Create array of “deep” struct (scalar) fields

Question

Jorge 2019-8-11

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/475708-create-array-of-deep-struct-scalar-fields

回答： Jorge 2019-8-12

采纳的回答： Jorge

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Hi! 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!

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Stephen23 2019-8-11

There is no way to avoid this.

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

https://www.mathworks.com/matlabcentral/answers/421867-accessing-all-lower-elements-of-a-structure

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

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

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Answer 1

Jorge 2019-8-12

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https://ww2.mathworks.cn/matlabcentral/answers/475708-create-array-of-deep-struct-scalar-fields#answer_387169

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Thanks for the answers & comments; turns out it's possible to use arrayfun() to this purpose!:

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

This achieves the desired effect with a single line of code :)

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Answer 2

TADA 2019-8-11

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As always, you can use a small utility function to drill down the object hierarchy

It's probably not the most elegant solution because you have to write the field names as strings, so don't expect any intellisence there, but hey, subsref does the same thing so...

function b = drillref(a, s)
    fields = strsplit(s, '.');
    b = a;
    for i = 1:numel(fields)
        b = [b.(fields{i})];
    end
end
A = drillref(my_struc, 'points.fieldA')
A =
   100   200   300
B_M = drillref(my_struc, 'points.fieldB.subfieldM')
B_M =
    10    20    30
C_N_Z = drillref(my_struc, 'points.fieldC.subfieldN.subsubfieldZ')
C_N_Z =
     1     2     3