Problem with a function

hi,

that's my problem (excuse me I'm a beginner with matlab!)

I introduce the input variables in my workspace x=[1;2;3;4]; t=5;

and I give a numerical value to the others constants that are in the next function (for example K1=1 m=1,2 etc...)

then I write in my command window

(x,t)

where the function is

function xdot = Fun1(x,t)
global K1 m M K2 w F2 c2 c1 g X;
A=[0,0,0,1;0,0,1,0;K1/m,-K1/m,-c1/m,c1/m;-K2/M-K1/M,K1/M,c1/M,-c1/M-c2/M];
B=[0;0;X*g*sin(w*t);-F2/M-X*g*sin(w*t)];
xdot=A*x+B;
end

In substance I expect to get in output a column vector with four lines but I obtain it with only two lines

someone could help me? Thank you in advance!

Answer 1

[Star Strider]

I don’t see the problem. When I run it (with arbitrary constants instead of the global call but otherwise the same code), I get a (4x1) vector.

However I encourage you to avoid global variables. Pass them as extra parameters instead:

function xdot = Fun1(x, t, K1, m, M, K2, w, F2, c2, c1, g, X)
A=[0,0,0,1;0,0,1,0;K1/m,-K1/m,-c1/m,c1/m;-K2/M-K1/M,K1/M,c1/M,-c1/M-c2/M];
B=[0;0;X*g*sin(w*t);-F2/M-X*g*sin(w*t)];
xdot=A*x+B;
end

Then call it from your script or from your ODE solver as:

[K1, m, M, K2, w, F2, c2, c1, g, X] = deal(1, 1, 2, 2, 3, 3, 2, 2, 3, 3);   % Define Constants
x=[1;2;3;4]; t=5;
Fun2 = @(x,t) Fun1(x, t, K1, m, M, K2, w, F2, c2, c1, g, X);
output = Fun2(x,t)

that here produces:

output =
     4.0000
     3.0000
     6.8526
   -12.8526

[Walter Roberson]

The cause of the problem is the use of global variables. In particular the use of global variables that have not been assigned a value and so continue to have the value [].

Answer 2

[Walter Roberson]

If either w or t are empty (as is the case by default for all global variables that have not been otherwise assigned a value), then sin(w*t) would be empty, and then X*g*sin(w*t) would be empty, and then -F2/M-X*g*sin(w*t) would be empty. When you concatenate empty results, the emptiness is "squeezed out", so the result of your concatenation of the four items would be just the two 0's.