What is the simplest way to partition a list based on a criteria?

Question

Let's say, we have an object definition like this,

class MyObject {
    int id;
    string name;
}

and we have a list of MyObjects.

List<MyObject> objectList

Now, I need to partition this objectList into 2 sub-lists based on which objects have a non-null name field.

So, after the operation, I need to have 2 lists, objectsWithName where name field is non-null and objectsWithoutName where name field is null (this is the immediate criteria, but I'm looking more towards partitioning into 2 groups using a predicate).

What is the simplest way to achieve this? Can I do this in a single operation? Using LINQ is permitted.

for loop with if statement would be simplest, LINQ is overhead here — Sandip Bantawa, Nov 21 '18 at 09:03
This would help you https://stackoverflow.com/questions/419019/split-list-into-sublists-with-linq — Hamza Haider, Nov 21 '18 at 09:10
@brykneval - what makes you think Linq is an overhead -- have you compared the IL generated by a foreach loop with that generated by Linq? I think you'll find they are not very different. — Chris F Carroll, Nov 21 '18 at 09:17

score 2 · Accepted Answer · answered Nov 21 '18 at 09:06

2

Two LINQ statements would do:

var nameNotNull = objectList.Where(o => !string.IsNullOrEmpty(o.Name));
var nameNull = objectList.Where(o => string.IsNullOrEmpty(o.Name));

Of course, you could use GroupBy, or a more efficient foreach statement.

To show the foreach option:

List<MyObject> nameNotNull = new List<MyObject>();
List<MyObject> nameNull = new List<MyObject>();

foreach (MyObject o in objectList)
{
    if (!string.IsNullOrEmpty(o.Name))
    {
        nameNotNull.Add(o);
    }
    else
    {
        nameNull.Add(o);
    }
}

answered Nov 21 '18 at 09:06

Patrick Hofman

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I was looking to partition the list in a single iteration - so I would go with the foreach solution. – WaughWaugh Nov 21 '18 at 10:12
That is perfectly fine. @SayanBose – Patrick Hofman Nov 21 '18 at 10:30

score 1 · Answer 2 · answered Nov 21 '18 at 09:16

public class MyObject
{
    public int Id { get; set; }
    public string Name { get; set; }
}

public class MyObjectLists
{
    private readonly List<MyObject> _objects;

    public List<MyObject> NullNameObjects
    {
        get
        {
            return _objects.Where(x => x.Name == null).ToList();
        }
    }

    public List<MyObject> NonNullNameObjects
    {
        get
        {
            return _objects.Where(x => x.Name != null).ToList();
        }
    }

    public MyObjectLists(List<MyObject> objects)
    {
        _objects = objects ?? throw new ArgumentNullException(nameof(objects));
    }
}

Using the code:

var list = new List<MyObject>
{
    new MyObject
    {
        Id = 1,
        Name = "John"
    },

    new MyObject
    {
        Id = 2
    },

    new MyObject
    {
        Id = 3,
        Name = "Mary"
    },

    new MyObject
    {
        Id = 4
    }
};

var objects = new MyObjectLists(list);

foreach (MyObject myObject in objects.NonNullNameObjects)
{
    Console.WriteLine($"Object with Id {myObject.Id} has a non-null name");
}

foreach (MyObject myObject in objects.NullNameObjects)
{
    Console.WriteLine($"Object with Id {myObject.Id} has a null name");
}

score 0 · Answer 3 · answered Nov 21 '18 at 09:16

0

I think you're looking for something like this example:

class MyObject 
{
    int id;
    string name;
}

var objectList = new List<MyObject>();

objectList.Add(new MyObject { name = "item 1" });
objectList.Add(new MyObject { name = string.Empty });
objectList.Add(new MyObject { name = "item 3" });

var objectsWithName = objectList.Where(x => !string.IsNullOrEmpty(x.name));
var objectsWithoutName = objectList.Except(objectsWithName);

I've created 2 lists that the first list contains the MyObject items with the name property is NOT null, otherwise to the second's.

answered Nov 21 '18 at 09:16

Tân

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`Except` is too expensive here. – Patrick Hofman Nov 21 '18 at 09:37
1

Because it will create a hashset to compare values, while there is absolutely no need for. – Patrick Hofman Nov 21 '18 at 09:50

What is the simplest way to partition a list based on a criteria?

3 Answers3