Subtract HashSets (and return a copy)?
I guess I should clone it first? How do I do that?
var universe = new HashSet<int>();
var subset = new HashSet<int>();
...
// clone the universe
var remaining = new HashSet<int>(universe);
remaining.ExceptWith(subset);
Not as simple as with the Except
extension method, but probably faster (you should run a few performance tests to make sure)
How about Except()
?
var x = new HashSet<int>();
var y = new HashSet<int>();
var xminusy = new HashSet<int>(x.Except(y));
I benchmarked Linq's Except
method against cloning and using the HashSet-native function ExceptWith
. Here are the results.
static class Program
{
public static HashSet<T> ToSet<T>(this IEnumerable<T> collection)
{
return new HashSet<T>(collection);
}
public static HashSet<T> Subtract<T>(this HashSet<T> set, IEnumerable<T> other)
{
var clone = set.ToSet();
clone.ExceptWith(other);
return clone;
}
static void Main(string[] args)
{
var A = new HashSet<int> { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };
var B = new HashSet<int> { 2, 4, 6, 8, 10 };
var sw = new Stopwatch();
sw.Restart();
for (int i = 0; i < 1000000; ++i)
{
var C = A.Except(B).ToSet();
}
sw.Stop();
Console.WriteLine("Linq: {0} ms", sw.ElapsedMilliseconds);
sw.Restart();
for (int i = 0; i < 1000000; ++i)
{
var C = A.Subtract(B);
}
sw.Stop();
Console.WriteLine("Native: {0} ms", sw.ElapsedMilliseconds);
Console.ReadLine();
}
}
Linq: 1297 ms
Native: 762 ms
http://programanddesign.com/cs/subtracting-sets/