How to get an array of all field elements of a 1xN structure with many fields

In this thread. @Sebastian asked how to get an array of all field elements of a 1xN structure. @AdamDanz answered, but his answer only applies to a single specified field of a struct. I want be able to loop through all fields of a struct and exact all elements of each field.

For example

S(1).a = 1; S(2).a = 2 ; S(1).b = 3 ; S(2).b = 4;  S(1).c = [ 1, 2]; S(2).c = [ 3, 4]

From @AdamDanz's answer, I can write

[S(:).a]
[S(:).b]

etc., but naturally one wants to be able to loop thru all fields of S, as in

Fields = fieldnames(S);
for ii= 1:numel(Fields);
    field = Fields{ii};
    eval(['[S(:).' field ']']);
end

There has to be a less kludgy way of doing this!!! Thanks!

[Stephen23]

"There has to be a less kludgy way of doing this!!! "

For a start, you can trivially remove that very ugly and inefficient EVAL by using dynamic fieldnames:

https://www.mathworks.com/help/matlab/matlab_prog/generate-field-names-from-variables.html

i.e. replace this slow, inefficient, complex, anti-pattern code:

eval(['[S(:).' field ']']) % ugh, do NOT do this!

with this neat, simple, and very efficient code:

[S(:).(field)]

or equivalently just this:

[S.(field)]

See also:

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

Using a FOR loop is probably the most efficient solution to your original question.

Answer 1

[Matt J]

I would recommend the attached file

S(1).a = 1; S(2).a = 2 ; S(1).b = 3 ; S(2).b = 4;

T=scalarize_struct(S)

T = struct with fields:
    a: [1 2]
    b: [3 4]

[Leo Simon]

Thanks very much @Matt J, unfortunately my example was too simplistic, because some fields are vectors rather than scalars. I'll edit my original question to be more general. I wonder if the file

arrayify_struct

that is referred to at the top of

scalarize_struct.m

would work for the more general case? I searched for that name on the web, but couldn't find it.

[Matt J]

scalarize_struct works for your modified example, too

S(1).a = 1; S(2).a = 2 ; S(1).b = 3 ; S(2).b = 4;  S(1).c = [ 1, 2]; S(2).c = [ 3, 4];

T=scalarize_struct(S)

T = struct with fields:
    a: [1 2]
    b: [3 4]
    c: {[1 2]  [3 4]}

[Leo Simon]

scalarize_struct woks perfectly, thanks very much!

Answer 2

[Jan]

Fields = fieldnames(S);
for ii= 1:numel(Fields);
    field = Fields{ii};
    [S(:).(field)]
end

[...] is the horizontal concatenation, so the line [S(:).(field)] is equivalent to:

cat(2, S(:).(field))

Maybe you want to join the vectors vertically, then use cat(1, ...). If the arrays have different sizes, you need a cell array:

{S(:).(field)}

[Leo Simon]

Thanks @Jan, I accepted @Matt's answer because was a bit easier to work with in my case.