Problem when using set.property method on a property of matrix type ? How to access the value of only one element of the matrix within the set.property method?

Hi everyone,

This is a general question on using the set.property method when the property is a matrix (contains several elements and thus several values)

Problem: I have a Class "Test" with a property "Matrix". This property is a nx1 matrix. During code execution I create an instance of this class and the code modifies each row of this property "Matrix" one after the other. I would be interested in adding a listener in order to detect when one of the Matrix element exceeds a given value. However, when setting-up the set.Matrix method I cannot access a particular row as my "Row_ID" indicator is not defined within the class. I can only access the property "Matrix" entirely but looses information on the current Row_ID.

Do I have to include the "Row_ID" as one of the class property to be able to access ii within set.Matrix ? Even if using other properties within a set property method is not recommended (Matlab help) ?

Any suggestion on a way to integrate this listener would be much appreciated...

Ben

[Andrew Newell]

What you seem to be saying is that, as you modify the matrix (really a vector) element by element, you want the listener to know when *any* element exceeds a certain value. Is that right? Do you need to know which element exceeds the value, or will it do to look at MAX(ABS(MATRIX))?

[Benjamin Guinot]

Hi Andrew,

Looks like I haven't been clear enough... It's actually quite hard to explain...

Basically this 'vector type' property (you're right it is actually more a vector than a matrix) is of size n (nx1 double). I have a separate script which uses an instance of the class described above. This script is a for-loop type where Row_ID is used to designate the current row of the 'vector property'. At each time step (referenced by Row_ID) the value of the 'vector property' for this particular row is calculated. What I would like is a listener capable, at each time step, to detect if the value just calculated (at Row_ID) has exceeded a given value (+ I need to know which Row it is). The problem is that in the set.property method I can access the 'vector property' as a whole but since Row_ID is defined in the script and not in the class I cannot use it within the set.property method

The only solution I see would be to define a property within the class (let's call it Row_ID2) and at each time step to assign the Row_ID value to Row_ID2 (My_Obj.Row_ID2 = Row_ID)

Any better suggestion? (Thanks anyway for your time)

Answer 1

[Andrew Newell]

@Benjamin, this seems like a very tricky way to implement something that could be done with a simple for loop. I hope there some deeper purpose to this. You can't feed Row_ID to set.Matrix because the get/set interface insists on property/value pairs. I can't think of a better way to do this than to index the row within the class. Here is an example without the listener:

classdef testTimeSequence < hgsetget

    properties
        x % your vector
    end
    properties (Access=private)
        rowNumber = 0;
    end
    methods
        function obj = testTimeSequence(x0)
            obj.x = x0;
        end
        function set.x(obj,newValue)
            n = get(obj,'rowNumber');
            if n == 0
                obj.x = newValue;
            else
                obj.x(n) = newValue;
            end
            set(obj,'rowNumber',n+1)
        end
    end
end

You would set this up with a command like

h = testTimeSequence(zeros(100,1));

and each step of the loop would involve a command like

set(h,'x',newValue)

[Benjamin Guinot]

Hi Andrew,

Thanks for your help. I have to admit that it might look tricky. But thinking again you are right and I can probably include the test "if x(i) > maxValue" within the for loop as you did. But by doing so I was wondering if the code would run slower if at each time step it has to test the variable x(i) (if x(i) > maxValue) (the vector might contain more than 500,000 rows). Would the use of a listener take the same time? (my knowledge are quite limited on this point and both solutions are probably totally equivalent from a running time point of view)

[Andrew Newell]

The loop is probably much faster. For the if test, all it has to do is look up an address. There is much more overhead in calling the class.

Answer 2

[Andrew Newell]

And here is a simple for loop that does everything you want:

x = zeros(100,1);
maxValue = 10;
for i=1:length(x)
  x(i) = input('Next number: ');
  if x(i) > maxValue
      break
  end
end
disp(['Maximum exceeded at index number ',num2str(i)])
disp(['Value at this point = ',num2str(x(i))])