Matlab, function handle auto use parameter

Question

Hi I have a question about how to achieve the following behaviour in Matlab.

A.x=pi
A.sin=@()sin(A.x)

A.sin() % Returns  1.2246e-16, essentially 0 so all good so far.

% Now for the problem
A.x = pi/2
A.sin() % Returns  1.2246e-16, meaning the new A.x is not used. It should return 1.

Does anyone have any ideas how to achieve this. I know I could define A.sin as @(x) sin(x) then provide A.x but would rather find an alternative.

Thanks.

score 1 · Answer 1 · answered Sep 09 '13 at 16:32

Create this class:

classdef mytrig    
    properties
        x
    end    
    methods
        function out = sin(obj)
            out = sin(obj.x);
        end
    end
end

Then at the command line:

>> A = mytrig;
>> A.x = pi;
>> A.sin
ans =
  1.2246e-016
>> A.x = pi/2;
>> A.sin
ans =
     1

The way you're doing it at the moment won't work, because when you create the function handle A.sin=@()sin(A.x), the function handle captures a copy of the current workspace, including x, which then remains fixed afterwards, even if you subsequently change x. If you want to be able to change x afterwards yourself, these best way would be to be implement a class as above.

Hope that helps!

score 0 · Accepted Answer · answered Sep 09 '13 at 16:19

Once you assign a value to a variable in Matlab, it is fixed.

If you want to have something that is updated automatically, please look into classes.

If you don't like classes, you could also define a function, for example

myAsin = @()sin(A.x)

Can't test it now, but as it is a function you should be getting the updated value when you call it after A.x is updated.

Matlab, function handle auto use parameter

2 Answers2