Incorrect use of '=' operator. Assign a value to a variable using '=' and compare values for equality using '=='.

Question

Nadya Shalsabila salman 2021-10-24

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1570363-incorrect-use-of-operator-assign-a-value-to-a-variable-using-and-compare-values-for-equalit

评论： Nadya Shalsabila salman 2021-10-26

trycurvefitI.m

Asking for help.

File: trycurvefitI.m Line: 45 Column: 17

ERROR "Incorrect use of '=' operator. To assign a value to a variable, use '='. To compare values for equality, use '=='."

I already followed instruction above but my dcdt(2) plot become a linear line.Any little suggestion or solutions or comments are very welcomed! Thank you so much!

This is my code

function trycurvefit
function C=kinetics(theta,t)
c0=[0.1143; 0.9659; 0.04406];
[T,Cv]=ode45(@DifEq,t,c0);
%
    function dC=DifEq(t,c)
        z = [1
             2
             3
             4
             5
             6
             7
             8
             9
             10
             11
             12
             13
             14
             15
             16
             17
             18
             19
             20];
         
        j = 1;
        for j = 1 : 20   
        if z(j) <= 4 
            I = 45; 
        elseif z(j) <= 7
            I = 90;
        elseif z(j) <= 11
            I = 120;
        elseif z(j) <= 15
            I = 150;
        else 
            I = 180;
        end
    
        dcdt=zeros(3,1);
        dcdt(1)= (theta(1).*(c(2)/(theta(2)+c(2)+(c(2).^2/theta(3))))*(I/(theta(4)+I)).*(1-c(1)/theta(5)).*(1-c(3)/theta(6)));
        -dcdt(2)=(1/theta(7).*dcdt(1))+(1/theta(8).*dcdt(3))+(theta(9).*c(1));
        dcdt(3)= (theta(10).*dcdt(1)+theta(11).*c(1)).*(c(2)./(theta(12)+c(2)+(c(2).^2/theta(13)))).*(c(3)/(theta(14)+c(3))).*(I/(theta(15)+I));
        dC=dcdt;
        end
    end
C=Cv;
end

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Answer 1

Steven Lord 2021-10-24

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1570363-incorrect-use-of-operator-assign-a-value-to-a-variable-using-and-compare-values-for-equalit#answer_815053

-dcdt(2)=(1/theta(7).*dcdt(1))+(1/theta(8).*dcdt(3))+(theta(9).*c(1));

Assigning to element 2 of the variable dcdt is allowed.

Assigning to the expression -dcdt(2) is not allowed.

Either eliminate the - entirely or move it to the right side (essentially multiply both sides of the line of code by -1.)

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Steven Lord 2021-10-25

I'm guessing that you're not specifying the tspan vector as a vector with more than 2 elements to evaluate the solution of the ODEs at the times for which you have data that you're trying to fit the curve. See these documentation pages:

https://www.mathworks.com/help/optim/ug/fit-ode-problem-based-least-squares.html
https://www.mathworks.com/help/optim/ug/fit-differential-equation-ode.html
https://www.mathworks.com/help/optim/ug/optimizing-a-simulation-or-ordinary-differential-equation.html

Nadya Shalsabila salman 2021-10-26

@Steven Lord I have tried to specifying the tspan, but still not work. I think the problem more complicated and i still don't get it to solve this. It will be my pleasure if you could help me out about my problem.

*Note: below is the problem i want to simulate in matlab. if there is an fault in my code, please let me know

I am trying to plot graphs (Substrate vs time, Biomass vs time, and Product vs time) from these modified kinetic equations below:

dX/dt = miumax * S/(Ks*S*(S^2/Kxi)) * I/(KXI+I) * (1-X/Xm) *(1-P/Pm)

-dS/dt = 1/Yxs*(dX/dt) + 1/Yps*(dP/dt) + miusx*X

dP/dt = (Ypx*(dX/dt) + miupx*X) * S/(Kps+S+(S^2/Kpi)) * P/(Kp+P) * I/(KPI+I)

where miumax, Ks, Kxi, KXI, Xm, Pm, Yxs, Yps, miusx, Ypx, miupx, Kps, Kpi, Kp, and KPI are kinetic parameters; S, X, P are substrate concentration, biomass concentration, and product concentration respectively. Besides the graphs, i also want to have the kinetic parameters as the output. The initial concentrations for each X, S, and P are 0.1143; 0.9659; 0.004406

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