Problem
You want to eliminate duplicate values from a list.
Solution
There are several possible solutions for this problem. Here are some of these:
Using lists:usort
The lists module contains a wealth of list processing functionality.
One possible solution to this problem is to use the lists:usort
function, which takes a list and returns a sorted copy of the original
list, with all duplicates removed:
1> UL = [1,2,8,7,8,10,3,12,3,99,188,3,2,1,3,5,15,72]. [1,2,8,7,8,10,3,12,3,99,188,3,2,1,3,5,15,72] 2> lists:usort(UL). [1,2,3,5,7,8,10,12,15,72,99,188] |
Using the sets Module
Erlang standard libraries includes a module, sets, with a variety of
functions related to generating, creating, and manipulating mathematical
sets.
10> Set = sets:from_list(UL).
{sets,12,
16,
16,
8,
80,
48,
{[],[],[],[],[],[],[],[],[],[],[],[],[],[],[],[]},
{{[],[99,3],[],[],"/274/f",[15],[2],[5],"H/b",[],[],[1],[],[7],"/n",[]}}}
11> sets:to_list(Set).
[3,99,12,188,15,2,5,8,72,1,7,10]
|
Note that sets:to_list(sets:from_list(L)) produces an unreliably arranged list.
Using a General Balanced Tree Set (gb_set)
Erlang's standard libraries includes an implementation of Professor
Arne Andersson's General Balanced Trees. These structures are more
costly than sorting lists for small sets, but this is a much more
efficient implementation when working with large sets of data.
The gb_set:from_list function will produce an ordered set of
elements (dropping duplicates). The set can then be extracted back to a
list for other use:
3> GBSet = gb_sets:from_list(UL).
{12,
{10,
{5,{2,{1,nil,nil},{3,nil,nil}},{8,{7,nil,nil},nil}},
{72,{15,{12,nil,nil},nil},{188,{99,nil,nil},nil}}}}
4> gb_sets:to_list(GBSet).
[1,2,3,5,7,8,10,12,15,72,99,188]
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