struct to cell array

I was working with structs and cell array and I came across something I didn't quite understand.

If I have a struct and its transpose -

s = struct('A',{1;2;3}) % 3x1 struct
sT = struct('A',{1,2,3}) % 1x3 struct

Then using dot notation to select a single field will produce a cell array with the same dimensions either way -

d = {s.A} % 1x3 cell

dT = {sT.A} % 1x3 cell

Why aren't d and dT transposes of each other?

[Stephen23]

"Why aren't d and dT transposes of each other?"

Comma-separated lists are a powerful and useful feature, once their concept is understood.

The reason why is because the comma-separated list that you generated is not one variable as you seem to think (which can have a size or orientation) but is simply a list of scalar variables, and such a list does not have anything like a size or orientation. It is just a list, separated by commas. Both of your examples produce exactly the same comma-separated list:

1,2,3

and then call the cell array constructor with that (same) comma-separated list:

{1,2,3}

The size of your output (if any) depends entirely on the function that you are entering that comma-separated list into as input arguments (i.e. the cell array constructor). Of course with matrices/arrays the order of the comma-separated list can be different, depending on the orientation of the array, but for vectors their orientation makes absolutely no difference.

So, simply put, a comma-separated list does not have size, therefore the output size depends only on the function that you are using that comma-separated list with.

Read these to know more:

https://www.mathworks.com/help/matlab/matlab_prog/comma-separated-lists.html

https://www.mathworks.com/matlabcentral/answers/320713-how-to-operate-on-comma-separated-lists

https://www.mathworks.com/matlabcentral/answers/344486-generate-comma-separated-list-in-single-line-of-code

Answer 1

[per isakson]

Why? Because Matlab works that way.

>> nums.f
ans =
     1
ans =
     2
ans =
     3

outputs a list and

     {}     

concatenates the list horisontally into a cell array.

See Access Elements of a Nonscalar Struct Array and Comma-Separated Lists

Answer 2

[Walter Roberson]

What you are doing is structure expansion, which is comma separated lists. So you are getting

{sT(1).A, sT(2).A, sT(3).A}