How to use parfor to create an array of SimulationInput objects?

I have a piece of code used to generate an array of SimulationInput objects, it works in a normal for loop, but when I switch to parfor loop, it throws error. The code named `generate_simin` is like the following:

model_name = 'my_model';
params_list = linspace(0,1,100);
numSims = length(params_list);
inCmd(1:numSims) = Simulink.SimulationInput(model_name); % initialize an array of simulationInput objects

parfor idx = 1:numSims % replaced for with parfor for performance
    inCmd(idx) = setVariable(inCmd(idx),'parameter_1',params_list(idx));
end

Then I received error:

Error generate_simin/parfor%supply_1
Index exceeds the number of array elements (0).

Error in generate_parsim>generate_simin (line 5)
parfor idx = 1:numSims % replaced for with parfor for performance

I followed the principle that each parfor iteration should be independent and in non-deterministic order, and that is why I initialized an array as a place holder to store the results for each iteration. The error seems to say that my pre-initialized array of SimulationInput objects 'inCmd' has 0 elements. But this is not true because if I type 'size(inCmd)' in my MATLAB command window, I will get:

size(inCmd)

ans =

           1        100

Clearly, it is an array with 100 elements. So why is parfor throwing error saying:

Index exceeds the number of array elements (0).

Any help or hint is appreciated!

Edit: I found out by myself that the problem was actually not with the syntax of SimulationInput, but stems from my illegal use of parfor loop. In my parfor loop, I had some if conditions. And in those if conditions, different variables were called, some exists in one type of upstream data, some exists in another type of upstream data. Something like this:

parfor idx = 1:numSims
    if data_type == 1
        params_list = type_1_specific_params_list;
    end
    if data_type == 2
        params_list = type_2_specific_params_list;
    end
    inCmd(idx) = setVariable(inCmd(idx),'parameter_1',params_list(idx));
end

But each time I run this parfor loop, only one of the two variables type_1_specific_params_list or type_1_specific_params_list were defined. And this was actually why I had the error:

Index exceeds the number of array elements (0).

[Edric Ellis]

Which version of MATLAB are you using? I tried your reproduction steps in R2022a, and they appeared to work correctly.

[Tong Zhao]

Hi Edric,

Thank you for your time trying it. Actually it was my fault. I identified the problem, and it was not this part of the code, but rather something else. It was because I was using if else conditions inside the parfor loop, and for some of the if conditions, although they will be false, the variables under those conditions were not initialized. For for loops this was completely fine as it did not check the existence of variables if the conditions are not met; however in parfor, there seems to be some mechanism demanding that all variables insided the parfor loop are defined or initialized.

I will answer my own question and modify the question accordingly.

Answer 1

[Tong Zhao]

My own answer to the problem:

Actually there's no syntax error regarding SimulationInput here. The error was due to another reason: I called variables in a parfor loop that was not previously defined.

In my parfor loop, I had some if conditions. And in those if conditions, different variables were called, some exists in one type of upstream data, some exists in another type of upstream data. Something like this:

parfor idx = 1:numSims
    if data_type == 1
        params_list = type_1_specific_params_list;
    end
    if data_type == 2
        params_list = type_2_specific_params_list;
    end
    inCmd(idx) = setVariable(inCmd(idx),'parameter_1',params_list(idx));
end

But each time I run this parfor loop, only one of the two variables type_1_specific_params_list or type_1_specific_params_list were defined. And this was actually why I had the error:

Index exceeds the number of array elements (0).