using linq query to find value that differs from previously found value

Question

Say i have a class that contains these items publicly accessible via properties:

class MyClass
{    
    int switch1; //0 or 1
    int switch2; //0 or 1
    int switch3; //0 or 1
}

This class represents switch states, and each time a switch state changes, i would like to add it to my transition list

I have a large sorted list that contains instances of this class and would like to use a query to capture only the entries in my list where the switch state for any switch changes.

Is this possible using a linq query?

Answer 1

try this:

Assuming your class looks like:

public class State
{
    public int Id { get; set; }
    public int Switch1 { get; set; }
    public int Switch2 { get; set; }
    public int Switch3 { get; set; }

    public override bool Equals(object obj)
    {
        var other = obj as State;

        if (other != null)
        {
            return Switch1 == other.Switch1 &&
                   Switch2 == other.Switch2 &&
                   Switch3 == other.Switch3;
        }

        return false;
    }
}

I just added an Equals() to compare flags and my Id field is purely to demonstrate which items changed.

We can then craft a LINQ query like:

    State previous = null;
    var transitions = list.Where(s =>
                                    {
                                        bool result = !s.Equals(previous);
                                        previous = s;
                                        return result;
                                    })
        .ToList();

Not elegant, but it works, if you had this data set:

    var list = new List<State>
                {
                    new State { Id = 0, Switch1 = 0, Switch2 = 0, Switch3 = 0 },
                    new State { Id = 1, Switch1 = 0, Switch2 = 0, Switch3 = 0 },
                    new State { Id = 2, Switch1 = 1, Switch2 = 0, Switch3 = 0 },
                    new State { Id = 3, Switch1 = 0, Switch2 = 1, Switch3 = 0 },
                    new State { Id = 4, Switch1 = 0, Switch2 = 1, Switch3 = 0 },
                    new State { Id = 5, Switch1 = 0, Switch2 = 1, Switch3 = 0 },
                    new State { Id = 6, Switch1 = 1, Switch2 = 1, Switch3 = 0 },
                    new State { Id = 7, Switch1 = 0, Switch2 = 0, Switch3 = 1 },
                    new State { Id = 8, Switch1 = 0, Switch2 = 0, Switch3 = 1 },
                    new State { Id = 9, Switch1 = 0, Switch2 = 0, Switch3 = 0 },
                };

And ran the query, the list would contain your state transitions at items: 0, 2, 3, 6, 7, 9

Answer 2

I would do as follow:

class MyClass
{    
    int ID; //needs for recognize the message
    int switch1; //0 or 1
    int switch2; //0 or 1
    int switch3; //0 or 1
    public int Pattern
    {
       get { return switch1 + switch2 << 1 + switch3 << 2; }
    }
}

Then it must be declared a dictionary with the previous-state messages:

Dictionary<int, int> _prevStates;

each cell has for key the ID, and for value the "Pattern" of the message. At this point, let's suppose that the new incoming message stream is a list of MyClass:

IEnumerable<MyClass> incoming = ...

var changed = from msg in incoming
              where _prevStates.ContainsKey(msg.ID)  //what to do?
              where _prevStates[msg.ID].Pattern != msg.Pattern
              select msg;

Finally, you must update the dictionary with the changed patterns.

Cheers