences in syntax between Erlang and most other programming
languages except Prolog is that the less-than-or-equals operator is not written “