using interp1 on ode45

i try to implement interpolation on ode45 but mostly error pop up, plus i dont really understand how it works. these are codes used...

function dxdt = forced(t,x)
dxdt_1 = x(2);
dxdt_2 = -100*x(2)-250000*x(1)+ sin(500*t);
dxdt = [dxdt_1;dxdt_2];

tspan=[0:0.1:100];
initial_x=0;
initial_dxdt=0;
[t,x]=ode45(@forced,tspan,[initial_x initial_dxdt]);
figure
plot(t,x(:,1));
grid on

this will result to having x data..

now, i want to interpolate x data into f(see below function)...

function dxdt = forced(t,x)
dxdt_1 = x(2);
dxdt_2 = -100*x(2)-250000*x(1)+(25000*(f(t)^3));
dxdt = [dxdt_1;dxdt_2]; 

tspan=[0:0.1:100];
initial_x=0;
initial_dxdt=0;
f=interp1(x1,p,x);
[t1,x1,f]=ode45(@forced,tspan,[initial_x initial_dxdt]);

but it doesnt work, and error pops such as Index exceeds matrix dimensions.

Error in interp1>sortxv (line 423)
        V = V(idx,:);

Error in interp1 (line 186)
    [X, V] = sortxv(X,V);
...

any body has any suggestion or correct my codes, thanks.

Answer 1

[Star Strider]

The problem as I see it is that you haven’t defined ‘x1’, ‘p’, or ‘x’ before this line:

f=interp1(x1,p,x);

at least in the code you posted. Also, this line:

[t1,x1,f]=ode45(@forced,tspan,[initial_x initial_dxdt]);

will likely throw an error.

I would not interpolate the output of ode45, instead I would simply define a ‘tspan’ vector with the points I wanted the ODE to be evaluated.