# ? Input argument y is undefined.

1. Apr 23, 2007

??? Input argument "y" is undefined.

HII Friends
Iam little bit new to matlab and iam trying to use ode45 and wrote code as

function dy = sampleode(t,y)

dy=zeros(2,1);

dy(1)=-0.192*y(1)*log(y(1)/y(2).^3);

dy(2)=(1/3)*(5.85-0.00873*y(2).^2-0.15*0*y(2)-0.02*y(2));

[t,y]=ode45('sampleode', [0:5:40],[1;1]);

BUt iam getting the below error:

??? Input argument "y" is undefined.

Error in ==> sampleode at 3
dy(1)=-0.192*y(1)*log(y(1)/y(2).^3);
I saved my mfile as sampleode.m .please suggest me any suggestions. Thanks in advance

2. Apr 24, 2007

### Glass

Hi,

I'm also a newbie in MATLAB, but two things strike me as odd here
1) In the following line you multiply by 0. Did you really mean to do that?
dy(2)=(1/3)*(5.85-0.00873*y(2).^2-0.15*0*y(2)-0.02*y(2));
2) I was under the impression ode45 required a function handle not a string containing the name of the function (though I may be wrong there)
Moreover, what are you inputing as your y as you didn't give the command you use to run the function.
Regardless, sorry I can't be much help.

3. Apr 24, 2007

### AiRAVATA

Matlab is strange when working with functions.

One solution is to define the function (and save it in an .m file) and then running ode45 in console. The other (wich i prefer) is to work with nested functions.

In the first one, you should do a sampleode.m file with the following code

Code (Text):
function dy = sampleode(t,y)
dy=zeros(2,1);
dy(1)=-0.192*y(1)*log(y(1)/y(2).^3);
dy(2)=(1/3)*(5.85-0.00873*y(2).^2-0.15*0*y(2)-0.02*y(2));
end
and then in console type

Code (Text):
[t,y]=ode45(@sampleode, [0:5:40],[1;1]);
(nothe the @).

The second solution works better when you are using a more sophisticated code and would be using nested functions. Create a file named ord.m with the following code:

Code (Text):
function ord

[t,y]=ode45(@sampleode, [0:5:40],[1;1]);

function dy = sampleode(t,y)
dy=zeros(2,1);
dy(1)=-0.192*y(1)*log(y(1)/y(2).^3);
dy(2)=(1/3)*(5.85-0.00873*y(2).^2-0.15*0*y(2)-0.02*y(2));
end

end
That way, when you type
Code (Text):
ord
in console, Matlab will perform ode45 on the function sampleode. If you add the line
Code (Text):
plot(y,U);
after ode45 but before defining sampleode, then Matlab will perform ode45 and plot the solution every time you type
Code (Text):
ord
in console.

You should check the diference between local and global variables, it could provide another way of solving your problem.

Last edited: Apr 24, 2007