Nesting depth and the error "Expected one output from a curly brace or dot indexing expression, but there were x results."

Consider this toy example:

a(1).x.y=1
a(2).x.y=2
[a.x.y]

Why does this produce the infamous error

Expected one output from a curly brace or dot indexing expression, but there were 2 results.

..instead of just listing all values from across all indexed structure elements, as in this other example where the nesting is at level 2 instead of level 3:

a(1).x=1
a(2).x=2
[a.x]
>> [a.x]

ans =

1     2

[Stephen23]

"Why does this produce the infamous error"

https://www.mathworks.com/matlabcentral/answers/329538-why-do-i-get-this-error-for-this-non-scalar-structure#answer_258438

https://www.mathworks.com/matlabcentral/answers/1656435-tutorial-comma-separated-lists-and-how-to-use-them#answer_966455

Answer 1

[Bruno Luong]

a work around if you insist on oneline

a(1).x.y=1

a = struct with fields:
    x: [1×1 struct]

a(2).x.y=2

a = 1×2 struct array with fields:
    x

axy = [struct([a.x]).y]

axy = 1×2

1 2

Answer 2

[Jan]

R2022a creates a different error:

a(1).x.y=1;
a(2).x.y=2;
[a.x.y]

Intermediate dot '.' indexing produced a comma-separated list with 2 values, but it must produce a single value when followed by subsequent indexing operations.

"instead of just listing all values from across all indexed structure elements"

Think twice. [a.x] is an array already with 2 elements. The dot operator cannot handle an array as input, but a scalar struct only. This is plausible. Consider, that there is no logical decision for the dimensions of the output. It is also unclear, what you call "just listing all values".

[z8080]

I meant (and was expecting to see) pooling all values from the end field (`y`), under all possible index values at every level of nesting.

But the 2022a error message is more helpful - thanks.

[Image Analyst]

Is there a bracket/brace/parentheses solution to this, or is the only way a simple but intuitive for loop

a(1).x.y=1;
a(2).x.y=2;
all_y = zeros(numel(a), 1);
for k = 1 : numel(a)
    all_y(k) = a(k).x.y;
end

or possibly a cryptic call to structfun or some other weird function

[Stephen23]

"The dot operator cannot handle an array as input, but a scalar struct only."

???

a(1).x.y=1;
a(2).x.y=2;
tmp = [a.x] % array struct, not scalar struct

tmp = 1×2 struct array with fields:
    y

[tmp.y] % dot indexing accepts an array without any problem

ans = 1×2

1 2