How can I use dynamic field names instead of setfield inside a function call?

In the following call, I have an existing structure Syst0. I want to call a function with as an argument Syst0 with a field set to a value:

HwErr1 = TracePeriodicSys(setfield(Syst0,'AngleError',Err1));

This line generates a yellow caution triangle with the note "Use dynamic fieldnames with structures instead of SETFIELD". How do I do this without creating a temporary structure (e.g.,

SystTemp=Syst0;SystTemp.AngleError=Err1;HwErr1 = TracePeriodicSys(SystTemp);

I think maybe this caution is erroneous.

[Walter Roberson]

There is a subsasgn equivalent but it is pretty messy.

Most of the times that I encounter setfield, it is in the form

variable = setfield(variable, fieldname, value)

and that is the kind of code that the caution is intended to address.

Using side effects the way you do is not recommended, but it is legal.

[Irl Smith]

Thanks for the quick response! I am, though, puzzled about your comment that this is "using side effects". Can you explain further? I thought this was a straightforward application of setfield.

[Walter Roberson]

The side effect is in the temporary variable that MATLAB creates.

Your code is equivalent to

HwErr1 = TracePeriodicSys(subsasgn(Syst0, struct('type', '.', 'subs' {'AngleError'}), Err1));

This does not modify Syst0: it creates a temporary variable to do the assignment into, and passes the result as output.

[Irl Smith]

Thank you much for that explanation. It sounds like using setfield in the way I did doesn't necessarily save time or memory, but at least the code is a little easier (for me, anyway) to read.