Function handle in structure field generating data from other fields

Hi everyone,

I have a structure and in one of its fields I want to store some data. It is large data, but generation of it is very fast. Since the structure needs to be saved as matfile, generating the data when needed (i.e. when accessing the struct field) would be much better than storing it with the struct. Is this possible to solve this, maybe with function handles?

Lets assume a function

function[DataOut]=MyFunction(Locs, Peaks) 
% some random stuff happening to generate DataOut
end

and a struct with the fields

MyStruct.Locs=[1 3 5 7 9];
MyStruct.Peaks=[40 50 60 70 80];
MyStruct.ProcessedData=@MyFunction

Is is somehow possible to just access the field MyStruct.ProcessedData and to get the data that have been produced by using the fields "MyStruct.Locs" and MyStruct.Peaks" ? That means that I would have to tell the function handle which data to use when making the handle... But how?

I think this is close to something like object oriented programming and a special method invoked when trying to access a field. But I have not much knowledge about this kind of topics. You help is very much appreciated!

Thanks in advance!

[Guillaume]

Things to bear in mind if you save a function handle in a mat file:

• When you load the mat file, the function that handle points to must be on the path.
• If a different function with the same name is on the path when you load the mat file, that function will be called instead.

Instead of a function handle, you could store an anonymous function. Variables in scope are permanently bound to an anonymous function when it is created. e.g:

a = 5;
b = 3;
fh = @(x) a+x-b;
fh(1) %return 3

%at this point a and b are bound to the anonymous function.
%changing their value does not affect the anonymous function
a = 0;
b = Inf;
fh(1) %still return 3

%even deleting them does not affect the anonymous function
clear a b
fh(1) %still return 3

Whether or not that can work for you, I'm not sure. It really depends on what the inputs to your function are.

[Maximilian Rüppell]

Thanks for the comment, but I do not really see where this could help me. The inputs to the function are basically 3 vectors that are stored in other fields of the structure. So in the moment when the field is adressed, these values need to be retrieved (they can change in between calls) and passed on to the function which then generates an output...

[Steven Lord]

It sounds to me like you want an object instead of a struct with a function handle as one of its fields. The function would then become an instance method of the class (not a Static method) and as such would need to be called with an instance of the class as one of its inputs.

[Guillaume]

So I'm a bit unclear on what you are trying to do. In the mat file, do you want to store:

• all the required inputs necessary to recreate the missing data as fields of the structure

or

• somehow, a function that recreates the missing data, without explicitly storing the data as fields of the structure?

[Maximilian Rüppell]

I am trying to describe better then: A structure "MyStruct" holds data, should be as small as possible because being stored, reloaded, worked on, stored again.

Multiple fields of "MyStruct" contain data. One function "MyFunc" takes these data and processes them (for example some baseline substraction or similar). Storing the results in MyStruct as well is possible (and I could easily do so). But the data could also be generated on the fly/ whenever the field is accessed to make MyStruct smaller.

Is it possible to make a field of a struct behave as if the was data stored but instead generating the data at the moment of accessing the field? Someone accessing the struct field should see no difference to a "normal" field and should just be able to retrieve the data.

[Maximilian Rüppell]

Steven, I am not sure since I did not fully understand. Would I be able to just access that field like any other field and get the processed data as return value, although their only generated that moment?

And would it be possible to integrate this functionality into an already existing struct?

Answer 1

[Guillaume]

With a structure, it's not possible to do what you want.

With a class, yes, using dependent properties.

classdef DemoClass
    properties
        Locs;
        Peaks;
    end
    properties (Dependent)
        SomethingElse;
    end
    methods
        function value = get.SomethingElse(this)
            %code to calculate SomethingElse.
        end
    end
end

SomethingElse will be calculated every time it is accessed by the user.

[Maximilian Rüppell]

Thank you for your help!