Pass out a dependent variable from ode15s without taking the derivative of it

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Andreas Skovhøj 2020-3-27

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https://ww2.mathworks.cn/matlabcentral/answers/513338-pass-out-a-dependent-variable-from-ode15s-without-taking-the-derivative-of-it

编辑： darova 2020-3-27

I have search for hours but i can't seem to figure out how it is possible to exstract a variable depending on x (x is derrived using the ode15s)

My solution to a system is (simplified)

function [y,t,rr] = simulation(t,initial)
    options = odeset('RelTol', 1e-5, 'AbsTol', 1e-7);   
    [y,t,rr] = ode15s(@kinetics, t, initial, options); 
end

The kinetics function is as follows (simplified)

function [dxdt,x,rr] = kinetics(t,x,init)
%code desribing i different phenomenas all depending on "x" e.g:
Ph1 = numaxG * x(1,1) * x(8,1) / (KSPG + x(1,1) + (x(1,1)^2)/KiPG);
%constants like numaxG has been defined..
rr(1,1)  = Ph1 * Ph5 * Ph7 * Ph10 * Ph16 * Ph19a;                      
rr(2,1)  = Ph2 * Ph6 * Ph8 * Ph11 * Ph17 * Ph19b * Ph21;          
rr(3,1)  = Ph3;                                                       
rr(4,1)  = Ph12 *  Ph9;                                                
rr(5,1)  = Ph14;                                              
rr(6,1)  = 0;                                                          
rr(7,1)  = 0;
% some other code describing dxdt
end

right now rr is also integrated using the solver, but i would like it to get as an ouput without it being solved. For me this is difficult as it is dependent on x which is derived.

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Torsten 2020-3-27

You used three output parameters in your code, not two.

Andreas Skovhøj 2020-3-27

no but that is my problem.. i know that i should only use two but i added "rr" because i need it as a output from the function.

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采纳的回答

darova 2020-3-27

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/513338-pass-out-a-dependent-variable-from-ode15s-without-taking-the-derivative-of-it#answer_422343

在 MATLAB Online 中打开

You should calculate rr variable after solving ode45

An example

function main
[t,u] = ode45(@f,[0.1 3],[1 1]);
u1 = u(:,1);        % extract u1
u2 = u(:,2);        % extract u2
rr1 = u1 - u2 ...
end
function du = f(t,u)
du(1,1) = u(2);
du(2,1) = sin(u(1))/t;
end

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Andreas Skovhøj 2020-3-27

Here are my two files i think you will need to understand by problem

darova 2020-3-27

编辑：darova 2020-3-27

在 MATLAB Online 中打开

Here is an idea

Your current kinetics.m

function [dxdt,rr,x] = kinetics(t,x,p,init,t_load,a,mode)
    % long code
    % computations with 'x' and other parameters
    rr(1:7,1) = ... % calculations
    dxdt = ...      % derivatives
end

Write an additional function

function par = mycalc(p,x)
    % long code
    % computations with 'x' and other parameters
    % ... return all needed results for further calculation as struct
    par.Ph1 = Ph1;
    par.Ph2 = Ph2;
end
function dxdt = kinetics(t,x,p,init,t_load,a,mode)
    par = mycalc(p,x)
    rr(1:7,1) = ... % calculations
    dxdt = ...      % derivatives
end