MATlab code line doesnt run

I am having the following MATlab code and keep running into the error of my xdot(i) line.

Error: Check for missing argument or incorrect argument data type in

call to function 'R'.

Error in 360Second (line 14)
    xdot(i)=[x(3);x(4);-u*x/R(i)^3;-u*y/R(i)^3];

can you please check if you see anything else wrong with it after I fix that error.

a= 7000;
e= 0.1;
time= (60*60*24);
u= 3.986*10^5;
dt=0.1;
t=(0:.1:86400);

Rp= a*(1-e);
Vp=sqrt(u*(2/Rp-1/a));
x=zeros(4);
x=[Rp;0;0;Vp];
i=1;
while t(i)<=86400
    xdot(i)=[x(3);x(4);-u*x/R(i)^3;-u*y/R(i)^3];
    x=x+xdot*dt;
    i=i+1
    R(i)=sqrt(x(1)^2+x(2)^2);
    V(i)=sqrt(x(3)^2+x(4)^2);
    E(i)=V(i)^2/2-u/R(i);
    
end

plot(R,t);
plot(V,t);
plot(E,t);
title('random'), xlabel('random'), ylabel('random'); 
grid on        

[the cyclist]

Please copy/paste the actual code, or upload it in an M-file.

Except for absolutely trivial errors that can be spotted immediately, images are useless, because we would need to type all that code ourselves, to debug it.

[Oskar Kinat]

a= 7000;
e= 0.1;
time= (60*60*24);
u= 3.986*10^5;
dt=0.1;
t=(0:.1:86400);

Rp= a*(1-e);
Vp=sqrt(u*(2/Rp-1/a));
x=zeros(4);
x=[Rp;0;0;Vp];
i=1;
while t(i)<=86400
    xdot(i)=[x(3);x(4);-u*x/R(i)^3;-u*y/R(i)^3];
    x=x+xdot*dt;
    i=i+1
    R(i)=sqrt(x(1)^2+x(2)^2);
    V(i)=sqrt(x(3)^2+x(4)^2);
    E(i)=V(i)^2/2-u/R(i);
    
end

plot(R,t);
plot(V,t);
plot(E,t);
title('random'), xlabel('random'), ylabel('random'); 
grid on        

[the cyclist]

When I run your code, I get the error message:

Unrecognized function or variable 'R'

Have you defined R? Can you share that function?

[Oskar Kinat]

I moved R(i) above the xdot and added R=[Rp,o]

R= [Rp,0];
Rp= a*(1-e);
Vp=sqrt(u*(2/Rp-1/a));
x=zeros(4);
x=[Rp;0;0;Vp];
i=1;
while t(i)<=86400
    R(i)=sqrt(x(1)^2+x(2)^2);
    xdot(i)=[x(3);x(4);-u*x/R(i)^3;-u*y/R(i)^3];
    x=x+xdot*dt;
    i=i+1
    
    V(i)=sqrt(x(3)^2+x(4)^2);
    E(i)=V(i)^2/2-u/R(i);
end

[Oskar Kinat]

Now I get a different error

"

Unable to perform assignment because the indices on the left side are not compatible with the size of the right side.

"

[the cyclist]

You don't define the variable y anywhere in this code, so it does not run for me.

Can you please upload the complete code, and first verify that it gives the error you are getting.

[Oskar Kinat]

I' am sorry I just added the y=0 I believe thats right.

a= 7000;
e= 0.1;
time= (60*60*24);
u= 3.986*10^5;
dt=0.1;
t=(0:.1:86400);
y=0;

R= [Rp,0];
Rp= a*(1-e);
Vp=sqrt(u*(2/Rp-1/a));
x=zeros(4);
x=[Rp;0;0;Vp];
i=1;
while t(i)<=86400
    R(i)=sqrt(x(1)^2+x(2)^2);
    xdot(i)=[x(3);x(4);-u*x/R(i)^3;-u*y/R(i)^3];
    x=x+xdot*dt;
    i=i+1
    
    V(i)=sqrt(x(3)^2+x(4)^2);
    E(i)=V(i)^2/2-u/R(i);
    
end

plot(R,t);
plot(V,t);
plot(E,t);
title('random'), xlabel('random'), ylabel('random'); 
grid on      

Answer 1

[Neeraj Rajpurohit]

DISCLAIMER: These are my own views and in no way depict those of MathWorks.

Greetings,

I see that you are accessing arrays R, V and E, without initialization in the line .To make the given script work, you will have to do the initialization of these variables. Also, to access the variables with indices, you will have to preallocate the arrays with appropriate sizes.