Changing a variable when calling a function

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Jesse Olson
Jesse Olson 2018-10-2
编辑： Stephen23 2018-10-2
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');
  1 个评论
KALYAN ACHARJYA
KALYAN ACHARJYA 2018-10-2
You assigned r=5 within the function, also you listed r in the function inputs lists, why??

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采纳的回答

Walter Roberson
Walter Roberson 2018-10-2
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.

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