Structures with multiple, conntected anonymous functions

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Kyle Johnson 2020-5-13

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/525050-structures-with-multiple-conntected-anonymous-functions

评论： Kyle Johnson 2020-5-13

采纳的回答： Ameer Hamza

I have a rather complicated workflow, however, I am having trouble identifying a way for it to work.

I start with a json of the form:

{
    "x":3,
    "y":6,
    "fx":"@(a) a^2+y",
    "fy":"@(a,b) y*fy(b)+b*x"
    }       

This gets read in and evaluated such that:

S.x = 3
S.y = 6
S.fx = @(a) a^2+y
S.fy = @(a,b) y*f(b)+b*x

I have tried both eval and str2funct to create the anonymous function.

Later in the workflow, this structure gets packaged into another structure

dS.S = S;

This is to help with passing a model around between functions.

The problem is, that later on, this command will give an error:

dS.S.fy(1,1)

The error states that the function fx cannot be found. However,

dS.S.fx(1)

does work, WITHOUT there being an "y" defined in the workspace.

Can anyone shed some light on this?

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Ameer Hamza 2020-5-13

What is f(b) in this line

y*f(b)+b*x

is it supposed to be fx(b)?

Kyle Johnson 2020-5-13

@James Tursa, they do not need to pick up S.x or S.y. These values will not change once they are read in.

@Ameer Hamza, yes that is a typo

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采纳的回答

Ameer Hamza 2020-5-13

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编辑：Ameer Hamza 2020-5-13

Ok. I know "eval is evil" but here it makes everything quite simple in this case. You can define a helper function like this

function s = helperFun(s)
    names = fieldnames(s);
    for i=1:numel(names)
        eval(sprintf('%s=s.%s;', names{i}, names{i}));
    end
    for i=1:numel(names)
        if ischar(s.(names{i}))
            s.(names{i}) = eval(s.(names{i}));
        end
    end
end

and then call it on your json file like this

S = jsondecode(fileread('test.txt'));
S = helperFun(S)
ds.S = S;

Then call the functions

>> ds.S.fx(1)
ans =
     7
>> ds.S.fy(1, 2)
ans =
   246

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Kyle Johnson 2020-5-13

This is exactly what I have done. Thank you. I have marked this at the accepted answer for future readers.

Ameer Hamza 2020-5-13

I am glad to be of help.

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the cyclist 2020-5-13

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Are you sure that y didn't exist in the workspace when you defined the structures? Because in a fresh workspace,

S.x = 3
S.y = 6
S.fx = @(a) a^2+y
S.fy = @(a,b) y*f(b)+b*x
dS.S = S
dS.S.fx(1)

gives the error

Unrecognized function or variable 'y'.
Error in @(a)a^2+y

Does that shed light?

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James Tursa 2020-5-13

编辑：James Tursa 2020-5-13

Function handles take snapshots of everything in the workspace that is part of them at the time they are built. It doesn't matter if you subsequently change those variables or even clear them downstream in your code because the function handle has stored inside of it those snapshots. The only way to change those snapshots is to rebuild the function handle from scratch. E.g.,

>> a = 4;
>> b = 5;
>> fx = @(x)a*x+b; %<-- fx stores snapshots of a and b inside itself
>> fx(2)
ans =
    13
>> a = 6; % <-- this changes a, but does not change the snapshot of a inside fx
>> b = 7; % <-- this changes b, but does not change the snapshot of b inside fx
>> fx(2)
ans =
    13

And then suppose we create a new function handle fy:

>> fy = @(x,y) fx(x)+a  % <-- fy stores current snapshots of fx and a
fy =
  function_handle with value:
    @(x,y)fx(x)+a
>> fy(2,3)
ans =
    19

Now change fx:

>> fx = @(x)b*x+a  % <-- fx stores current snapshots of a and b
fx =
  function_handle with value:
    @(x)b*x+a
>> fy(2,3)
ans =
    19

Notice that fy didn't change. That is because fy is working with it's shapshot of fx as fx existed at the time that fy was created. It doesn't matter that you subsequently changed fx downstream in your code.

You could do this to get fx defined properly in fy:

x =3;
y = 6;
S.x = x
S.y = y
fx = @(a) a^2+y  % <-- fx stores shapshot of y
fy = @(a,b) y*fx(b)+b*x  % <-- fy stores snapshots of fx and x and y
S.fx = fx
S.fy = fy

BOTTOM LINE: Function handles always work with shapshots of workspace variables at the time of their creation. If you want different behavior, use actual function files that will go looking for functions in realtime instead of using function handles.