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

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

Answer 1

[Steven Lord]

-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.)

[Nadya Shalsabila salman]

@Steven Lord thank you so much for answering my question! I have tried move (-) to the right side, but when i run the code, the program just keeps running without stopping. It will be my pleasure if you could help me out about my problem! Thank you

[Steven Lord]

Is your problem stiff? Try a stiff solver.

[Nadya Shalsabila salman]

@Steven Lord I have tried with ode 15s, ode 23s, ode 23t, and ode 23b. But the program still error and said

Unable to meet integration tolerances......

Arrays have incompatible sizes for this operation.

Error in lsqcurvefit/objective (line 303)

F = F - YDATA;

Error in snls (line 329)

newfvec = feval(funfcn{3},xcurr,varargin{:});

Error in lsqncommon (line 179)

snls(funfcn,xC,lb,ub,flags.verbosity,options,defaultopt,initVals.F,initVals.J,caller, ...

Error in lsqcurvefit (line 295)

lsqncommon(funfcn,xCurrent,lb,ub,options,defaultopt,optimgetFlag,caller,...

Error in trycurvefitIcoba2 (line 96)

[theta,Rsdnrm,Rsd,ExFlg,OptmInfo,~,Jmat]=lsqcurvefit(@kinetics,theta0,t,c);

Related documentation

[Steven Lord]

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]

@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