Undefined function 'isnan' for input arguments of type 'Nodes2'

I have created a class Nodes that creates a node with properties value, next, and previous. Next and previous are set to nan and Nodes 2 takes in an item which is the value.

I have created a seperate handle class that uses Nodes2, it has an obj.head that is set to nan as its property. I have attached some troubleshooting I have below to confirm my filepath is correct.

I am trying to run

if (~isnan(obj.head))

which is where the error occurs and where the code stops.

I am not sure what to do and how to move forward.

Thanks for any ideas!

Answer 1

[Walter Roberson]

if I understand correctly, obj.head is either a class member or else NaN for the case where the queue is empty.

In the case where obj.head is indeed NaN because the queue is empty then isnan applied to it would be fine because isnan applied to a number is fine.

But consider the case where the queue is not empty so that the head is a class object. Then isnan is applied to the class object and that fails because the class does not define an isnan method.

Is there a particular reason you are representing empty-queue as nan and not as [] or an empty object of the class?

[Steven Lord]

You could also define an isnan method for the class that checks if the value property of the object is NaN. You could implement the nan function for your class and use nan('Nodes') to make a "placeholder" object to use as your head and tail nodes.

Answer 2

[J. Alex Lee]

For reference, a related question that shows more of the actual classes in question: https://www.mathworks.com/matlabcentral/answers/1817710-inserted-value-failing-to-show-up-in-list?s_tid=mlc_ans_men_view&mentions=true#comment_2398560

I second @Walter Roberson's question about if you can use an empty object:

classdef DLL < handle 
    properties
        head; % head node of the DLL
    end
    methods
        function obj = DLL()
            obj.head = Nodes2.empty();
        end
    end
end

And in Nodes2

classdef Nodes2
    properties
        value % the value that will be stored in our DLL 
        previous % pointer to previous node
        next % pointer to next node 
    end 
    methods
        
    end
        function obj = Nodes2(item)
            obj.value = item;
            obj.next = Nodes2.empty;
            obj.previous = Nodes2.empty;
        end
end

So you can test with the vanilla "isempty"?

[Walter Roberson]

Using an empty object seems like a good approach to me.