Strange behavior when initializing empty cell in struct()

Question

0 个投票

Structs initialized with an empty cell using both {} and cell.empty within a struct() returns an empty struct object.

The following function reproduces that:

function print_hello_world()
    obj = struct(MyProp=123, MyOtherProp={}); % or MyOtherProp=cell.empty
    disp(obj)
    disp(isempty(obj))
end

The obj variable is empty when running print_hello_world.

I have resorted to the following to go around this issue:

function print_hello_world()
    obj = struct(MyProp=123);
    obj.MyOtherProp = {};
    disp(obj)
    disp(isempty(obj))
end

The value of obj is as expected when running the above.

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Stephen23 2024-6-20

"Structs initialized with an empty cell using both {} and cell.empty within a struct() returns an empty struct object."

Answer 1

Aquatris 2024-6-20

编辑：Aquatris 2024-6-20

1 个投票

If you want to define the property as empty, then you should use '[]' instead of '{}'.

obj = struct(MyProp=123, MyOtherProp=[]); 
disp(obj)
         MyProp: 123
    MyOtherProp: []

This is an intended behaviour. Here is the relevant part of the documentation:

s = struct(field1,value1,...,fieldN,valueN) creates a structure array with multiple fields.

...
If any value input is an empty cell array, {}, then output s is an empty (0-by-0) structure. To specify an empty field and keep the values of the other fields, use [] as a value input instead.

Stephen23 2024-6-20

Or if an empty cell array is required:

obj = struct(MyProp=123, MyOtherProp={{}});