MATLAB: Changing a variable when calling a function

calling functionchanging variables

So I have a value 'r' that I'm trying to change from a constant (r=5) to time-dependent (r=1.5*t) when I call a function and redefine 'r'. So far, no luck, I'm new at MATLAB and still don't know a lot of syntax. Here's one of my attempts (the third plot has the changing r):
%function file:
function ydot = ode5_39 (t,y,r)
r=5;
L = 1;
g = 9.81;
ydot(1) = y(2);
ydot(2) = (1/L)*(r*cos(y(1))-g*sin(y(1)));
ydot=ydot';
%plot file:
[t,y,r]=ode45('ode5_39', [0 10], [0.5 0]);
subplot(3,1,1);
plot(t,y(:,1));
ylim([-0.5 1]);
hold on
plot(t,y(:,2));
title('part (a)')
legend('x','xdot');
[t,y,r]=ode45('ode5_39', [0 10], [3 0]);
subplot(3,1,2);
plot(t,y(:,1));
hold on
plot(t,y(:,2));
title('part (b)')
legend('x','xdot');
[t,y,r]=ode45('ode5_39', [0 10], [3 0]);
r=1.5*t;
subplot(3,1,3);
plot(t,y(:,1));
hold on
plot(t,y(:,2));
title('part (c)')
legend('x','xdot');

Best Answer

function ydot = ode5_39 (t, y, rf, rc)
r = rf*t + rc;
L = 1;
g = 9.81;
ydot(1) = y(2);
ydot(2) = (1/L)*(r*cos(y(1))-g*sin(y(1)));
ydot=ydot';
with
[t, y] = ode45(@(t,y) ode5_39(t, y, 0, 5), [0 10], [0.5 0]);  %r = 0*t + 5
and
[t, y] = ode45(@(t,y) ode5_39(t, y, 1.5, 0), [0 10], [3 0]);  %r = 1.5*t + 0
If you wanted something more complicated than linear for finding r, then probably the easiest way to do that would be to use an anonymous function. No point in describing that unless you need it, though.
Related Question