I do not understand why my code still gives me errors

Question

DAVIDE TRONO 2019-7-23

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/473036-i-do-not-understand-why-my-code-still-gives-me-errors

编辑： Jan 2019-7-24

采纳的回答： Jan

在 MATLAB Online 中打开

I'm basically setting a Runge-Kutta of order 4 to solve a sort of 'pred-prey' model. So, my three programs are:

1.

             function [t,y]=RK(fun,tspan,y0,n,A,b,c)
             h=(tspan(2)-tspan(1))/n;
             y(:,1)=y0;
             t(1)=tspan(1);
             s=length(b);
             for j=1:n 
     $              k(:,1)=feval(fun,t(j),y(:,j));
                            for i=2:s
                                    k(:,i)=feval(fun,t(j)+c(i)*h,y(:,j)+h*k(:,1:i-1)*A(i,1:i-1)');
                   end
             end

2.

             function dy=pred_prey(t,y)
             function s=cos(x)
             k=1;
             a=2/3;
             d=4/3;
             r(t)=int(s(x^2),x,0,t);
             mu(t)=13/20-(3/5)*exp(-(3/x));
             dy(1)=(y(1)+k)*r(t)-a*y(1)*y(2);
             dy(2)=-mu(t)*y(2)+d*y(1)*y(2);
             dy=dy';

3.

function []=driver_pred_prey()
            close all
           tspan=[0 100];
           n=1000;
           fun='pred_prey';
           y0=[0 -1]';
           A=[0 0 0 0; 1/2 0 0 0 ; 0 1/2 0 0; 0 0 1 0];
           b=[1/6 1/3 1/3 1/6]';
           c=[0 1/2 1/2 1]';
 $$     [t,y]=RK('pred_prey', tspan, y0, n, A, b, c);
           figure (1)
           plot(t, y(1,:), 'k-'), hold
           plot(t, y(2,:), 'k-.')
           legend('resources',ppl')
           figure (2)
           plot(y(1,:),y(2,:))
           title('orbits)

but I really do not understand what errors there are at $ and $$

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Jan 2019-7-24

在 MATLAB Online 中打开

What about:

% Integral cos2(x) dx = 0.5 * (cos(x)*sin(x) + x)

This is only the part of the messages, which shows where the error is, but not what it is:

Error in RK (line 8) k(:,1)=feval(fun,t(j),y(:,j)); (this is the $ line)
Error in driver_pred_prey (line 14) [t,y]=RK('pred_prey', tspan, y0, n, A, b, c);

Please post the complete messages.

For the first message:

Output argument "dy" (and maybe others) not assigned during call to "pred_prey".

Did you remove the orphaned "function s=cos(x)" as you have been instructed already? Then this message should disappear.

DAVIDE TRONO 2019-7-24

在 MATLAB Online 中打开

function [t,y]=RK(fun,tspan,y0,n,A,b,c)
h=(tspan(2)-tspan(1))/n;
y(:,1)=y0;
t(1)=tspan(1);
s=length(b); %numero di l
for J=1:n
    k(:,1)=feval(fun,t(j),y(:,j));
    for i=2:s
        k(:,i)=feval(fun,t(j)+c(i)*h,y(:,j)+h*k(:,1:i-1)*A(i,1:i-1)');
    end
end

code number 1, it is called RK

function dy=pred_prey(t,y)
k=1;
a=2/3;
d=4/3;
f=@(x)cos(x.^2)
r=@(t)integral(f,0,t);
mu=@(t)13/20-(3/5)*exp(-(3/t));
dy(1)=(y(1)+k)*r(t)-a*y(1)*y(2);
dy(2)=-mu(t)*y(2)+d*y(1)*y(2);
dy=dy';

this is the number 2, called pred_prey

function []=driver_pred_prey()
close all
tspan=[0 500];
n=1000;
fun='pred_prey';
y0=[0 -1]';
A=[0 0 0 0; 1/2 0 0 0 ; 0 1/2 0 0; 0 0 1 0];
b=[1/6 1/3 1/3 1/6]';
c=[0 1/2 1/2 1]';
[t,y]=RK(fun, tspan, y0, n, A, b, c);
figure (1)
plot(t, y(1,:), 'k-'), hold
plot(t, y(2,:), 'k-.')
legend('resources','population')
figure (2)
plot(y(1,:),y(2,:))
title('orbits')

and this is the 3rd one, called driver_pred_prey. The second one has been changed so now the syntax should be right but yes, when I try to run the third(which calls the first two) basically I get reported this:

Index exceeds the number of array elements (1).
Error in RK (line 8)
k(:,1)=feval(fun,t(j),y(:,j));
Error in driver_pred_prey (line 14)
[t,y]=RK(fun, tspan, y0, n, A, b, c);

Those are all the error messages MatLab reports, but I really don't see where is the problem!

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Answer 1

Jan 2019-7-24

1
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/473036-i-do-not-understand-why-my-code-still-gives-me-errors#answer_384583

在 MATLAB Online 中打开

Index exceeds the number of array elements (1).
Error in RK (line 8)
    k(:,1)=feval(fun,t(j),y(:,j));

After the line:

t(1)=tspan(1);

t is a scalar.

If you run this code:

    for J=1:n
      k(:,1)=feval(fun,t(j),y(:,j));

another error is expected: The for loop uses an uppercase J as counter, but inside the loop there is a lower case j. Because it is not defined, j is the imaginary unit and cannot be used as index.

If you fix this to for j = 1:n, there is the problem, that t is a scalar and you cannot access t(2).

Maybe you want this instead:

t = linspace(tspan(1), tspan(2), n);

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DAVIDE TRONO 2019-7-24

在 MATLAB Online 中打开

Well as you said there's no problem when defining

t=linspace(tspan(1),tspan(2),n);

but now a new problem is reported:

Index in position 2 exceeds array bounds (must not exceed 1).

(the other two are the same as before). I guess there's just a little mistake with the indexes.

Jan 2019-7-24

编辑：Jan 2019-7-24

Again: Please post a complete copy of the error message. The details matter.

In which line does the error occur? There is an index in position 2, which exceeds the limits. This should help you to find and to fix the problem.

With some guessing: The code tries to access y(:,j), but this has not been defined anywhere before. The Runge Kutta code calculates the k 's only, but does not create the values for the actual trajectory.

You find many working Runge-Kutta-codes in the net.

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