Today’s Erlang Thursday is on ordsets:union/2.

ordsets:union/2 takes two ordered sets and returns an merged ordered set of the arguments.

SetA = ordsets:from_list([1, 1, 2, 3, 5]).
% [1,2,3,5]
SetB = ordsets:new().
% []
SetC = ordsets:from_list([3, 1, 4, 1, 5, 9]).
% [1,3,4,5,9]
SetD = ordsets:from_list([a, b, c, d, e]).
% [a,b,c,d,e]
UnionAB = ordsets:union(SetA, SetB).
% [1,2,3,5]
UnionAC = ordsets:union(SetA, SetC).
% [1,2,3,4,5,9]

And because a string in Erlang is just a list of characters, we can also create ordered sets from strings, and then get a union of the unique characters that are in two strings.

ordsets:from_list("Kermit").
% "Keimrt"
ordsets:from_list([75, 101, 114, 109, 105, 116]).
% "Keimrt"
ordsets:from_list("Mississippi").
% "Mips"
ordsets:union(ordsets:from_list("Kermit"), ordsets:from_list("Mississippi")).
% "KMeimprst"

The ordsets modules also contains ordsets:union/1, which takes a list of ordered sets and returns the union of all the ordered sets in the list.

UnionAC = ordsets:union([SetA, SetC]).
% [1,2,3,4,5,9]
UnionABC = ordsets:union([SetB, SetC, SetA]).
% [1,2,3,4,5,9]
UnionABCD = ordsets:union([SetB, SetC, SetA, SetD]).
% [1,2,3,4,5,9,a,b,c,d,e]
UnionCD = ordsets:union([SetC, SetD]).
% [1,3,4,5,9,a,b,c,d,e]

WARNING: While the representation for an ordered set is just a list, if you pass a list to ordsets:union/2 you will not get what you expect, as it expects the items in each “ordered set” to actually be ordered and a set.

ordsets:union([1, 2, 3], [a, b, c]).
% [1,2,3,a,b,c]
ordsets:union([1, 1, 2, 3, 1, 2], [a, b, c]).
% [1,1,2,3,1,2,a,b,c]
ordsets:union([1, 1, 2, 3, 1, 2], [1, a, b, c]).
% [1,1,2,3,1,2,a,b,c]
ordsets:union([1, 1, 2, 3, 1, 2], [1, a, b, c, 1]).
% [1,1,2,3,1,2,a,b,c,1]

–Proctor