How can I access all values of a field in an array of nested structs

Question

Michael 2023-8-7

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/2005767-how-can-i-access-all-values-of-a-field-in-an-array-of-nested-structs

评论： Michael 2023-8-7

The following code creates an array of nested structs (length = 4).

nested_struct = struct(...
    'a', 1, ...
    'b', 1 ...
    );
s = struct( ...
    'c', 0, ...
    'nested', nested_struct ...
    );
array_of_nested_struct = repmat(s, 4, 1);

How can I easily access an array of all of the values of the field a ?

For example, I'd like to compute the sum as

% sum(array_of_nested_struct.nested.a)

However, this does not work because

array_of_nested_struct.nested
ans = struct with fields:
    a: 1
    b: 1
ans = struct with fields:
    a: 1
    b: 1
ans = struct with fields:
    a: 1
    b: 1
ans = struct with fields:
    a: 1
    b: 1

does not produce a struct array.

My only solution seems overly cumbersome, and requries usage of intermediate variables:

% The following is not an array of structures
array_of_nested_struct.nested;
% So convert it into a struct array
nested = [array_of_nested_struct.nested];
% Do the same for the next level
a_value_array = [nested.a];
% Finally compute sum
sum(a_value_array)
ans = 4

is there a better syntax to achieve this result? The syntax should also support code generation without any data copies

Thanks!

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Stephen23 2023-8-7

"is there a better syntax to achieve this result?"

Don't spread data over lots of different structures if it is intended to be processed together.

Michael 2023-8-7

Thanks, Stephen. In this case, the data naturally wants to be this way. I actually have two substructs that need to go two different processing components.

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Answer 1

Benjamin 2023-8-7

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/2005767-how-can-i-access-all-values-of-a-field-in-an-array-of-nested-structs#answer_1284437

编辑：Benjamin 2023-8-7

在 MATLAB Online 中打开

% Given code to create the array_of_nested_struct
nested_struct = struct(...
    'a', 1, ...
    'b', 1 ...
);
s = struct( ...
    'c', 0, ...
    'nested', nested_struct ...
);
array_of_nested_struct = repmat(s, 4, 1);
% Use arrayfun to extract 'a' field values and compute the sum directly
sum_a_values = sum(arrayfun(@(x) x.nested.a, array_of_nested_struct));
% Display the sum
disp("Sum of 'a' values: " + sum_a_values);

So this here will give you the array:

array = arrayfun(@(x) x.nested.a, array_of_nested_struct)

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Michael 2023-8-7

在 MATLAB Online 中打开

Thanks, @Benjamin! This is much better, though still a little uneasy on the eyes.

Unfortunately, in codegen it still makes a copy of the data before performing the sum, instead of just referencing the values in place.

An example of the generated code follows (Note it's approximate - I changed some variable names).

        for (c_k = 0; c_k < 4; c_k++) {
          temp[c_k] = b_varargout_2[c_k].nested.a;
        }
        bsum_re = temp[0];
        for (xblockoffset = 0; xblockoffset < 3; xblockoffset++) {
          bsum_re += temp[xblockoffset + 1];
        }