CODEWITHSUNDEEP
matlaboctave
embert
embert

Reputation: 7592

Why is the function argument undefined?

Apparently this might be a banal error, but I cannot figure out how to do it correctly (my first day with octave). I want to define the second order ODE d²x/dt² + M/L * dx/dt + 5x = 0 as system of ODEs. ODE2.m looks like this:

function dz = ODE2(z,t)
  %% d^2x/dt^2 + M/L * dx/dt + 5x = 0
  M = 2;
  L = 10;
  dz = zeros(2,1);
  dz(1) = z(2);       % <--- apparently here's something wrong?!
  dz(2) = -(M/L)*z(2)-5*z(1);
end

Then I do

t = linspace(0,1,100);
z = lsode(ODE2,[1;1],t);
...

But I get

error: 'z' undefined near line 6 column 11

However, I thought z is defined as argument in the first line of ODE2.m. Maybe the problem is that z is a vector and this is unknown at definition time, but how to do it properly than? How to define the equation and solve it with lsode?

Upvotes: 4

Views: 2639

Answers (1)

Jonas
Jonas

Reputation: 74940

In the call z=lsode(ODE2,[1;1],t);, ODE2 is interpreted as a function call, which will evaluate the function and then run into the issue that there are no input arguments. Note you will get the same error if you split the statement into fun=ODE2; z=lsode(fun,[1;1],t);

Assuming that Octave works the save way as Matlab, you need to pass the reference to ODE2 as a function handle, z=lsode(@ODE2,[1;1],t); will work.

Upvotes: 5

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