Create array of "deep" struct (scalar) fields

Question

How can I collapse the values of "deep" struct fields into arrays by just indexing?

In the example below, I can only do it for the "top-most" level, and for "deeper" levels I get the error:

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

The only workaround I found so far is to unfold the operation into several steps, but the deeper the structure the uglier this gets...

clc; clear variables;

% Dummy data
my_struc.points(1).fieldA = 100;
my_struc.points(2).fieldA = 200;
my_struc.points(3).fieldA = 300;
my_struc.points(1).fieldB.subfieldM = 10;
my_struc.points(2).fieldB.subfieldM = 20;
my_struc.points(3).fieldB.subfieldM = 30;
my_struc.points(1).fieldC.subfieldN.subsubfieldZ = 1;
my_struc.points(2).fieldC.subfieldN.subsubfieldZ = 2;
my_struc.points(3).fieldC.subfieldN.subsubfieldZ = 3;
my_struc.info = 'Note my_struc has other fields besides "points"';

% Get all fieldA values by just indexing (this works):
all_fieldA_values       = [my_struc.points(:).fieldA]

% Get all subfieldM values by just indexing (doesn't work):
% all_subfieldM_values  = [my_struc.points(:).fieldB.subfieldM]
% Ugly workaround:
temp_array_of_structs   = [my_struc.points(:).fieldB];
all_subfieldM_values    = [temp_array_of_structs.subfieldM]

% Get all subsubfieldZ values by just indexing (doesn't work):
% all_subsubfieldZ_values   = [my_struc.points(:).fieldC.subfieldN.subsubfieldZ]
% Ugly workaround:
temp_array_of_structs1  = [my_struc.points(:).fieldC];
temp_array_of_structs2  = [temp_array_of_structs1.subfieldN];
all_subsubfieldZ_values = [temp_array_of_structs2.subsubfieldZ]

Output:

all_fieldA_values =

   100   200   300

all_subfieldM_values =

    10    20    30

all_subsubfieldZ_values =

     1     2     3

Thanks for any help!

[my_struc.points(:).fieldA] is the same as [my_struc.points.fieldA]. — Cris Luengo
– Cris Luengo, Commented Aug 11, 2019 at 13:15
"but the deeper the structure the uglier this gets" -> Deep structures are ugly. Avoid using them, then you will avoid this problem! :) — Cris Luengo
– Cris Luengo, Commented Aug 11, 2019 at 13:16
Indeed. In particular, I haven't found a way to initialize them outside loops where their data (e.g. the "points(:)" fields in the example) is generated, so currently they are growing inside the loop! Is there a way to get around this? I know in advance how many points I'll have, but each one has a complex subfields hierarchy so initialization by hand is not an option, I guess... :( — dontpanic
– dontpanic, Commented Aug 12, 2019 at 9:49
If they all have the same subfield hierarchy, you can create one of them, then use deal to assign the same template to all points: [my_struct.points(1:N)] = deal(template). — Cris Luengo
– Cris Luengo, Commented Aug 12, 2019 at 12:26

rinkert · Accepted Answer · 2019-08-11 10:32:45Z

1

You can use arrayfun to have acces to each individual 'point', and then acces its data. This will return an array with the same dimensions as my_struc.points:

all_subfieldM_values = arrayfun(@(in) in.fieldB.subfieldM, my_struc.points)
all_subsubfieldZ_values = arrayfun(@(in) in.fieldC.subfieldN.subsubfieldZ, my_struc.points)

Not optimal, but at least it's one line.

answered Aug 11, 2019 at 10:32

rinkert

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