# My code runs but the plots do not show any line

2 views (last 30 days)
DAVIDE TRONO on 24 Jul 2019
Answered: Jay Patel on 24 Jul 2019
I'm using a Runge-Kutta of order 4 to resolve a system of differential equations. My codes 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
y(:,j+1)=y(:,j)+h*k*b;
t(j+1)=t(j)+h;
end
end
then
2
function dy=pred_prey(t,y)
a=2/3;
d=4/3;
r=(2/3)*(1/(exp(-0.5*t)+1)-0.5)*(1+(sin(t)/t));
mu=13/20-(3/5)*exp(-(3/t));
dy(1)=y(1)*r-a*y(1)*y(2);
dy(2)=-mu*y(2)+d*y(1)*y(2);
dy=dy';
end
and
3
function []=driver_pred_prey()
close all;
tspan=[0 5000];
n=1000;
fun='pred_prey';
y0=[2 5]';
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')
end
When I run driver_pred_prey everything is okay, except that no lines are showed in the plots.
Star Strider on 24 Jul 2019
If you use:
ynans = nnz(isnan(y))
inside ‘driver_pred_prey’, you will find that the result is 2000. So the result of every computation is NaN, which is of course the reason nothing shows up on the plots.
NaN values are the result of 0/0, Inf/Inf and similar operations, and they propagate through iterative computations so that if one result is NaN, everything following it is also NaN.
You have to find that and correct it.

Jay Patel on 24 Jul 2019
Hi,
You have an indeterminate form expression in your code, please debug your code line by line and correct it. It might solve your problem.

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