Changing a variable when calling a function

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');

[KALYAN ACHARJYA]

You assigned r=5 within the function, also you listed r in the function inputs lists, why??

Answer 1

[Walter Roberson]

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.