How would I check to see if a struct field exists OR is empty?

I'd like to program a conditional statement that checks if an optional events function for an ODE was triggered. This would look something like:

if ~isfield(sol,'xe') || isempty(sol.xe)
    fprintf('Event Not Triggered\n')
else
    fprintf('Event Triggered\n')
end

However, this fails if sol.xe doesn't exist because no events function was specified. In the interest of shorter/cleaner code, I'd like to avoid this solution:

if isfield(sol,'xe')
    if isempty(sol.xe)
        fprintf('Event Not Triggered\n')
    else
        fprintf('Event Triggered\n')
    end
else
    fprintf('Event Not Triggered\n')
end

Is there a better way to do this?

[Fangjun Jiang]

If .xe does not exist in sol, the first code still prints 'Event Not Triggered\n'. What do you mean 'fails'?

[Darren Wethington]

Many apologies, I wrote this up and did not test it. Comparing it to my actual code, I had something that mimicked this instead:

if isempty(sol.xe) || ~isfield(sol,'xe')
    fprintf('Event Not Triggered\n')
else
    fprintf('Event Triggered\n')
end

This generates an error when tested, so it seems that ordering matters here; the second condition is only checked if the first is not met.

Thank you for helping me to understand my code and Matlab better, and I apologize for the inconvenience. Thanks to @Walter Robinson as well.

[Walter Roberson]

Conditions are evaluated left to right except when there are (), in which case everything before the () is evaluated and then the inside of the () is evaluated as a group.

When the || or && operators are used, the values on the two sides must be scalar (even empty is not permitted), and the calculation within that group stops as soon as it figures out the answer-- so

if A||B

declares success if A is true, without executing B, but continuing on to B if A is false; and

if A&&B

declares failure if A is false, without executing B, but continuing on to B if A is true.

This is not the case for the & or | operators: they execute both sides anyhow.

Answer 1

[Walter Roberson]

You might find this clearer:

if isfield(sol,'xe') && ~isempty(sol.xe)
    fprintf('Event Triggered\n')
else
    fprintf('Event Not Triggered\n')
end

However, your existing code is fine. In your existing code, if there is no xe field then the ~isfield is true and the || short-circuits and you get the Not Triggered. If there is an xe field then the ~isfield is false but you are in a || so the second test is performed which is about emptiness. You can only reach the Triggered message if there is an xe field and it is not empty, which is what you want.

Answer 2

[Andrei Cimponeriu]

Hi,

For a structure named TT having one of the fields called FF, you could try this:

if ismember('FF',fieldnames(TT)) ...

Best regards,

Andrei