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

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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.
  4 Comments
Walter Roberson
Walter Roberson on 17 Oct 2019
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
Irl Smith on 17 Oct 2019
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.

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