How to write an efficient overloaded subsasgn?

Question

Jan Siegmund 2020-5-19

1
链接

此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/526536-how-to-write-an-efficient-overloaded-subsasgn

评论： James Lebak 2021-10-13

I want to write a class which defines subsasgn. The class holds a numeric value val and the subsasgn should target val only.

I wrote the following test:

function [] = test()
    num = 1000;
    a = randi([0 255],[1000 num]);
    b = Subsasgntest(randi([0 255],[1000 num]));
    c = randi([0 255],[1 num]);
    tic;
    for i = 1:length(c)
        a(2,i) = c(i);
    end
    toc
    tic;
    for i = 1:length(c)
        b(3,i) = c(i);
    end
    toc
    b = b.setKind(1);
    tic;
    for i = 1:length(c)
        b(4,i) = c(i);
    end
    toc
end

classdef Subsasgntest
    
    properties
        val
        kind = 0;
    end
    
    methods
        function obj = Subsasgntest(in)
            obj.val = in;
        end
        
        function obj = setKind(obj,kind)
            obj.kind = kind;
        end
        
        function obj = subsasgn(obj,sub,varargin)
            %SUBSASGN Subscripted assignment
            if(isequal(sub.type,'()'))
                if isequal(obj,[])
                    error('Not yet implemented!');
                end
                if(isa(varargin{1},'Subsasgntest'))
                    switch obj.kind
                        case 0
                            obj.val = builtin('subsasgn',obj.val,sub,varargin{1}.val);
                        case 1
                            obj.val = subsasgn(obj.val,sub,varargin{1}.val);
                    end
                else
                    switch obj.kind
                        case 0
                            obj.val = builtin('subsasgn',obj.val,sub,varargin{1});
                        case 1
                            obj.val = subsasgn(obj.val,sub,varargin{1});
                    end
                end
            else
                error('This is not supported for this class!');
            end
        end
    end
end

Results are;

Elapsed time is 0.000019 seconds.
Elapsed time is 4.477900 seconds.
Elapsed time is 4.449843 seconds.

I know self-written subsasgn can be slow, but 235680x slower? Is there any way to achieve a similar level of optimization as in matlabs standard subsasgn?

This guide tells how to branch for different input subscripts, but not how to write the subsasgn syntx itself: https://www.mathworks.com/help/matlab/matlab_oop/code-patterns-for-subsref-and-subsasgn-methods.html

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Jan Siegmund 2020-5-19

在 MATLAB Online 中打开

Update: I added a third kind of subsasgn:

...
case 2
    obj.val(sub.subs{1},sub.subs{2}) = varargin{1}.val;
...

which does not cover all cases, but works for the little example and is much faster:

Elapsed time is 0.028915 seconds.

I think the key here is making matlab realize, that in this call:

obj.val = subsasgn(obj.val,sub,varargin{1}.val);

the rvalue obj.val is equal the lvalue obj.val, so it does not create a temporary obj.val.

Any ideas how I could do this?

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Answer 1

Jan Siegmund 2020-5-19

0
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/526536-how-to-write-an-efficient-overloaded-subsasgn#answer_433404

在 MATLAB Online 中打开

I may have a solution but do not know if this covers all cases:

classdef Subsasgntest
    
    properties
        val
    end
    
    methods
        function obj = Subsasgntest(in)
            obj.val = in;
        end
        
        function obj = subsasgn(obj,sub,varargin)
            %SUBSASGN Subscripted assignment
            if(isequal(sub.type,'()'))
                if isequal(obj,[])
                    error('Not yet implemented!');
                end
                if(isa(varargin{1},'Subsasgntest'))
                    obj.val(sub.subs{:}) = varargin{1}.val;
                else
                    obj.val(sub.subs{:}) = varargin{1};
                end
            else
                error('This is not supported for this class!');
            end
        end
    end
end