Using a matrix as an input argument for a function

Hello,

I'm currently trying to solve a problem in Matlab that involves using two input arguments in a function, one of which is a matrix. My problem is that within the Matrix are the variables y, z, Vy and Vz which Matlab recognises as undefined.

The code I'm having trouble with is this:

[dotx]=me12ttdP4(t, x)

x=([y;z;Vy;Vz]);

Vdoty=(K2(z-y)+B(Vz-Vy))/M;

Vdotz=(K1(u-z)+K2(z-y)-B(Vz-Vy))/m;

Ydot=Vy;

Zdot=Vz;

dotx=[Ydot;Zdot;Vdoty;Vdotz];

I get the error in the command window when I input

>> me12ttdP4(3, [1;2;0;3])

The error is:

Undefined function or variable 'y'.

Error in me12ttdP4 (line 6)
x=([y;z;Vy;Vz]);

Thank you for your time

Answer 1

[Image Analyst]

Why do you think that the me12ttdP4() function should know what y is when you didn't pass it in or define it inside me12ttdP4() itself? It won't know what it is until you give it a value.

Answer 2

[Thiran]

The variable y, z, Vy and Vz don't have a constant value. The input of:

>> me12ttdP4(3, [1;2;0;3])

into the main window is just to test whether or not the script genereates a vector for dotx.

Answer 3

[Sagar Damle]

I think,this will give you correct answer.
[dotx]=me12ttdP4(t, x)
y = x(:,1); z =x(:,2);  Vy =x(:,3);  Vz =x(:,4);
Vdoty=(K2(z-y)+B(Vz-Vy))/M;
Vdotz=(K1(u-z)+K2(z-y)-B(Vz-Vy))/m;
Ydot=Vy;
Zdot=Vz;
dotx=[Ydot;Zdot;Vdoty;Vdotz];

But my question is - you have not defined values of K1,K2,M and m in function definition of 'mel2ttdP4()'.How can you get correct output without this? Oh,are these global variables or such a thing?

[Thiran]

They are indeed global variables, I didn't include the whole of the code :) I will try your solution thank you

[Sagar Damle]

I have assumed that variables y,z,Vy and Vz are column vectors. If they are scalars(matrix with 1*1 dimension),then you can use this line instead of one given above -

y = x(1); z =x(2); Vy =x(3); Vz =x(4);

[Thiran]

I tried your solution and got the error:

Attempted to access x(:,2); index out of bounds because size(x)=[4,1].

Error in me12ttdP4 (line 6)
y=x(:,1); z=x(:,2); Vy=x(:,1); Vz=x(:,1);

So I switched it around a bit to:

y=x(1,:); z=x(2,:); Vy=x(3,:); Vz=x(4,:);

this now produced the error:

Index exceeds matrix dimensions.

Error in me12ttdP4 (line 23)
Vdoty=(K2(z-y)+B(Vz-Vy))/M;

[Sagar Damle]

Sorry for my mistake! I think you want to get product of two terms - K2 and (z-y).So you have to write this as below -

Vdoty=(K2*(z-y)+B*(Vz-Vy))/M;

Similarly, Vdotz=(K1*(u-z)+K2*(z-y)-B*(Vz-Vy))/m;

Answer 4

[Mohit Kumar]

try this

[y;z;Vy;Vz]=x;

[Stephen23]

@Mohit Kumar: please provide a link to the documentation, where this syntax is explained?:

[y;z;Vy;Vz]=x;