Setting a Value in an Array in a Class

classdef myclass_t

    properties
        my_array double
    end

methods
        
        function self = myclass_t( array_size ) 
            self.my_array = zeros(1, array_size);            
        end %constructor
        
        function self = set.my_array(self, my_val, my_index)
            self.my_array(my_index) = my_value;
        end
    end
end

I want to set the value of my array with the value my_value at my_index. Lets assume there are no type errors.

However when I do this, I get the following error: Set Methods must have exactly two inputs

How can I set the value of this array at the desired index?

[Matt J]

What syntax are you trying to enable with this?

[A. B.]

I don't know. I just want to set the damn value lol

[Matt J]

What's wrong with simply doing a direct assignment to my_array all the time,

obj=myclass_t(10);
obj.my_array(5)=6;

Answer 1

[Matt J]

Use an ordinary method:

classdef myclass_t
    properties
        my_array double
    end
methods
        
        function self = myclass_t( array_size ) 
            self.my_array = zeros(1, array_size);            
        end %constructor
        
    function self = set_my_array(self, my_val, my_index)  %<---changed set. to set_
            self.my_array(my_index) = my_value;
        end
    end
end

[A. B.]

When I get back from that function call however, nothing has changed.

[Matt J]

You must call it with an output argument,

obj = obj.set_my_array(my_val, my_index) 

Alternatively, you can write this as a handle class,

classdef myclass_t < handle  %<---NOTICE
    properties
        my_array double
    end
methods
        
        function self = myclass_t( array_size ) 
            self.my_array = zeros(1, array_size);            
        end %constructor
        
        function set_my_array(self, my_val, my_index)  %<---NOTICE
            self.my_array(my_index) = my_value;
        end
    end
end

but make sure you know how handle classes work, and how they behave differently from value classes:

https://www.mathworks.com/help/matlab/handle-classes.html

[Guillaume]

Careful there! That set method for the handle class is not valid.

Property set methods do not support indexing into the property. You have to replace the whole array.

So with the value class, a valid set method would be

function self = set.my_array(self, value)
    self.my_array = value;
end

and for a handle class, the equivalent:

function set.my_array(self, value)
    self.my_array = value;
end

Note that in both case, you never call the set method directly. It's invoked for you by matlab when you do:

obj.my_array = [1, 2, 3];  %will invoke set.my_array(obj, [1, 2, 3]) if the method is defined

With this particular example, the set method is completely pointless. It's useful if you want to do additional validation or something more complex than just assigning the input to the property.

If you do want to index into the property, then either don't define a set method so you can access directy the public property, or create a normal method as per matt's first example.

[Matt J]

Yep. I've made the appropriate edits.