Checking if same object is already present in a list

Question

Here is the story:

Im trying to make a list of different clusters... I only want to have the necessary clusters... And Clusters can be the same.

How can I add this to a list by checking if the list contains the object (I know objects cant be passed here)

This is my sample quote:

foreach (Cluster cluster in clustersByProgramme)
{
    if (!clusterList.Contains(cluster))
    {
        clusterList.Add(cluster);
    }
}

Answer 1

If you're using .NET 3.5, use a HashSet to do this.

HashSet<Cluster> clusterList = new HashSet<Cluster>();
foreach (Cluster cluster in clustersByProgramme)
{
     clusterList.Add(cluster);
}

In this case, also make sure that if cluster1 == cluster2, then

cluster1.Equals(cluster2);
cluster2.Equals(cluster1); //yeah, could be different depending on your impl
cluster1.GetHashCode() == cluster2.GetHashCode();

Answer 2

Your example is about as simple as it is going to get. The only thing I could possibly recommend is that you use the Exists method:

The Predicate is a delegate to a method that returns true if the object passed to it matches the conditions defined in the delegate. The elements of the current List are individually passed to the Predicate delegate, and processing is stopped when a match is found.

This method performs a linear search; therefore, this method is an O(n) operation, where n is Count.

Answer 3

Your code should work; if it hasn't, you might be using different object instances that represent the same actual cluster, and you perhaps haven't provided a suitable Equals implementation (you should also update GetHashCode at the same time).

Also - in .NET 3.5, this could be simply:

var clusterList = clustersByProgramme.Distinct().ToList();

---

As an example of class that supports equality tests:

class Cluster // possibly also IEquatable<Cluster>
{
    public string Name { get { return name; } }
    private readonly string name;
    public Cluster(string name) { this.name = name ?? ""; }
    public override string ToString() { return Name; }
    public override int GetHashCode() { return Name.GetHashCode(); }
    public override bool Equals(object obj)
    {
        Cluster other = obj as Cluster;
        return obj == null ? false : this.Name == other.Name;
    }
}

Answer 4

Your code is correct, but it is not very efficient. You could instead use a HashSet<T> like this:

HashSet<Cluster> clusterSet = new HashSet<T>();
foreach (Cluster cluster in clustersByProgramme)
  clusterSet.Add(cluster);

In this case, also make sure that if cluster1 == cluster2, then

cluster1.Equals(cluster2);
cluster2.Equals(cluster1); //yeah, could be different depending on your impl
cluster1.GetHashCode() == cluster2.GetHashCode();

Answer 5

why not just use dictionary?

It is n(1) as long as your items have a good hash.

Seems a simple solution

Ie dictionary.Contains(key) is n(1)

you can then update existing if at all or add new