Multiple unit steps in ode solver

Hi there,

I am trying to run an odesolver where in the differential equation, there is a function that uses a for loop.

The code is something like this:

for N=1:10
  Iapp=Io*(heaviside(20*N+2-t)-heaviside(20*N-t));
  hold on
end
dydt(1,1) = Iapp/C;

Iapp is meant to generate multiple heaviside step functions that only lasts for 2s in each step. The number of step functions is controlled by the N.

I then created a separate file for the odesolver. Something like this:

x = [0,500];
[t,y] = ode15s(@filename,[x],[the IC]);

The problem I get after this is that the Iapp always comes out as 0. Thus not resulting in the result that I need.

I hope my issue is clear enough and I would be grateful if someone can help me with this, as I am very stuck.

Thanks in advance!

Answer 1

[Walter Roberson]

"hold on" only has to with graphics routines.

When you have a "for" loop in which you are not using a reduction variable and you are not indexing the output, then each iteration of the "for" loop overwrites all of the output of the previous, giving a result that is the same as if only the last iteration had been run. Your

 for N=1:10
  Iapp=Io*(heaviside(20*N+2-t)-heaviside(20*N-t));
  hold on
 end

is the same as if you had written

 for N=10
  Iapp=Io*(heaviside(20*N+2-t)-heaviside(20*N-t));
 end

Possibly you wanted

 for N=1:10
  Iapp(N)=Io*(heaviside(20*N+2-t)-heaviside(20*N-t));
 end

but that would create a vector Iapp. I speculate that you want to sum all of them together,

dydt(1,1) = sum(Iapp)/C;

If so then you could write

N = 1 : 10;
Iapp = Io*(heaviside(20*N+2-t)-heaviside(20*N-t));
dydt(1,1) = sum(Iapp)/C;

with no loop.