Can you update a constant in a function.

Question

Lee Luderman 2018-1-22

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评论： Kevin 2021-10-7

I currently have 3 functions which rely on a constant I have created inside a file called cConstants.m

classdef cConstants
   properties (Constant)
    a = 3
    m = 1
   end
end

The 3 functions which this constant are myv.m, mydv.m and myd2v.m

function [ v ] = myv( x )
import constants.*;
v = 2*exp(-(x^2)/2)+(cConstants.m/(cConstants.m+(x-cConstants.a)^2));
end

With mydv.m and myd2v.m being the first and second derivative of myv.m

I have a function which finds the minimum points of myv.m.

I now want to create a function which finds the minimum points of myv.m as the constant cConstants.a varies over an interval.

Something similar to this.

function f = PlotMin(a,b,h) %%a = starting value of a, b = end value of a and h is increase interval
hold on
while a <= b
cConstants.a = a;
x = FindMin(-10,10,0.5,4);
y = mydv( x );
plot(x,y)
a = a+h;
end

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Adam 2018-1-22

You can set a default value for any property so if you do that and never change it then it would act like a constant from a usage perspective, but it gives the additional ability to alter it if you need to, which can be good and bad since it allows for accidental changes that a constant doesn't, but works for what you want with deliberate changes.

Lee Luderman 2018-1-22

How would I go about defining this property across multiple function files?

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Answer 1

Walter Roberson 2018-1-22

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https://ww2.mathworks.cn/matlabcentral/answers/378337-can-you-update-a-constant-in-a-function#answer_301176

https://www.mathworks.com/help/matlab/matlab_oop/properties-with-constant-values.html

"Use constant properties to define constant values that you can access by name. Create a class with constant properties by declaring the Constant attribute in the property blocks. Setting the Constant attribute means that, once initialized to the value specified in the property block, the value cannot be changed."

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Lee Luderman 2018-1-22

Thank you for the prompt reply, what options are there to achieve what I'm looking to do?

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Answer 2

Guillaume 2018-1-22

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By definition a constant property is ... constant. Matlab is not designed so that these properties can change.

To be perfectly honest, it is possible to design the class so that you could change the constant properties but that would involve having to call clear classes each time you want to update the constant values. This could have some unpleasant side effects.

The workaround would be to create a singleton non-constant class. You'd be in effect creating a class that acts as a global so I'm not sure it is a good idea either.

I think you'd be better off having a normal class with non-constant properties and pass instances of the class to your mydv and co. functions.

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Guillaume 2018-1-22

在 MATLAB Online 中打开

This is what I would recommend:

classdef Coefficients
   properties    
      a = 3;
      m = 1;
   end
end
   function v = myv(x, coeffs)
      v = 2*exp(-(x^2)/2)+(coeffs.m/(coeffs.m+(x-coeffs.a)^2));
   end
function f = PlotMin(a,b,h) %%a = starting value of a, b = end value of a and h is increase interval
  coeffs = Coefficients;
  hold on
  while a <= b
     coeffs.a = a;
     x = FindMin(-10,10,0.5,4);
     y = mydv(x, coeffs);
     plot(x,y)
     a = a+h;
  end
end

But to be honest, you may as well use a structure instead of a class for the above.

----

If you really want to have a class that acts as a global:

classdef Coefficients < handle
   properties
      a = 3;
      m = 1;
   end
   methods (Static)
      function singleton = getinstance   %function to retrieve the singleton
          persistent singleton;
          if isempty(singleton)
             singleton = Coefficients;
          end
      end
   end
   methods (Access = private)   %private constructor. Can only be invoked by getinstance
      function this = Coefficients
      end
   end
end
   function v = myv(x)
      coeffs = Coefficients.getinstance;
      v = 2*exp(-(x^2)/2)+(coeffs.m/(coeffs.m+(x-coeffs.a)^2));
   end
function f = PlotMin(a,b,h) %%a = starting value of a, b = end value of a and h is increase interval
  coeffs = Coefficients.getinstance;
  hold on
  while a <= b
     coeffs.a = a;
     x = FindMin(-10,10,0.5,4);
     y = mydv(x);
     plot(x,y)
     a = a+h;
  end
end

But this suffers the same problem as any global: Any code, anywhere, can change the value of the coefficients under your feet.

Guillaume 2018-1-22

编辑：Guillaume 2018-1-22

The getGlobalx functions are a bit pointless. They don't give any kind of safety above just fetching the global directly.

Using globals (or my singleton class emulating globals) is really not recommended (in any language). It really makes it hard to debug code when any function anywhere can change the value of a variable. It's a maintenance nightmare when you have to review every single line of code in every function when you edit a global variable and have to check which piece of code it will affect.

The proper way to do what you want is to pass these not really constant anymore values to the function, as proper arguments.

Kevin 2021-10-7

Thank you so much for your explanation, this problem has troubled me for a long time.

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Can you update a constant in a function.

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Can you update a constant in a function.

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