Why contructing a MATLAB struct object with an empty cell it creates an empty struct instead?

Question

I want to construct an struct object with three properties:

arg1 = 42;
arg2 = 'test';
arg3 = cell(0);

But if I try to initialize that object:

struct('arg1', arg1, 'arg2', arg2, 'arg3', arg3);

It returns an empty struct:

ans = 

  0×0 empty struct array with fields:

    arg1
    arg2
    arg3

I figured out the empty cell is the culprit, so if i initialize it without the empty cell it returns a correct value:

ans = 

  struct with fields:

    arg1: 42
    arg2: 'test'
    arg3: []

But I need my code to work with empty cells, and I don't know if or where they will be in one of the fields.

Is there a way to get out of this problem?

Cris Luengo · Accepted Answer · 2023-08-27 13:36:04Z

3

@Wolfie's explanation for the behavior is correct.

The workaround is to put the data in cell arrays like so:

>> struct('arg1', {arg1}, 'arg2', {arg2}, 'arg3', {arg3})

ans = 

  struct with fields:

    arg1: 42
    arg2: 'test'
    arg3: {}

This works because of this line in the documentation:

If any of the value inputs is a nonscalar cell array, then s has the same dimensions as the nonscalar cell array.

So we make all the value inputs cell arrays. {arg3} is the same as {{}}, a cell array with one element: an empty cell array.

Note that it is possible to create a struct array with this syntax:

>> struct('arg1', {1,2,3}, 'arg2', {arg2}, 'arg3', {arg3})

ans = 

  1×3 struct array with fields:

    arg1
    arg2
    arg3

Because the 'arg1' argument is a cell array with 3 elements, the created struct array also has 3 elements. The cell arrays with a single element are replicated across all cell arrays.

edited Aug 27, 2023 at 13:36

answered Dec 14, 2018 at 14:16

Cris Luengo

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Community · Accepted Answer · 2020-06-20 09:12:55Z

2

This is documented behaviour:

s = struct(field,value) creates a structure array with the specified field and values. The value input argument can be any data type, such as a numeric, logical, character, or cell array.

If any of the value inputs is a nonscalar cell array, then s has the same dimensions as the nonscalar cell array. [...]

If value is an empty cell array {}, then 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

The take-away for you is the last line.

To get around this, you will have to do checks like

if iscell( argX ) && isempty( argX )
    argX = [];
end

If you always just have 3 items in your struct then this is fairly simple to implement.

edited Jun 20, 2020 at 9:12

CommunityBot

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answered Dec 14, 2018 at 12:41

Wolfie

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