Replacing a field in an structure that is subset within a larger structure

I'm currently trying to resolve a scripting issue with EEG data, whereby I want to replace a field (type) within a structure (event) that is part of a larger structure (i.e., EEG.event.type).

The replacement data is the same length as EEG.event.type, and is currently stored as type cell, called 'codes'.

At present my code looks like this:

EEG.event = rmfield(EEG.event, 'type') %remove field 'type' from EEG.event structure

EEG.event.type = codes;

this does not work, and instead gives me the error 'Scalar structure required for this assignment.'

However, if I try to add codes to the structure EEG, this seem to work using identical code:

EEG.new = codes;

So specifically, I seem to be having an issue with creating a new field in a structure that is subset within another structure.

Any input very gratefully recieved!

[Jon]

Could you please attach a small, self contained runnable example that reproduces the problem, so we can see exactly what is going wrong

[Stephen23]

"I seem to be having an issue with creating a new field in a structure that is subset within another structure"

No, it is entirely due to the size of the structure that you are trying to assign to:

S = struct('X',{1,2,3})

S = 1×3 struct array with fields:
    X

S.Y = {1,2,3}

Scalar structure required for this assignment.

The fact that the structure is stored within another structure makes no difference.

Answer 1

[Stephen23]

If EVENT is non-scalar them you will get that error. The correct approach is to use a comma-separated list.

Lets try it right now:

codes = {7,8,9};
EEG = struct('event', struct('type',{1,2,3}, 'blah',{4,5,6}))

EEG = struct with fields:
    event: [1×3 struct]

EEG.event = rmfield(EEG.event, 'type')

EEG = struct with fields:
    event: [1×3 struct]

[EEG.event.type] = codes{:}; % comma-separated list -> no error!
EEG.event.type % checking the elements of EVENT

ans = 7

ans = 8

ans = 9

Whereas your original approach throws an error:

EEG.event = rmfield(EEG.event, 'type');
EEG.event.type = codes; % this assignment will not work.

Scalar structure required for this assignment.

See also:

https://www.mathworks.com/help/matlab/matlab_prog/access-multiple-elements-of-a-nonscalar-struct-array.html

https://www.mathworks.com/matlabcentral/answers/1656435-tutorial-comma-separated-lists-and-how-to-use-them

[Jen]

Thank you so much Stephen, that's done it! I had no idea as to the correct approach for the scalar structure error but this makes total sense now.

Thanks again!

[Stephen23]

"I had no idea as to the correct approach for the scalar structure error..."

Yes, that message is not very clear. As far as I can tell, the "scalar structure" refers to what should be on the LHS of the assignment, in order for that assignment to work:

S = struct('X',pi) % scalar structure

S = struct with fields:
    X: 3.1416

S.Y = {1,2,3}

S = struct with fields:
    X: 3.1416
    Y: {[1]  [2]  [3]}

The error message could make that advice clearer, and give some advice on non-scalar structures too.