Proper way to use exist function.

I am using the ode23 function to call another function. The problem is that I need to change the time value each time the function is called in order to change another variable. Here is the part of the function that is not working to count time:

function f = wdotz(t,w,M,K,Q)
% -------------------------------------------------------
% time counter
AKA = exists(i);
if AKA == 0
    i = 0
end
i = i+1;
time = (0:.001:22.5)';

treal = time(i);
% -------------------------------------------------------

when I run the code that calls this function it gives me this error:

Error: File: wdotz.m Line: 7 Column: 5

"i" previously appeared to be used as a function or command, conflicting with its use here as the name of a variable.

A possible cause of this error is that you forgot to initialize the variable, or you have initialized it implicitly

using load or eval.

[Stephen23]

"Proper way to use exist function."

I would argue that the "proper" way to use exist on a variable is to NOT use it: introspective programming is inherently inefficient, which is why experienced users and the MATLAB documentation warn against using it: "Avoid functions that query the state of MATLAB such as inputname, which, whos, exist(var), and dbstack. Run-time introspection is computationally expensive."

source: https://www.mathworks.com/help/matlab/matlab_prog/techniques-for-improving-performance.html

In any case, it is not clear what you expect your code to do. Please explain the expected behavior.

[Walter Roberson]

i will exist because it is a function that returns sqrt(-1)

Answer 1

[Image Analyst]

In your main, calling code, you do this:

f = wdotz(t,w,M,K,Q)

Note that none of the variables is called i. So there is never ever any i in your code, until you assign it.

exists() is not a function, but exist() is, and you'd need to pass in a second argument of 'var':

iExists = exist(i, 'var')

time() is a built-in function so don't use that as a variable name.

You're passing in t, which is perhaps a "time so far". If you're trying to return the elapsed time since t just do this

f = toc(t);

and get rid of everything else except the function line.

Answer 2

[Walter Roberson]

The time value is being received into t and the ode routines will modify it as necessary to achieve integration tolerances.

[Jesse Crotts]

I have discovered that I can use the time variable built into the ODE function. The time variable also changes a force input. Thats why I needed it. However, I am wondering why the exist function does not work the way that I am using it?

[Stephen23]

Because i always exists:

https://www.mathworks.com/help/matlab/ref/i.html

And if that i is not what you expect to be testing for, what do you expect to test the existence of?